Accessories & Oxides
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Transcript of Accessories & Oxides
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Accessory(silicates)andOxideMinerals
Commonaccessoryandmetaloxidespeciesareoftenvaluableoreminerals
orimportantphasesforpetrographicinterpretation.
Anaccessorymineral
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Titanite(sphene) CaTi[SiO4](O,OH,F)
Anesosilicatewherethedominantstructural
unitarecornersharingTiO6 octahedra.
WhatisspecialabouttheCa
coordination?
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Sphene is monoclinic, +ve, with a 2V = 27.
Pale pinkish-brown to dark brown with 4th order birefringence.
It has a very distinctive form, particularly in thin section, and is a commonaccessory phase in many magmatic rocks. It will also form large crystals
under correct metamorphic terranes.
Its occurrence in alkaline (syenites nepheline) can be of economic value.
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Zircon Zr SiO
,
mineralworld,
because
of
its
textural,
chemical
andisoto iccom lexit
It
is
also
extremely
stable:o p ys ca yan c em ca y.
systemtobesaturatedwithZrandSiO2.
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These
very
small
crystals
contain
very
small
concentrations
of
238Uand235U.
Theelementsareradiogeneicanddecayto206Pband207Pb
respectively.
Itispossibletomeasuretheratiosoftheseelementsinazircon
againstaknownstandard(normallymanmade205Pb).Thenwe
canuse
the
very
well
constrained
half
lives
and
decay
constants
of238Uand235U,tocalculatetheageofthatgrain.
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Thegreatstabilityofzirconmeansthatafterthe238Uand235Uare
enclosedinthestructure,aninternalclockisstartedandifoften
notstoppeduntilageologistwantsitstopped.
Himalaya ZirconsSIPHidraEast Greenland Naantali
10-25 Ma
a380 Ma ~1.8 Ga
Acasta
Gneiss~
Eastern Papua
New Guinea.4.3 Ma
Jack Hills
~4.4 Ga
Basement
460 Ma
Thetechnique
is
robust
from
shortly
after
Earths
formation
(4.5
Ga)
until
recently.
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Zirconistetragonal(+ve)
an
growsin
many
differentforms.
, ,
andwhere
enclosed
in
phyllosilicates,maygiveriseto
Undercorrectconditions
itwill
form
perfect
.
3 cm
euhedralbipyramids.
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Spinel Group (AB2O4)
Thespinelgroupmineralsconsistof6endmembers,with
variableoccupancyoftheA andBsites.
Spinel,(MgAl2O4)
and
hercynite,
(FeAl2O4)
make
up
the
simplest
solid
solution.
OthervariationsinthisseriesincludeZnandMnendmembers.
Analternative
substitution
occurs
at
.
Themostcommonformbeing2+ 3+
4
.
ThereisalsoaCr3+ substitution.
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Ilmenite (FeTiO3)
Ilmenitehasahexagonalstructurecomparabletocorundum(Al2O3).
Itis
fairly
ubiquitous
in
many
igneous
and
metamorphic
rocks,
occurringasanaccessoryphase,orinveinsorstratiformlayers,
w ereitcanoccuraseconomic eposits.
Compositionally,it
will
partake
in
solid
solution
with
hematite
(Fe2O3).
uncommon,most
ores
are
dark
grey
andnondescript,whilegrainsinthin
sectionareo a ue.
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ReflectedLightMicroscopy
Thisisatechniquewhichisapplicabletoopaquemineralsandusesasetof
observationsremarkablysimilartothosemadeduringtransmittedlight
work.
Colour Simpleobservation,orbycomparisonwithknowns.
Reflectivity Theamountoflightreflectedbacktophotocell.
.
Texture Cleavagetextures,orthogonalvsobliqueandpitting.
Bireflectance a
property
akin
to
pleochroism;
mineral
changes
colour
on
rotation.
Anisotropy apropertyakintobirefringenceincrosspolarisedlight.
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Ilmenitetorutile
Whileilmeniteisfairlycommoninmostgreenschist,amphiboliteandeven
granulitefaciesrocks,itisfarlesscommonatpressuresabove~11kbar.
Atthese
pressures
it
is
typically
converted
to
rutile
(TiO2)
TiO2
Rutile
(tetragonal + ve)
Anatase
(tetragonal - ve) Brookite
parallel
perpendicular
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Optical
properties
of
ilmenite
are
difficulttodetermineintransmitted
light.Possibleindicatorsareafaint
purpletingewhenviewedwithintense
ig t.
Rutile,on
the
other
hand,
has
a
characteristicdeepredbrowncolour,
extreme re r ngencean g re e .
.TiO2 isavaluableore,butitslimited
paragenesismakeeconomicdeposits
unviable.Instead,largeilmenitedeposits
areexp o te .
s
asan
a e
onus
ofhighpurityFeorebyproducts.
, ,
mostcommonlyasapigment.
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Other important, but uncommon oxides.
Fe2O
3- Hematite
SnO2 - Cassiteritee r
2 4- rom e
Al2O
3
Corundum
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HydroxideMinerals
Therearetwoimportanthydroxidemineralsthatyoushouldbeawareof.
Brucite(Mg[OH]2),
because
of
itspotentialroleinCO2
Gibbsite(Al[OH]2),
because
it
definestheoctahedrally
itdefinestheoctahedrally
coordinated
site
in
the
dioctahedralphyllosilicates,and
itistheworldsformostsource
. .
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SulphidesandSulphatesImportantmineralsinoredepositgeologyandtheir
occurrenceiscontrolled,primarily,bytheREDOX conditions
.
,
thelossofanelectron.Oftentheyoccurtogether,"linked",inwhatare
referredto
as
reduction/oxidation
(REDOX)
reactions.
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Anoxicenvironments sulphidesThebiggestgeneratorofsulphidemineralsareblacksmokers.
2
whichaccumulates
in
stratiform
deposits
that
may
result
inMississippiValleyTypeeconomicdeposits.
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,largeeconomicdepositsisa
commonaccessorymineralismany
i neous,sedimentar and
metamorphicrocks.
Ithasahighmetalliclustre,isoftenyellow/gold
, . , .
ItsstructureiscloselyrelatedtoNaCl.
Pyriteisacommonmineralinfossils,using
replacetheskeletalstructureofdeadorganisms.
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OtherSulphideMineralsCommonly occurring together are sphalerite (ZnS) and Galena (PbS)
Thesetwomineralsaretheprincipleoremineralsforbothmetalsandalso
.
operations
will
also
look
to
recover
significant
Cu,
Ag,
Sb
and
Bi.Observablepropertiesnotdissimilar,butPbSismuchheavier!
Znisthe4thmostcommonmetalinuse. Pb,perhapshasmoreformerusesthan
presentuses,butisstillimportant.
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Sulphates hydrousoranhydrous
100sofspeciesknown,twoarerockforming,andonlyfour
willbecommonlyfoundinrockformingstrata.
Anhydrite CaSO4Celestite SrSO
Gypsum CaSO4.2H2O
EvaporiteDeposits
Baryte(Barite) BaSO4isaprimaryconstituentofwhite
.