American Mineralogist, Volume 66, pages 610-625, l98l

The crystal chemistry of the uranyl silicate minerals

FRANcES V. SroHr, eNo DneNB K. SunH

Department of Geosciences, The Pennsylvania State (JniversityUniversity Park, Pennsylvania I 6802

Abstract

The uranyl silicate minerals have been divided into three groups on the basis of their ura-nium to silicon ratios. The l: I group includes uranophane, beta-uranophane, boltwoodite,sodium boltwoodite, kasolite, sklodowskite, and cuprosklodowskite. A structure refinementof uranophane, a structure determination of boltwoodite, and previously reported structuredetermiaations o.f most of these minerals indicate that they are composed o1 uranyl silicatechains made of edge-shared uranium pentagonal bipyramidal groups and silicate tetrahedra.These chains have the composition (uorxsio4)h2" and are crosslinked by a bridging oxy-gen atom to form a uranyl silicate sheet. These sheets are crossbonded by the additional cat-ions in the structure. The uranyl minerals with a uranium to silicon ratio of I :3 include week-site and haiweeite. A partial structure analysis of weeksite suggests that the structure type forthis group consists of uranyl silicate chains, similar to those found in the I : I group, that arecrosslinked by the additional silicate tetrahedra in the structure. The uranyl miniral groupwith a uranium to silicon ratio of 2:l contains only the mineral soddyite. This structure iscomposed of uranyl silicate chains that are crossbonded by sharing a common silicon to givea three-dimensional framework structure. A new triclinic uranyl silicate mineral was discov-ered during this study, although there is not enough sample to describe it adequately. Thelocations of the uranium atoms in this structure indicate that it may not be composed of ura-nyl silicate chains such as those found in all the other uranyl silicate minerals.

Introduction

The known uranyl silicate minerals can be dividedinto several categories on the basis of their uraniumto silicon ratios (Table l). Three categories, with ura-nium to silicon ratios of l: l, l:3, and 2: l, are welldefined as reported by Stohl (1974) and Stohl andSmith (1974). The minerals listed in Table I are theonly accepted uranyl silicates as indicated by Fleis-cher (1980).

I : I Uranyl silicate group

Structure determinations were carried out for sixof the members of the group with a uranium to sili-con ratio of 1: l. The structure of uranophane wasoriginally determined by Smith et al. (1957). and isrevised in this study. The structure of beta-ura-nophane was determined by Smith and Stohl (1972).A structure analysis of boltwoodite was carried out

rPresent address: Organization 4731, Sandia National Laborato-ries, Albuquerque, New Mexico,87 185.

0003-004x/8 I /0506-06 I 0$02.00

during this study. The structure of kasolite was origi-nally determined by Huynen et al. (1963), and wasrevised by Mokeeva (1965), and by Rosenzweig andRyan (1977). The sklodowskite structure was ana-lyzed by Mokeeva (1959), and refined by Huynenand Van Meerssche (1962), by Mokeeva (1964), andby Ryan and Rosenzweig (1977). The cuprosklo-dowskite structure was originally determined by pi-ret-Meunier and Van Meerssche (1963), and was re-vised by Rosenzweig and Ryan (1975). The formulaslisted in Table I for uranophane and boltwoodite arebased on this work whereas the other formulas comefrom the most recent structural papers for each rrin-eral.

The description, properties, and cell constants ofsodium boltwoodite were feported by Chernikov etal. (1975). There has not been any structure work onthis mineral. Its cells constants, however, are verysimilar to those of the other I : I uranyl silicate min-erals, indicating that it probabty contains the sameuranvl silicate sheets.

6 t0

STOHL AND SMITH: URANYL SILICATE MINERALS 6II

Table l. Members ofthe uranyl silicate mineral group separated on the basis oftheir uranium to silicon ratios

Uran ium: S i l i con , Rat ios

I : 3Ques t ionablecompos i tLon

Uranophaneca (HrO) r (uor) z$ io i z ' 2Ezo

Beta-uranophaneca (Uor) (uooH) (si04 ) (Sio30H)' 4H20

Bo1 twoodi ceK(H3o) (uo2) (s io4)

Na bo l twood i te(N.0 .

ZKo. : ) (H30) (uo2) (s i04) 'H20

K a s o l l t ePb (uo2) (s io4) .H2o

Sklodowsk i te

Ms (H30) 2 (Uo2 ) 2

(s i04 | 2 . 4H2c.

Cupros k lodowsk i tecu[ (uor ) , (s io3oH)

2 ] ' 6H2o

Weeks i teKz{oo)2( s i2o5)3 .4H20

Haiwee i teCa (uor) , (Sl r0r)

r 'Suro

Soddy i te(uo2) 2 (sro4) '2H2o

New rnineral

I :3 Uranyl silicate group

The formulas for the two known I :3 uranyl sili-cate minerals that are listed in Table I were takenfrom Fleischer (1980). The mineral weeksite, whichwas originaily called gastunite by Honea (1959), wasdescribed by Outerbridge et al. (1960) and wasnamed for Dr. Alice Weeks. The name weeksite wasretained due to the many conflicting descriptions ofgastunite that have been reported. The compositionof this mineral is usually written with SLO,' groupson the basis of infrared data and chemical analyses.

Haiweeite was named for its type locality near theHaiwee Reservoir in the Coso Mountains, California,and was described by McBurney and Murdock(1959). The composition was originally reported asCaO' 2UOr' 6SiO,' 5H2O, although these authorsindicated that the spherulitic aggregates of crystalsare probably made up of two minerals with varyingwater contents. The inner part of the spherulite, withhigher indices of refraction, was considered to bemeta-haiweeite and can be produced from the outermaterial by heating. However, meta-haiweeite hasnot been adequately described and has therefore notbeen accepted as a mineral (M. Fleischer, Smithso-nian Institution, personal communication). Out-erbridge et al. (1960) showed that there is a verystrong similarity between the powder patterns ofweeksite and haiweeite.

The mineral ranquilite, which was reported byAbeledo et al. (1960), is actually haiweeite and hasbeen discredited (Fleischer, 1980). Ursi l i te(Ca,Mg),(UO,)r(Si3O,o)' 9HrO (Chernikov et al.,1957) has not been adequately described and is notaccepted as a mineral (M. Fleischer, Smithsonian In-stitution, personal communication). There are nopreviously reported structure determinations in thel :3 group.

2: I Uranyl silicate grouP

The only known mineral that occurs in the 2: Iuranium to silicon group is soddyite. This mineralwas originally described by Schoep (1922) from Ka-solo, Zaire, and was named in honor of the Englishradiochemist Frederick Soddy. Gorman (1952) de-scribed the physical properties, morphology, andpowder pattern of soddyite. He indicated that it usu-ally occurs as zoned transparent and opaque mate-rial, both of which have the same.powder patterns.The crystal structure of soddyite was originally pro-posed by Stohl (1974) and Stohl and Smith (1974) onthe basis of a partial structure analysis of a synthetichydrated uranyl germanate analogue that was re-ported by Legros et al. (1972). The proposed soddyitestructure is essentially identical to the completed ura-nyl germanate structure reported in Legros and Jean-nin (1975). The proposed soddyite structure was sub-

6t2

stantiated by a structure analysis carried out on asynthetic soddyite crystal by Belokoneva et al.(1979). The formula for soddyite in Table I is basedon this crystal structure.

New mineral

The new mineral that was discovered during thisresearch has an appearance and powder pattern verysimilar to soddyite. A chemical analysis cannot becarried out due to the small amount of sample.

The I : I group of uranyl silicates

The refnement of uranophane

The composition CaO.2UO..2SiOr.5HrO occurs intwo polymorphic forms as the minerals uranophaneand beta-uranophane. The structure of uranophanewas originally determined by Smith et al. (1957), al-though they were not successful in locating the watermolecules. They suggested the possible presenoe ofhydronium ions to balance the charge. A structureanalysis of beta-uranophane by Smith and Stohl(1972) indicated that the extra hydrogen atoms in thestructure that are necessary to balance the chargemight be bonded to the free uranyl oxygen atomsand the free oxygen atoms of the silicate tetrahedronas indicated by the formula in Table l. Therefore. astructure refinement of uranophane was carried outin order to locate the water molecules and therebyclarify the relationships between these two minerals.

A very high quality, clear crystal from Shinko-lobwe, Katanga was selected for the structure refine-ment. This very small crystal, measuring lOxl2x215,rm, was chosen to minimize absorption effects. Therefined cell constants, which were obtained duringthe alignmgnt segment of data collection on a PickerFAcs-l four-circle diffractometer, are shown inTable 2. This structure refinement substantiated theP2, space group that was reported by Smith et c/.(1957). The F2,/a pseudosymmetry was also sub-stantiated because the (ft0l) reflections with ft odd aremuch weaker than the (ftOl) reflections with & even. Adata set consisting of 525 independent reflectionswith a maximum 20 valte of 35o was measured usingMoKa radiation. Measurable intensities could not beobtained above 35o 20 because of the small crystalsize. Thirty-two of the reflections were unobserved,and their observed structure factors were set al zeto.The intensities were corrected for absorption effectsand Lorentz-polarizaton effects using the programACAC by wuensch and Prewitt (1965). The atom lo_cations reported by Smith et al. (1957) were refined

ST:OHL AND SMITH: URANYL SILICATE MINERALS

using the full matrix least squares program onFLS byBusing et al. (1962). These refined atom positionswere used to calculate a three-dimensional Fouriermap and a three-dimensional difference Fourier mapwhich showed the locations of four possible oxygens,from water molecules, in the general positions of p2,around the calcium atoms at distances of approxi-mately 2.4 to 2.64. Throughout this analysis, all theatoms in the sheet were refined u5ing the restrictionsof the pseudosymmetry, whereas the calcium atomsand oxygens from the water molecules were refinedusing the P2, space group. A refinement of this entirestructure with isotropic temperature factors gave R :0.081. The observed and calculated structure factorsare given in Table 3.'?The hydrogen atoms could notbe located. The largest electron diferences in the fi-nal difference Fourier were less rhan l/2 electron/Ar.Table 4 lists the atomic coordinates and isotropictemperature factors for the atoms of uranophane,and Table 5 shows the bond distances and angles inthe structure. The high temperature factors shown inTable 4 are normal for uranyl silicate structure deter-minations. Figure I shows a stereo view of the sheetstructure.

Structure determination of boltwoodite

The sample used in this analysis is from the NewMethod Mine in Amboy, California, and was lent tothe authors by Paul Moore of the University of Chi-cago. The identification of this sample was verifiedby comparing its Guinier powder pattern to a patternof the type material from the Delta Mine on thesouthern edge of the San Rafael Swell, EmeryCounty, Utah (U.S. Museum sample numberll27l0). This comparison proved that the sampleused in this analysis was boltwoodite, and also sub-stantiated the fact that the powder pattern of bolt-woodite reported by Honea (1961) (PDF 13-218 and29-1026) contained several extra tines (7.53A.3.91A.3.75A, and 2.08A). The 7.53A line is probably a gyp-sum peak and the 3.91A line is probably a brochan-tite peak. These two minerals are found in associa-tion with the type material. Table 6 shows the d-spacings measured from Guinier powder patterns ofthe New Method Mine sample and the type material,and a pattern that was calculated from the present

2To receive a copy of Table 3, order Document AM-gl-156from the Business Office, Mineralogical Society of America, 2000Florida Avenue NW, Washington, DC 20009. please remit $1.00in advance for the microfiche.

STOHL AND SMITH: URANYL SILICATE MINERALS

Table 2. The crystallographic data for the l: I minerals. The listings of the cell constants for beta-uranophane and cuprosklodowskite

have been shifted so that they can be easily compared to the structurally equivalent values in the other minerals

6 1 3

1Uranophane-

a = l s . 858 t0 .015 ib = 6 . 9 8 5 1 0 . 0 0 7c = 6 . 6 4 1 1 0 . 0 0 5S = 9 7 " 3 3 ' ! 2 1- - 1

Fiber paral le l Dsheet paral le l (100)

Kasol l te5

a = 5.?ol+to.oozib = 6.93zto.ooac = I3.2r2tO.OO7I = t o 4 ' 1 3 ' t 2 '

P2rlc

Flber para1lel bstreet paratlel ( too)

Beta-uranophane 2

b = r s .39a to .o14 id = 1 3 . 8 9 8 1 0 . 0 1 1c = 6 . 6 0 9 1 0 . 0 0 5B = 9 L " 2 5 ' ! l 'P2 I l d

Fiber paral le l astreet paral le l (010)

Sklodowski te6

4 = r?.382to.oo6;b = T .04Zt0 .oorc = 6 .5 r0+o.oo2B = ro5 '51+ ' t r2 '

C2/n

I l b e r p a r a l . t e L

S h e e t p a r a l l e l ( 1 0 0 )

Bol twood i te3

a = 7.ot3to.oozLb = 7 . 0 6 4 1 0 . 0 0 1c = 6 . 6 3 8 1 0 . 0 0 1B = IO5 . l +5 ' r I 'p)- - 1

Flber paral le l bSheet paral le l (100)

Cuprosklodowskl te7

ob = 9 . 2 6 7 ! 0 . 0 0 8 Aa = 7 . 0 5 2 1 0 . 0 0 5c = 6 . 6 5 5 1 0 . 0 0 5a = 1 0 9 o 1 4 r 1 3 'B = 8 9 o 5 0 r 1 3 'y = 1 1 0 ' 1 ' 1 4 'PI

Fiber paral le l aSheet paral le l (0 lO)

Na; bo l twood l te4

a = 27 ,4O !0 .O54b = 7 .O2 !0 .o2c = 6 , 6 5 ! 0 . 0 2

D 1 i a

Refe rences : Th i s wo rk ; 2 . Sm i th and Stohl( 1 9 7 7 ) i 6 .Rosenzweiq and Rvan

n y a n ( I 9 7 5 )

structure determination using a program from Smith(1963). The cell constants for boltwoodite (Table 2)that were used to calculate the d-spacings for thispowder pattern were obtained from a least squaresrefinement of the underlined Guinier powder datafor the type material. This refinement was carried outwith a program by Appleman and Evans (1973). Thecell constants determined during the alignment seg-ment of single crystal data collection are not reportedbecause the intensities for most reflections were veryweak.

All of the boltwoodite crystals analyzed by pre-cession and Weissenberg techniques were twinned.The two overlapping lattices, which had reflections ofthe type (hkD n coulmon, were related by reflectionacross (l0l). The only systematic extinctions were(0k0) with k :2n11, so that the possible spac€ groupswere nt or nt/m. The twinned crystal used in thisstructure determination measured 20xl5xll5 pmand had no sphenoidal terminations. A data set con-sisting of all possible reflections up to a 20 value of35o was collected from the twin lattice with thegreater intensity. A total of 223 independent reflec-tions, includng 22 multiple reflections with a com-ponent from each twin, was measured with MoKa ra-diation. A comparison of intensities of the twolattices indicated that the reflections of one latticehad an average intensity of approximately 25 percent

( 1 9 7 2 1 t 3 .Ryan and

Th is work ;Rosenzweig

4 . C h e r n i k o v e t a L . ( 1 9 7 5 ) t( f 9 7 7 ) ; 7 . R o s e n z w e i g a n d

of the intensity of the equivalent reflections of thepredominant lattice. In the initial stages of structureanalysis, 20 percent of the intensity of overlappingtwin reflections was subtracted, whereas in the finalstages of analysis, these reflections were omitted.Only an approximate absorption correction was car-ried out using the program AcAc because the twin-ning was not visible optically, and therefore the vol-ume distribution of the twins was unknown.

The structure was solved from a three-dimensionalPatterson map. The uranium, silicon, and oxygenatoms were located in positions which indicated asheet structure very similar to that in uranophane.The two potassium atoms in the unit cell were lo-cated in general positions binding the sheets togetherand proving that the space group had to be PZr.However, it was not possible to refine the y coordi-nates for the uranium and silicon atoms because the

J, coordinates kept switching back and forth acrossthe mirror plane which would be present if the sym-metry were H,/m.In addition, oxygens 3 and 38showed very high correlation coemcients betweentheir x and z coordinates, indicating that the uranylsilicate sheet has Hr/m pseudosymmetry. The pres-ence of the potassium atoms and hydronium ionstherefore causes the symmetry to degenerate fromHr/m to PZr. The final R-factor which was calcu-lated with isotropic temperature factors is 0.109. The

614 STOHL AND SMITH: URANYL SILICATE MINERALS

Atoh

Table 4. Atomic coordinates for uranophane. The space group isP2,. (Standard errors in parentheses.)

Structural comparison of the l:1 minerals

The cell constants, symmetry, fiber direction andsheet orientation of the various l: I uranyl silicateminerals are given in Table 2. All of the strucruresare composed of uranyl silicate chains that are cross-bonded by bridging oxygen atoms to form a uranylsilicate sheet (Figure l). The fibrous habit of theseminerals is due to the presence of the chains, andtheir perfect cleavage is due to the presence of thesheets. The sheets are linked by the additional cat-iorls in the structure.

The refinement of uranophane has clarified someof the relationships between this mineral and itsdimorph beta-uranophane. The two-fold screw axisin uranophane lies along b within the uranyl silicatechain. The adjacent uranyl silicate sheets are parallelto (100) and are related by the a glide plane of thepseudosymmetry, which occurs at y values of 0.25and 0.75. The equivalent uranium atoms and siliconatoms of adjacent sheets are separate dby a y value of0.06 so that adjacent sheets have almost identical ori-entations.

Table 5. Distances and angles in uranophane

1 . 7 8 + 0 . 0 5 A l1 . 8 2 F o . o s I2 . 2 1 T 0 . 0 62 . 2 6 T 0 . 0 52 . 3 4 T 0 , 0 62 . 4 3 F 0 . 0 6 i2 . 4 6 F 0 . 0 6 |

I . 5 6 + 0 . 0 7l . s 9 F o . o 61 . 5 9 F 0 . o G1 . 6 9 F 0 . 0 7

3 . L2+0 . 02

3 . 5 4 + 0 . 0 2

3 . 9 1 6 + 0 . 0 0 4

u(1)v ( 2 )s i ( 1 )s i ( 2 )o (1 )o ( 2 )o ( 3 )o ( 4 )o ( s )o (6 )o ( 7 )o (8 )o ( 9 )o (10 )o (11 )0 (12 )H20 (1 )

H20 (2)

H20 (3 )

H20(4)

0 . 2 s 5 7 ( 2 )-0 .2557 (2 )

0 . 2 8 4 ( 1 )- 0 . 2 8 4 ( 1 )

0 . 3 7 1 ( 3 )- 0 . 3 7 1 ( 3 )

0 . 1 4 3 ( 3 )- 0 . 1 4 3 ( 3 )

o . 2 7 r ( ! + )- 0 . 2 7 r ( 4 )

0 . 2 5 8 ( 4 )- 0 . 2 s 8 ( 4 )

o . 2 2 9 ( 3 )- o , 2 2 9 ( 3 )

0 . 3 8 1 ( 3 )- 0 . 3 8 1 ( 3 )

0 . 0 6 9 ( 3 )

0 . 9 9 1 ( 3 )

0 . 9 3 6 ( 3 )

0 . 9 9 3 ( 4 )

0 . 0 1 9 ( 1 )

0 . 7 8 2 2 ( 6 )- 0 . 7 8 2 2 ( 6 )

0 . 2 8 1 ( 4 )- 0 . 2 8 1 ( 4 )

0 . 7 9 s ( 9 )- 0 . 7 9 5 ( 9 )

o . 7 5 7 ( 9 )- o . 7 5 7 ( 9 )

0 . 4 5 3 ( 9 )- 0 . 4 s 3 ( 9 )

0 . 0 9 5 ( e )- 0 . 0 9 5 ( 9 )

o , 2 9 2 ( 9 )- o . 2 9 2 ( e )

o . 2 7 2 ( 9 )- 0 . 2 7 2 ( 9 )

0 . 3 4 5 ( 1 0 )

0 . 0 1 0 ( 9 )

0 . 6 1 0 ( 9 )

0 . 507 ( r -o )

0 . 6 7 2 ( 3 )

0 . 1 3 4 4 ( s )- 0 . 1 3 4 4 ( 5 )

0 . 3 3 9 ( 3 )- 0 . 3 3 9 ( 3 )

0 . 1 3 8 ( 8 )- 0 . 1 3 8 ( 8 )

0 . 1 2 3 ( 8 )- 0 . 1 2 3 ( 8 )

0 . 1 8 7 ( 9 )- 0 . 1 8 7 ( 9 )

0 . 1 8 1 ( 8 )-0 . 181 (8 )

0 . s 2 3 ( 8 )- 0 . 5 2 3 ( 8 )

0 . 4 3 2 ( 8 )- 0 . 4 3 2 ( 8 )

0 . 3 s 9 ( 8 )

0 . r e 6 ( 8 )

0 . s s 8 ( 8 )

o . 9 4 5 ( 7 )

0 . 2 8 0 ( 3 )

0 . 8 ( 1 )0 . 8 ( 1 )0 . 8 ( s )0 . 8 ( s )2 .7 ( r3)2 .7 ( r 3 )1 . 3 ( 1 2 )r . 3 ( r 2 )2 . r ( 13 )2 . 1 ( r 3 )2 . r ( r 3 )2 . 1 ( 1 3 )r . 0 ( 1 2 )1 . 0 ( 1 2 )2 . 2 ( 13 )2 .2 ( r3)3 . 6 ( 1 s )

1 . 1 ( 1 2 )

2 .9 ( 14 )

2 . 6 ( r 4 )

2 . e ( s )

observed and calculated structure factors for thisstructure determination of boltwoodite are given inTable 7.3 The final atomic coordinates and isotropictemperature factors are listed in Table 8, and the im-portant distances and angles are shown in Table 9.

Figure 2 shows a stereo view of this structure. andFigure 3 shows a view of the potassium coordinationsphere from Figure 2. The uranyl silicate sheets areparallel to (100) and are crossbonded by potassiumatoms and hydronium ions. The sheets are composedof uranyl silicate chains that are parallel to the D di-rection sf ths rrnit cell. The potassium-oxygen bonddistances can be divided into two groups: an innergroup of seven bonds with distances ranging up toapproximately 3.24 and an outer group of threebonds with distances between 3.3A and 3.5A. Thisarrangement is quite similar to the potassium coordi-nation sphere in the micas, in which there is an innercoordination sphere of six bonds with distances lessthan 3.1A and an outer coordination sphere of sixbonds with distances between 3.3A and 3.5A. How-ever, the geometry of the potassium coordinationsphere in the micas is much more regular than inboltwoodite. The locations of the extra hydrogenatoms in this structure could not be determined, al-though they are probably bound to the water mole-cules to give hydronium ions.

3To receive a copy of Tabte 7, order Document AM-gl-157from the Business Office, Mineralogical Society of America, 2000Florida Avenue NW, Washington, DC 20009. please remit $1.00in advance for the microfiche.

u - o ( 3 )- o ( 1 )- o ( 7 )_ o ( e )- o ( 5 )- u t > ,_ o ( 7 )

s i - o ( s )- o ( 9 )- o ( 1 1 )- o ( 7 )

u - s i

- s i

U - U

Uranyl fono-G-)-:-T-:---GT

Pentagona l R ingAround Uran imo ( 9 ) - u - o ( 7 )o ( 7 ) - u - o ( s )o ( 5 ) - u - o ( 7 )o ( 7 ) - u - o ( s )o ( 5 ) - u - o ( e )

Si I i ca Tet rahedra

uranyl oxygens

br idg ing oxygen

edge-shared to s i l i con

across edge-sharedpo lyhedra

across br idg ing oxygen

across edge-sharedpolyhedra

AngIes

t z 7+30

I o+2o6972062120o o + z8: FZo

O - O d i s t .

2 . 8 8 + 0 . 0 8 A2 . 6 2 1 0 . 0 82 . 5 1 + 0 . 0 9

3 . o 6 T 0 . o 9

o ( 7 ) - s io ( 7 ) - s io ( s ) - s io ( s ) - s io t 9 ) - s i

- o ( e )- o ( 1 r )- o ( e )- o ( r r )- o ( 1 1 )

l o r + 3 : 2 . 5 1 + 0 . 0 91 1 3 F 3 u 2 . 7 4 T O . O B1 L r T 3 0 2 . 7 o F O . O 8r r s T 3 9 2 . 6 6 T 0 . o g109F3 : 2 . s7To . o81 0 8 ; 3 " 2 . 5 7 T 0 . 0 7

STOHL AND SMITH: URANYL SILICATE MINERALS

Fig. l. Stereo view of the uranyl silicate sheet in uranophane.

6 1 5

Ooxygen

New Uethod TYPeHonea (1961) Mine @terial

d neas r/ Io d ueas d neas

Table 6. Powder patterns for boltwoodite The structure of beta-uranophane (Smith andStohl, 1972) is composed of sheets that are parallel to(010). In this case, however, the true symmetry isF2,/a and the a glide planes lie within the sheet at yequal 0.25 and 0.75, and the two-fold screw axes areperpendicular to these sheets at x :0.25, z : 0.0, etc.Therefore, adjacent sheets are related by a two-foldrotation and are not in similar orientations.

This difference in the stacking of sheets affects thecalcium coordination in these minerals, as shown inFigure 4. The uranyl silicate sheets for this orienta-tion are perpendicular to the drawing above and be-low the calcium coordination spheres. The calciumatoms in uranophane are in seven-fold coordination,whereas those in beta-uranophane are in eight-foldcoordination. The equivalent of the Ca-O4 bond inbeta-uranophane is missing in uranophane, so thatthe other bonds move away from the Ca-O3 refer-ence bond to compensate for the missing bond andgive a more regular coordination polyhedron for thecalcium atoms. The O3-Ca-91 angle in beta-ura-nophane straightens by 14' to give the analogousO3-Ca-O2 angle of uranophane. This causes an in-crease in the distance between the sheets in ura-nophane, which results in the larger a dimension inuranophane relative to the b dimension of beta-ura-nophane.

The reason for the doubling of the fiber a dimen-

Calculated pattern

d carc r lao (hkl)

7 . 5 3 A6 , 8 r6 . 4 05 . 4 54 . 7 44 . 3 24 . t I3 . 9 13 , 7 5

3 . 5 43 . 4 0

3 . 1 33 , 0 7

2 . 9 5

2 . 6 9

2 . 5 3

2 , 4 5

2 . 3 42 . 2 62 . 2 L

2 , 1 6

2 , 7 32 , I T

2 , 0 82 . 0 5

1 . 9 8 31 . 9 5 0

6 . 8 0 9 A6 . 3 9 85 . 4 6 34 . 7 4 14.3204 . L 3 7

J . J O O

3 . 5 3 43 . 4 1 1

3 . 1 3 63 , O 9 22 . 9 6 0

2 , 9 r 22 , 6 8 52 , 5 4 6

2 . 4 5 9

2 . 2 6 92 , 2 L 0

2 . t 6 2

2 . 1 3 L2,LL I

2 , 0 t 3

L . 9 9 4I . 9 8 0r . 9 5 6

6 . 8 0 9 A6 . 3 9 3s . 4 6 04 . 7 3 94 . 3 2 04 . 1 3 3

2 , 9 I I2 . 6 8 42 , 5 4 52 . 5 3 32 . 4 6 62 , 4 5 3

2.T6I

2 .1292.L09

2 , 0 4 4

1 . 9 9 3

t . 9 5 5

6.808A6 . 3 9 05 . 4 5 54 . 7 3 94 .3t74 . I34

3 . 5 6 83 , 5 3 23 . 4 1 33 . 4 0 43 . 1 3 53 , 0 9 12 . 9 6 52 . 9 5 32 . 9 L I2 .6852 . 5 4 42 . 5 3 42 . 4 6 62 . 4 5 42 . 4 5 12 . 3 4 92 . 2 6 92 . 2 0 9

2 . 1 6 2

2 . 1 3 02 . 1 r 0

2 . O 5 22 , 0 4 6r . 9 9 1 9r .98371 . 9 5 5 8

1 1 1020201200120o 2 Ll2lIT20L2I 2 I2 t22ILlL22 2 L2203013000 3 1r03131310003113r 2 2

3171 3 12r32r232r

21055A421t

79

5I

81tl"

3

5

100 r0031 001L7 10130 01114 l r t

3 101

3 . 5 6 73 . 5 3 43 . 4 0 9

3 . 1 3 73 . 0 9 12 . 9 5 9

2828t92 6

4

1 )

34275

l-34646

I 2

4

A

2

32

I3

223

2 . 3 4 82 , 2 6 82 . 2 0 9

* ( B = b r o a d r e f l e c t i o n )

6 t 6 ST,OHL AND SMITH: URANYL SILICATE MINERALS

Atom

Table 8. Atomic coordinates for boltwoodite. The space group lsP2,. Oxygens 3 and 38 are related by the mirror plane of thepseudosymmetry. (Standard errors in parentheses).

Honea (1961) indicated that there should be aclose structural relationship between boltwoodite andkasolite because of their similarities in formulas, cat-ion ionic radii, X-ray powder patterns, and infraredpatterns. Two independent structure determinationscarried out on kasolite by Huynen et al. (1963) andMokeeva (1965) are basically identical except for thepositions of the lead atoms. Huynen et al. (1963) re-ported the lead atoms in three-fold coordination.whereas Mokeeva (1965) r€ported that the y value forthe lead atoms determined by Huynen et al. (1963)was off by 0.5 so that the lead atoms really occurredin eight-fold coordination. A refinement of the kaso-lite structure carried out by Rosenzweig and Ryan(1977) showed that Mokeeva's structure is correct.The kasolite structure is very closely related to thestructure of boltwoodite. The geometric configura-tion of the nine bonds around the lead atoms of kaso-lite is basically identical to the configuration of nineof the ten bonds around the potassium atoms of bolt-woodite. The diference in coordination is explainedby the fact that in boltwoodite each of the watermolecules is bonded to two adjacent potassium atomsalong b, whereas in kasolite the water molecules areonly bonded 1e a 5ingle lead atom. The potassiumatoms in boltwoodite are crossbonded along D bysharing two uranyl oxygen atoms and a water mole-

Table 9. Distances and angles in boltwoodite

u - o ( 2 ) 1 . 7 4 + 0 . 0 8- o ( 1 ) 1 . 8 5 - + 0 . 0 s- o ( 5 ) 2 . 2 3 F 0 . 0 8- o ( 3 8 ) \ 2 . 3 s T 0 . 1 2- o ( 3 ) f- o ( 3 8 ) l 2 . 4 5 + 0 . 1 0_ o ( 3 ) /

s i - o ( 5 ) 1 . 5 3 + 0 . 0 8- 0 ( 6 ) 1 . 5 9 F 0 . 0 8- o ( 3 ) I r . 6 1 1 0 . r l- o ( 3 8 ) t

u 0 . 0 2 5 2 ( 1 )K 0 . 5 4 1 ( 5 )s i 0 . 9 3 3 ( 5 )o ( 1 ) 0 . 2 9 0 ( 1 1 )o ( 2 ) 0 . 2 2 7 ( 1 1 )o ( 3 ) 0 . 0 2 2 ( 1 4 )o (3B ) -O .O22 ( r4 )o ( 5 ) 0 . 0 6 0 ( t - 1 )o ( 6 ) 0 . 2 9 4 ( 1 1 )H2O 0 . 400 (14 )

o . 2 50 . s 4 8 ( 6 )o . 2 50 . 2 50 . 7 5

- 0 . 0 7 9 ( 1 7 )o . o 7 9 ( r 1 )

o . 7 50 . 4 1 3 ( 1 7 )

0 . 1 3 8 s ( 8 )0 . 1 s 3 ( 6 )0 . 6 3 i ( 5 )0 . r 4 7 ( 1 2 )0 . 8 8 s ( 1 2 )0 . 194 (1s )

-0 . 194 (15 )0 . 4 8 4 ( 1 2 )0 . 4 9 0 ( 1 2 )0 . 6 8 9 ( r 6 )

0 . e ( 2 )I . 4 ( r r )0 . e ( 7 )0 . 6 ( 1 4 )3 .9 ( 22 )4 .7 ( 2L )4 . 7 ( 2 r )0 . 9 ( 1 6 )3 . 7 ( 2 r )2 . 3 ( 2 3 )

sion in beta-uranophane relative to the fiber b di-mension of uranophane was discussed bv Smith andStohl (1972). In these structures, a single oxygenatom ofthe silicate tetrahedra points either above orbelow the plane of the sheet. In uranophane, theseoxygen atoms from adjacent tetrahedra along thesame side of the chain all point in the same direction(Figure l), whereas in beta-uranophane they alter-nately point up and down causing a doubling of thefiber dimension.

The c dimensions in these two minerals are vervsimilar and represent the crossbonding of uranyl sili-cate chains through a bridging oxygen atom. The Bangle in uranophane occurs between the a axis,which is approximately perpendicular to the sheets,and the c axis, which parallels the crossbonding be-tween the chains. The offset of the chains along c iscaused by the fact that all the bridging oxygen atomsare on the same side of the silicate tetrahedra. Inbeta-uranophane, the a angle, which corresponds tothe B angle of uranophane, equals 90o because theoxygen atoms bonding the chains together are fromalternate sides of the sitcate tetrahedra and thus donot cause a resultant offset. However, this alternationdoes add a substantial amount of distortion to thechains so that the B angle, which occurs within theuranyl silicate sheet, is 91"25,.

The structure of uranophane also shows many sim-ilarities to the structure of boltwoodite. Both miner-als have space group symmetry p2,, with the two-fold screw axes along the uranyl silicate chains andthe uranyl silicate sheets parallel to (100). The uranylsilicate sheets in uranophane have the pseudo-symmetry F2,/a so that adjacent sheets are relatedby the a gltde plane, whereas the sheets in bolt-woodite have the pseudosymmetry y2,/m and adja-cent sheets are identical with an offset of 1.92A alongc. Due to this offset, the potassium atoms attain afairly regular ten-fold coordination.

A \I

uranyl oxygens

bridging oxyqen

edge-shared to s i l i con

across edge-sharedpolyhedra

across br idg ing oxygen

across edge-sharedpolyhedra

u - s i

- si.

U - U

3 . 2 0 + 0 . 0 3

3 . 5 4 + 0 . 0 3

3 . 9 4 4 + 0 . 0 0 5

Uranv l Ion6'r1-]:=-T--b-rr-Pentagonal Ring

&t'j'fE=+bi?"ro ( 3 8 ) - U - o ( 3 )o ( 3 ) - u - o ( 3 8 )

S i l i c a T e t r a h e d r ao-T3j - si-- dGE)-o ( 3 ) - s i - o ( 5 )o ( 3 ) - s i - 0 ( 6 )o ( 5 ) - s i - 0 ( 6 )

AngIes

l 7 7 + 3 o

8 1 + 2 o7oT4059rao

I IS ;4O1 10T401 0 9 + 4 "

O - O d i s t .

z . s e + o . t z i2 . 7 4 + 0 . L 42 .4 ] -10 . I7

2 .4I+0 . I ' l2 . 6 5 F 0 . I 22 .6310 .L22 . 5 4 F 0 . 1 1

STOHL AND SMITH: UMNYL ilLICATE MINERALS

Fie. 2. Stereo view of the structure of boltwoodite.

Sil iconUranium

cule to give chainlike structures. Each potassiumatom in the chain is crosslinked along c to each oftwo potassium atoms in an adjacent chain by sharinga free oxygen ofa silicate tetrahedron. The free oxy-gen atoms of the sitcate tetrahedra are those that ex-tend above or below the uranyl silicate sheet (Figurel). In kasolite, the lead atoms are crossbonded alongb by sharing two uranyl oxygen atoms, which is very

Fig. 3. Potassium coordination sphere in boltwoodite showingbond distances in angstroms.

@ Potassium

@ Hyaronium

@ otys"n

similar to the configuration found in boltwoodite ifthe water molecules are not considered. However, thecrossbonding of the lead atoms along c in kasolite isvery different from the crosslinking of the potassiumatoms along c in boltwoodite. In kasolite, each leadatom is crosslinked along c to another lead atom bysharing two free oxygen atoms of the silicate tetra-hedra. This causes edge-sharing of these polyhderaalong c, which does not occur in boltwoodite. Thecrossbonding of the potassium coordination spheresin boltwoodite therefore gives rise to a chainlikestructure, whereas the crosslinking of the lead coordi-nation spheres in kasolite gives rise to a more planarconfiguration. This results in a symmetry of P2' forboltwoodite and F2r/c symmetry for kasolite. The cdimension in kasotte is double that of boltwooditedue to the c glide plane, and the a dimension in bolt-woodite is longer than the a dimension in kasolite be-cause of the larger ionic radius and lower charge onthe potassium atoms as compared to the lead atoms.

The structure of sklodowskite, as determined byMokeeva (1959), Huynen and Van Meerssche (1962),Mokeeva (1964), and Ryan and Rosenzweig (1977),is also very similar to the other l:l minerals. Theuranyl silicate sheets are parallel to (100), with adja-cent sheets related by the C-centering of the C2/msymmetry. This alternation of sheets results in a dou-bling of the a dimension. The a/2 value of 8.724,which is longer than the distance between the sheetsin the other structures of this group, is caused by the

6 1 8 STOHL AND SMITH: URANYL SILICATE MINERALS

o32.4

o32.4

H2O( l )2 .5

o22.4

orl2.4

H2o ( l )2.4

or22.6

H20(3)

linear arrangement, parallel to a, of the magnesiumatoms between the free oxygen atoms of the silicatetetrahedra of adjacent sheets. The magnesium atomsare in six-fold coordination. Ryan and Rosenzweig(1977) found a water molecule in this structure thatwas not bonded to the silicon, uranium, or magne-sium atoms.

The structure of cuprosklodowskite was deter-mined by Piret-Meunier and Van Meerssche (1963)and refined by Rosenzweig and Ryan (1975). Thisstructure is very similar to the structure of sklodows-

H2o(2 )2.4

H2o(a)2.5

H20(3)2 .4

o42.7

kite except that the copper atoms that occur betweensheets are bonded in a linear arrangement to a uranyloxygen atom from each ofthe adjacent sheets insteadofa free oxygen atom ofthe silicate tetrahdedra fromeach sheet, as in sklodowskite. The longer D dimen-sion of cuprosklodowskite, as compared to a/2 ofsklodowskite, is therefore a result of the approxi-mately linear crossbonding arrangement of cupro-sklodowskite (uranium-oxygen-copper-oxygen-ura-nium), which produces a longer distance betwen thesheets than the approximately linear crosslinking ar-

H2O(4)2.5 H2OO)

2.4

ol2.5 2.6

BETA - URANOPHAN EURANOPHANEFig' 4. Calcium coordination spheres in uranophane and beta-uranophane showing bond distances in angstroms.

.Sil icon6Uranium6Oxygen

Fig. 5. Stereo view of the uranyl silicate chains in weeksite.

STOHL AND SMITH: UMNYL SILICATE MINEMLS 619

rangement of sklodowskite (silicon-oxygen-magne-sium-oxygen-silicon). Rosenzweig and Ryan (1975)reported a weighted R-factor of 0.0263 although theycould not find the extra hydrogen atoms in the struc-ture. They did, however, locate an additional watermolecule that is not bonded to the silicon, uranium,or copper atoms.

The uranyl silicate sheet in the mineral group witha uranium to silicon ratio of l: I has the formula

[(Uor),(SiOo)"]"-o" where n is the number of groupsper unit cell. For kasolite, n : 0.5 so that the -2

charge from the sheet is completely balanced by thelead atoms. Boltwoodite and sodium boltwooditealso have a value ofn : 0.5. In these cases, however,the singly charged potassium and sodium atoms arenot sufficient to balance the -2 charge of the sheet.Crystal structure analyses do not show how thesecharges are balanced. Therefore, each of these struc-tures must have an additional hydrogen atom presentto balance the charge. Uranophane, beta-ura-nophane, sklodowskite, and cuprosklodowskite haven : I so that the charge on the sheet is -4. Thischarge is only half satisfied by the doubly chargedcation in each structure. Therefore, each of thesestructures must have two hydrogen atoms present perunit cell. The locations of these hydrogen atomscould not be determined in any of the structure de-terminations of the l:l minerals. However, the struc-ture analyses of each mineral indicated the mostprobable locations for the hydrogen atoms on thebasis of crystal chemical considerations. These areshown by the formulas in Table l. The formulas foruranophane and boltwoodite are based on this workwhereas the other formulas are from the most recentstructural paper for each mheral.

The degree of hydration and the role of the waterin the structures of the l:l uranyl silicate mineralshave been the subject of some controversy. Severaldi,fferent values have been reported for the hydrationstates of most of the l:l minerals on the basis ofchemical analyses. Uranophane and beta-ura-nophane have been reported with the following com-position: CaO'2UO;2SiOr'XHrO where X: 4.5 to7(Gorman and Nuffield, 1955; Smith and Stohl, 1972).The composition for sklodowskite has been reportedas MgO'2UO.'2SiO,'YH,O where Y is either 6 or 7(Gorman, I 957). The cuprosklodowskite compositionhas been reported as CuO'2UO3'2SiOr'6HrO (Rosen-zweig and Ryan, 1975). The original composition ofboltwoodite was determined by Frondel and Ito(1956) as K,O'2UO,'2SiOr'6HrO, although Honea(1961) indicated that the formula should be

K2O'2lJO..'2SiOr'3HrO with the possibility of someadditional zeolitic water. Some of the water in theseminerals is water-of-crysts.lliz31leq combined as partofthe first coordination sphere around the interlayercation. A small part of the water is involved in theelectronic charge balance ofthe uranyl silicate sheet.It either replaces terminal oxygens with hydroxylions (most likely as SiO3OH or UOOH) or occurs ashydronium ions between the sheets. The rest of thewater, which is probably the variable amount re-ported, is zeolitic in nature and occurs in the largeopen spaces between the sheets.

Five water molecules per two uranyl groups werefound in the structures of beta-uranophane (Smithand Stohl, 1972) and uranophane. The structure de-terminations of sklodowskite (Ryan and Rosenzweig,1977) and cuprosklodowskite (Rosenzweig andRyan, 1975) each yielded seven water molecules pertwo uranyl groups. The structure determination ofboltwoodite showed three water molecules per twouranyl groups. Any additional water in these miner-als would be present as zeolitic water.

The crystal structure determinations on the miner-als uranophane, beta-uranophane, boltwoodite, kaso-lite, sklodowskite, and cuprosklodowskite show thatthey all contain similar sheet structures. However,the sheets show varying amounts of distortion due tothe diflerent sizes and charges on the cations betweenthe sheets. An indication of the amount of distortionthat is present in these structures can be obtained bycomparing the unit cell dimensions parallel to the fi-ber direction. According to the data in Table 2, kaso-lite has the smallest flber dimension (D : 6.9324) in-dicating a large amount of chain distortion, andboltwoodite has the largest fiber dimension (D :

7.0644) indicating a low amount of chain distortion.Fitting a plane to each of two edge-shared UO'groups, the uranium and the five equatorial oxygens,and calculating the angle between these planes in-dicates the amount of distortion that occurs along the

Table 10. Atomic coordinates for weeksite. These values are based

on the partial structure arlalysisin the pseudocell with spac€ group

Atom

US i0 ( r )0 ( 2 )0 ( 2 8 )o ( 3 )0 ( 4 )

0 . 00 . 0o . 2 50 . 00 . 00 . 1 9 40 . 0

0 . 1 9 80 . 1 3 00 . 3 0 70 . 1 8 00 . 1 8 00 . 0 7 1o . 4 2 9

0 . 00 , 5 2 3o . 5 4 60 . 6 9 30 . 3 0 70 . 4 8 70 . 4 7 3

STOHL AND SMITH: I]RANYL SILICATE MINERALS

uranyl silicate chains. In boltwoodite, the atoms in aUO, group are statistically planar and the normals tothe planes through edge-shared UO, groups make anangle of 0.0o to each other. In kasolite, however.these UO, groups are not planar, and the normals tothe best fit planes gf edge-shared UO, groups makean angle of 20.6o to each other.

Structure determination of the 1:3 group mineralweeksite

Weeksite is the only member of the l:3 uranium tosilicon compositional group for which acceptablecrystals could be obtained. The sample used in thisanalysis was from the Anderson Mine, yavapaiCounty, Arizona, and was lent to the authors by Mr.Bill Hunt of Sun City, Arizona. The acicular crystalsmeasured approximately 25 x 25 x 200 trrm and oc-curred as radiating aggregates, up to I mm in diame-ter, on a sandstone matrix. The comparison of aGuinier powder pattern of this sample with a patterntaken of the type material number 115886 from theU.S. National Museum verified its identification.

The unit cell and space group of weeksite were de-termined by Dr. Joan Clark and Dr. George Ashbyof the U.S. Geological Survey, and were reported byOuterbridge et al. (1960). They reported that thismineral belongs to space grotp Pnnb and has an ex-tremely strong pseudosymmetry, with the dimensionsof the pseudocell half those of the real cell and withpseudocell extinctions which indicate the presence ofan A-centered lattice. Therefore, there are five pos-sible space groups for the pseudocelt Ammm, Amm2,Am2m, A2mm, or A222.

Many crystals from the Anderson Mine samplewere checked on a precession camera in an effort tofind a single crystal that was suitable for srructureanalysis. Alignmel photographs of these crystalsshowed elongated white radiation streaks and split-ting at the ends of the streaks, both of which are in-dicative of multiple crystals. A complete set of photo-graphs, taken on a large crystal, showed severalindividuals in very close orientations. These photo-graphs substantiated the l-centered pseudocell andverified the fact that all the dimensions of the realcell were double those of the pseudocell.

A complete set of precession photographs, takenon a crystal measuring 25 x 25 X 200 pm, showed avery slight splitting at the ends of the white radiationstreaks. The extinctions on these photographs in-dicated a face-centered lattice so that the possiblespace groups of the real cell were: Fmmm, Fmm2,Fm2m, F2mm, or F222. There were also additional

extinctions present that could not be explained onthe basis of these space groups. For example, the(hk0) zone had all indices even, but also had k: 4n.The previously reported Pnnb symmetry is a subsetof the possible face-centered space group and couldnot therefore be verified or disproved.

Two data sets were collected using a Picker FACS-I system with MoKc radiation and the slightly im-perfect 25x25x200 pm crystal. The first data set con-sisted of 270 reflections, with 2d less than 40o, thatwere permitted by the pseudocell symmetry. The cellconstants obtained during the alignment segment ofthis data collection were as follows: a : 7.106+0.0084, b : 17.90+0.02A, c : 7.0g7+0.00?A. Thecell constants for the real cell are double these values.The second data set consisted of 2641 reflections ofthe real cell with 2d less than 35o. This set had ll75unobserved reflections.

It is probable that the uranium atoms and most ofthe other atoms in weeksite do not contribute to thevery weak reflections that cause the doubling of allthe axes. Therefore, the pseudocell data set withoutan absorption correction was used in the initial stagesof structure analysis. An analysis of Harker sectionsand Harker lines in a three-dimensional Pattersonmap indicated that the symmetry of the pseudocellwas probably Amm2. The uranium atom locationswere determined from the Patterson map and wereused in the calculation of a three-dimensional Fou-rier map. The R-factor obtained from the refinementof only the uranium atoms in space group Amm2was0.22. The analysis of this map showed several pos-sible oxygen atom positions and a silicon atom posi-tion, which indicated a structural unit similar to theuranyl silicate chains in the I : I uranyl silicate miner-als. Several successive sets of least squares analysisand Fourier calculations gave locations for severaladditional oxygen atoms and the potassium atoms,and lowered the R-factor to 0.15.

The remainder of the structure analysis was car-ried out using the real cell data set. The analysis ofthese data again showed the presence of uranyl sili-cate chains but did not reveal the locations ofpotas-sium atoms or the extra silicon atoms in the struc-ture. Attempts were made to analyze these data usingthe five face-centered space groups as well as Pnnb.In each case, the possible extra silicon atom positionswould have required edge-sharing between silicatetetrahedra, which is very unlikely.

The aspects of this structure that were determinedare shown in Figure 5. This structure consists of ura-nyl silicate chains, similar to those found in the l:l

Table I l. The cell constants and space groups of soddyite and thesynthetic uranyl silicate and uranyl germanate

STOHL AND SMITH: URANYL SILICATE MINERALS

2 U 0 ^ ' G e O ^ ' 2 H ^ 0 " "J Z I

dyite and the 2:l mineral group. The uranium-ger-manium distances and uranium-uranium distancesin the partial structure analysis are very similar to theuranium-silicon and uranium-uranium distances inthe uranyl minerals with a uranium to silicon ratio ofI : I and I :3. Therefore, this structure is composed ofuranyl germanate chains that are very similar to theuranyl silicate chains. The structure is made up ofuranyl germanate chains that extend along the diago-nals of the unit cell, and that are crossbonded bysharing a single germanium tetrahedron (Figures 6and 7). This structure was originally proposed byStohl and Smith (1974) and has since been sub-

Table 12. The powder patterns of soddyite and the synthetichydrated uranyl silicate. The pattern for soddyite was measuredfrom a Guinier pattern and the intensities were visually estimated.

621

Soddyi te***

, I l n . C i d . t H n- - ' 3 " - 2 - " 2 -

a = 8 , 3 2 Lb = 7 L . 2 Ic = 1 8 , 7 I

T'M4

a = 8 . 2 9 A

c = 1 8 . 6 5

! aaQ

a = 8 . 1 7 9 1 0 . 0 0 1 Ab = 1 1 . 5 1 5 1 0 . 0 0 2e = 19 .397xO.0O2

I @

R e f e r e n c e s : * c o r m a n ( 1 9 5 2 ) ; * * L e g r o s e t a L . ( L 9 7 2 1 .

minerals, that are parallel to the c direction in (100)at x :0. In this case, however, adjacent chains arenot crosslinked to produce a sheet, but rather occuras separate units that are mirror images of eachother. The adjacent chains are separated by an oxy-gen-oxygen distance of approximately 2.64, which iscomparable to the oxygen-oxygen distance that oc-curs in a silicate tetrahedron. Therefore, it seems rea-sonable that some of the unlocated silicon atoms inthis structure occur between the chains and link thechains together in (100). Some of these unlocated sili-con atoms may also be crossbonded to give a two-di-mensional arrangement of silicon tetrahedra in the(010) plane between the chains. This two-dimen-sional layer of silicon tetrahedra would link togethersome adjacent chains along the a direction. The po-tassium atoms probably occur between the chainsalong a, in positions similar to the potassium atomsin boltwoodite. The atomic coordinates for the atomsin these chains, relative to the pseudocell, are shownin Table 10.

Structure of the 2:1 mineral soddyite

The similarity between soddyite and the synthetichydrated uranyl silicate and uranyl germanate ofLegros et al. (1972) can be seen by comparing theircell constants, space group (Table ll), and powderpatterns (Table l2). The powder pattern of soddyitewas obtained on a Guinier camera using samplenumber R16788 from the U.S. National Museum.The research on the hydrated uranyl germanate byLegros et al. (1972) included a partial structure anal-ysis in space group Fddd, wit}l^ the origin at pointsymmetry 222, which located the germanium atomsin special position 8a(0,0,0) and the uranium atomsin special position 169(0,0,2) with z : 0.1667. Theywere, however, unsuccessful in finding the oxygenatoms in this stucture. Due to the similarity betweensoddyite and the synthetic hydrated uranyl silicate, itis reasonable that the structure of this analogous hy-drated uranyl germanate should be a model for sod-

* Reference: Legros e t aL . (1972) .* * S = s t r o n g i n t e n s i t y

11 = modera te in tens i tyr r r L c r 1 5 r L J

VW = very weak in tens i tyB = b r o a d r e f l e c t i o n

Soddy i te

d neas r / ro**

t t r n , c i n . r q n J '- - "3 - ' -2 - "2 -

d meas T / Io

o6 . 298A4 . 8 0 5q . o o z

4 . 5603 . 8 0 3

3 . 3 4 83 . 2 6 22 . 9 9 22 . 8 0 62 , 7 2 0

2 . 4 9 32 . 4 7 7

2 . 3 3 52 . 2 5 8

2 . 0 9 9

2 . 0 4 61 . 9 8 r -1 . 9 1 3

6 , 2 9 6 A4 . 8 3 r4 . 6 6 54 . 5 5 81 ? q l

3 . 3 7 0

3 . 2 5 92 . 9 9 92 . 8 2 01 1 ) )

2 . 6 5 72 . 5 2 r2 . 4 8 72 . 4 7 3/ . q o J

2 . 4 r 22 . 3 3 r2 . 2 6 r2 , 2 L 42 . 1 8 02 . 1 6 02 , 0 9 72 . 0 7 22 . 0 5 2L , 9 7 91 q t ?

r . 9 0 5L . 8 9 7I . 8 8 71 . 8 7 81 . 8 6 51 . 8 4 1L . 7 9 7L . l 7 2| . 1 3 91 . 7 1 0

84? 1

I )

100t )

10802 0

2 IL 248

43

301 9a b

2L 2L J

832

206

102 27 7

261I

2 811

T 24

1 1

Mvws!'i^I

q

t4l

M

w

WBw

W

1 . 8 6 4

r . 1 7 2

1 . 7 0 6

vt^I

uVln

l'1^]

vll

622 STOHL AND SMITH: URANYL SILICATE MINERALS

I

II

ooUraniumOGermanium

Fig. 6. The three-dimensional chain structure for the hydrateduranyl germanate. The chain positions are shown by crosshatching and are superimposed on the uranium and germaniumatom locations given by Legros e/ al. (1912).

stantiated by Legros and Jeannin (1975) for the ura-nyl gernanate and by Belokoneva et al. (1979) forsoddyite. The atom positions reported by Belokonevaet al. (1979) in space grottp Fddd with the origin atthe center of symmetry are as follows: U(0.875, 0.875,0.0443), Si(0.875, 0.875, 0.875), O(l) (0.695, 0.963,0.0455), O(2) (0.026,0.044, 0.0663) and O(3) (0.87s,0.875,0.170).

New Mineral

A new uranyl silicate mineral was collected by D.K. Smith in 1955 from the Jackpile Mine, Laguna,New Mexico. It has an appearanoe very similar tosoddyite and occurs as opaque, yellow green, radiat-ing crystals on a sandstone matrix. Its powder patternis also similar to soddyite (Table l3). An emissionspectrographic analysis showed the presence of ura-nium and silicon with trace amounts of calcium. Theuranium to silicon ratio could not be determined bymicroprobe analysis because the mineral dis-integrates in an electron beam. A series of precessionphotographs indicated that the mineral was triclinic.The cell constants obtained during the alignmsalprocedure on a Picker FAcs-l system were as follows:

6io

o Silicon€Uranium

@otys"nFig. 7. Stereo view ofthe crossbonded chains ofthe uranyl germanate structure.

a : 5 . 5 3 5 1 0 . 0 0 4 A , b : 6 . 1 1 7 + 0 . 0 0 4 A , c :7.759+0.005A, d : 84"47'+2', B : 9l"ll'!2', y -

96n32'!2'. A data set consisting of 336 independentreflections with 20 values up to 35o was collected.The intensities of three standard reflections. whichwere measured throughout the data collection, de-creased by approximately 20 percent. Because theseintensities could not be brought back up to their orig-inal values by reorienting the crystal, we assumedthat the crystal structure was changing in the X-raybeam.

An analysis of this data set gave an R-factor of0.21. A uranium atom was located at x:0.87, y:0.16, z : 0.79, but the remainder of the structurecould not be determined because of the poor data setand the uncertainties in the composition and proper-ties of this new mineral. If, however, the space groupis Pl, the uranium-uranium distance due to the cen-ter of syrrmetry would be 3.89A, which is the dis-tance between the uranium atoms in edge-shareduranium pentagonal bipyramidal groups. In thiscase, there would only be one such distance associ-ated with each uranium atom rather than two as re-quired in the uranyl silicate chain. It is, therefore,possible that this structure may be unique among theuranyl silicates.

Summary

The known uranyl silicate minerals have been di-vided into three groups on the basis of their uraniumto silicon ratios. The dominant structural feature,which occurs in all three groups, is a uranyl silicatechain that is formed by edge-shared uranium pen-tagonal bipyramidal groups and silicate tetrahedra.

In the l: I uranyl silicate group, these chains arecrosslinked by bridging oxygen atoms to give uranylsilicate sheets of composition [(UO,), (SiOo),ho'.These sheets are crossbonded by the additional cat-ions in each structure. The diferences among thestructures in this group, such as the dimorphs ura-nophane and beta-uranophane, are caused by factorssuch as different stacking arrangements of the sheets.A structure refinement of uranophane showed thatthe calcium atoms are in seven-fold coordination incontrast to beta-uranophane where the calciumatoms are in eight-fold coordination. The mineralsboltwoodite and kasolite have very similar formulas,cation ionic radii, X-ray powder patterns and in-frared patterns. A structure determination of bolt-woodite showed that the main difference betweenthese minerals is due to the manner in which the cat-ion polyhedra between the sheets are crossbonded. Inboltwoodite, the crossbonding of the potassium coor-

623

Table 13. Guinier powder patterns of the new mineral andsoddyite (R16788). The intensities were visually estimated

STOHL AND SMITH: URANYL SILICATE MINERALS

New llinerald meas t l to

Soddr/l ted meas r/rox

7 . 783A6 . 2 9 86 . 0 4 15 . 5014 . 9 5 14 . 8 1 3

4 . 5 5 34 . 5 0 73 . 9 0 2J . d J I

3 . 7 L 43 . 3 4 83 . 2 5 8

2 . 9 9 2

2 . 7 4 42 . 7 2 02 . 5 9 5

1 . 9 8 0r . Y I I

r . 8 6 4

o6 .298A

4 . 8 0 54 . 6 6 24 . 5 5 0

3 . 8 0 3

3 . 3 4 83 . 2 6 2

2 . 9 9 22 . 8 0 6

2 . 7 2 0

2 . 4 9 32 , 4 7 7

2 . 2 5 82 . 0 9 92 . 0 4 61 . 9 8 1r . 9 1 3I . 8 6 4

I

75a

I Mvw5

vtJ

Sw

MI,J

!lBl'I1lvl^IvI^I'!'1,J

vt^lid

trl

10IO8

4

1

1

231

x q = c r r n n o i n t a n c i i v

U = modera te in tens i tyr r r L c r r s r L )

VW = very weak in tens i tyB = b r o a d r e f l e c E i o n

dination polyhedra gives rise to a chainlike structure,whereas in kasolite, ths sls55linking of the lead coor-dination spheres gives rise to a more planar configu-ration.

The extra hydrogen atoms, which must be presentin most of these structures in order to balance thecharge, were not located in any of these structuralanalyses. However, the structural formulas for sev-eral of these minerals show the presence of hydroxylor hydronium ions that have been proposed on thebasis of crystal chemical considerations.

A partial structure analysis of weeksite, which hasa uranium to silicon ratio of l:3, indicates that thestructure is composed of uranyl silicate chains thatoccur as separate units. These chains are separatedby an oxygen-oxygen distance of approximately2.6A, which is comparable to the oxygen-oxygen dis-tance in a silicate tetrahedron. Therefore, thesechains are probably linked together by the unlocatedsilicon atoms in this structure. The potassium atoms

624 STOHL AND SMITH: URANYL SILICATE MINERALS

probably occur between the chains in positions simi-lar to the potassium atoms in boltwoodite.

Soddyite is the only known uranyl mineral with auranium to silicon ratio of 2: l. Its structure consistsof uranyl silicate chains that are crosslinked by shar-ing a single silicate tetrahedron to give a three-di-mensional framework structure.

ReferencesAbeledo, M. J., Benyacar, M. R. and Galloni, E. E. (1960) Ran-

quilite, a calcium uranyl silicate. American Mineralogist, 45,1078-1086.

Appleman, D. E. and Evans, H. T. (1973) Iob9214: Indexing andleast-squares refinement of powder diffraction data. U. S. Na-tional Technical Information Service, PB 216 188.

Belokoneva, E. L., Mokeeva, V. I., Kuznetsov, L. M., Simonov,M. A., Makarov, E. S. and Belov, N. V. (1979) Crystal srructureof synthetic soddyite (UO2)2 [SiO4] (HrO):. Doklady AkademiiNauk SSSR, 246,1,93-6.

Busing, W. R., Martin, K. O. and Levy, H. A. (t962) oRFLS, Afortran crystallographic least-squares program. U. S. NationalTechnical Information Service, oRNL-TM-3OS.

Chernikov, A. A., Krutetskaya, O. V. and Sidelnikova, V. D.(1957) Ursilite - a new uranium silicate. Atomnaya EnergiyaVoprosy Geologii Urana, Supplement, 6, 73-7 (not seen; ex-tracted from Chemical Abstracts 53, 8955 h).

Chernikov, A. A., Shaskin, D. P. and Gavrilova, I. N. (1975) So-dium boltwoodite. Doklady Akademii Nauk SSSR, Z2l, 195-197 (not seen; extracted from Fleischer, M. (1976) New MineralNames. American Mineralogist, 6 I, 105,+- I 055.)

Fleischer, M. (1980) Glossary of Mineral Species 1980. Mineral-ogical Record, Inc., P. O. Box 35565, Tucson, Arizona 85740.

Frondel, C. and Ito, J. (1956) Boltwoodite, a new uranium silicate.Science, 124, 931.

Gorman, D. H. (1952) Studies of radioactive compounds: V-Sod-dyite. American Mineralogist, 37, 386-393.

Gorman, D. H. (1957) Studies of radioactive compounds: IX-Sklodowskite. Canadian Mineralogist, 6, 52-60.

Gorman, D. H. and Nufreld, E. W. (1955) Studies of radioactivecompounds: Vlll-Uranophane and beta-uranophane. Ameri-can Mineralogist, 40, 634-645.

Honea, R. M. (1959) New data on gastunite, an alkali uranyl sili-cate. American Mineralogist, 44, rc47-1056.

Honea, R. M. (1961) New data on boltwoodite, an alkali uranylsilicate. American Mineralogist, 46, 12-25.

Huynen, A. M. and Van Meerssche, M. (1962) Confirmation de lastructure de la sklodowskite. Bulletin De La Classe Des Sci-ences. Academie Royale De Belgique, 48,742-750.

Huyneq A. M., Piret-Meunier, J. and Van Meerssche, M. (1963)Structure de la kasolite. Bulletin De La Classe Des Sciences.Academie Royale De Belgique, 49, 192-201.

Legros, J. P. and Jeanain, Y. (1975) Coordination de I'uraniumpar I'ion germanate. II Structure du germanate d'uranyle dihy-

drate (UO2)2GeOa(H2O)2. Acta Crystallographica, B3l, 4,I 140-l 143.

Legros, J. P., Legros, R. and Masdupuy, E. (1972) Sur un silicated'uranyle, isomorphe du germanate d'uranyle et sur les solu-tions solides correspondantes. Application d I'etude structuraledu germanate d'uranyle. Bulletin de la Societe Chimique deFrance, 8, 305 l-3060.

McBurney, T. C. and Murdock, J. (1959) Haiweeite, a new ura-nium mineral from California. American Mineralogist, 44,839-843.

Mokeeva, V. I. (1959) The crystal structure of sklodowskite. Dok-lady Akademii Nauk sssn, 124,578-580 (transl. Soviet Phys-ics-Doklady, 4, 27 -29, 1959).

Mokeeva, V. L (1964) The structure of sklodowskite. Kristallo-grafiya, 9, 2, 277-278 (transl. Soviet Physics-Crystallography,9,2,2t7-218, t964).

Mokeeva, V. L (1965) The crystal structure of kasolite. Kristallo-grafiya, 9, 5, 7 38-7 40 ( transl. Soviet Physics--Crystallography,9, 5,62r-622, 1965).

Outerbridge, W. F., Staatz, M. H., Meyrowitz, R. and Pommer, A.M. (1960) Weeksite, a new uranium silicate from the ThomasRange, Juab County, Utah. American Mineralogist, 45,39-52.

Piret-Meunier, J. and Van Meerssche, M. (1963) Structure de Iajachimovite. Bulletin De La Classe Des Sciences. AcademieRoyale De Belgique. 49, l8l-191.

Rosenzweig, A. and Ryan, R. R. (1975) Refinement of the crystalstnrcture of cuprosklodowskite, Cu[(UO2)r(SiO3OH)r] . H2O.American Mineralogist, 60, M8453.

Rosenzweig, A. and Ryan, R. R. (1977) Kasoli te,Pb(UOrXSiO4). H2O. Crystal Structure Communications, 6,6t ' t-621.

Ryan, R. R. and Rosenzweig, A. (1977) Sklodowskite,MgO.2UO3.2SiO 2.7 H2O. Crystal Structure Communications,6 , 6 l l - 6 1 5 .

Schoep, A. (1922) Soddite, a new radioactive mineral. ComptesRendus Academie Des Sciences Paris, 174, 1066-1067.

Smith, D. K. (1963) A fortran program for calculating X-ray pow-der diffraction patterns. Lawrence Radiation Laboratory, Liver-more. Calif. UcRL-7196.

Smith, D. K. and Stohl, F.V. (1972) Crystal structure of beta-ura-nophane. Geological Society of America Memoir, 135, 281-288.

Smith, D. K., Gruner, J. W. and Lipscomb, W. N. (1957) Thecrystal structure of uranophane [Ca(HrO)2](UOr)2(SiO4)2.3HrO. American Mineralogist, 42, 59+618.

Stohl, F. V. (1974) The Crystal Chemistry of the Uranyl SilicateMinerals. Ph.D. Thesis, The Pennsylvania State University,University Park.

Stohl, F. V. and Smith, D. K. (1974) The crystal chemistry of theuranyl silicate minerals. (abstr.) American Crystallographic As-sociation Meeting, Program and Abstracts Set.2, Yol.2,271.

Wuensch, B. and Prewitt, C. T. (1965) Corrections for X-ray ab-sorption by a crystal of arbitrary shape. Zeitschrift Fiir Kris-tallographie, 122, 2+59.

Manuscript received, July 23, 1980;acceptedfor publication, January 12, 1981.