TasNat 1926 Vol1 No5 Pp7-12 Lewis OutlinesGeology

7/28/2019 TasNat 1926 Vol1 No5 Pp7-12 Lewis OutlinesGeology

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THE TASMANIAN NATURALIST

Outlines 0/ Tasmanian 'Geology.SECTION 19 (Continued).

MINERALOGY.Although the characwristic externalform of minerals taught by crystalogl'aphy is an uncertain gt:.ide to the student in the determination of the mineralspecies, the molecular structure is apparent through other physical characters. These are particularly useful asthey can be often ascertained without

elaborate equipment. and are applicablein the field. They all depend on crystalographic structure and are as fol lows:-I. Charaeters Depend,ing on Cohesionand Elasticity.(a) Cleavage---the tendency of a mineral to break in certain definite directions parallel to some possible face of thecrystal. The method of cleavage givesan indication of the crystalographic system.(b ) Gliding Planes-similarly are thedirections in whiCh a mineral parts inresPonse to pressure, as opposed to ablow which gives us the cleavage.(c) P c r < ~ L ~ s i o n figures and.. etching fig

u r e ~ are the distinctive marks producedrespectively by a blow and by a solvent.the effect on different minerals depending on Rtructure.(d) Fracture is the term used to describe the surfaee produced by breakingthe mineral in any dir,-ectlon other thanthe cleavage. I t may tie (i.) conchoidalor a series of curved cavities; (ii.) even;(iiL) uneven; (iv.) hackley, or sharp andj:lgged l i l ( ~ brok.en iron.(e) Hardness is the resistance that asmooth surface offem to abrasion.A scale is adopted ranging from H-1 toH-10, according to the following simpletests: 1 has a 6'Teasy feel, 2 can bescratched with the finger najl, 3 can becut with a knife, 4 can be easily scratched with a knife, 5 "an be scratched withsomll difficulty, 6 can only be scratchedwith a file. 7 can be scratched with afile but will scratch glass, 8 and 9 cannot be !'cratched except with diamond, 10diamond.

( f ) Tenacity indicates the behavior ofthe mineral when an attempt is made tocut it . and is classlftpd according to thefollowing simple tests: (1.) Bl'ittle--theparts separate in powder; (Ii.) sectile-can be cd with a knife, but pulverisem

under a blow; (iii.) malleable-can b ~cut but will fiatten under a blow; (iv.)flexible-will bend and remain bentwhen an attempt is made to cut it.(g ) Elasticity-the resistance it offel'sto change of shape and it s tendency toreturn to the original shape when thepressure is rllmoved. Elaborate apparatus is I!eC'essary to measure elasticity.2. Charaeters Depending on SpecificGravity or Relative Density.The specific gravity of a substance isthe ratiO of its' density to that of water,or in other words, the weight of the bodydivided by the weight of an equalvolume of water. Weight alone is jm material as this largely depends onquantit:J


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THE TASMANIAN NATURALIST(d) Lustet" (so usually spelt In geological works}-This depends on the nature ot tht: cl'Yst'al faces expoeed on abroktln surface of the mineral. and maybe ( i .) metallic as in t h ~ metala, or (il.)

nonmetallic. which includes (a) adamantine (e.g., diamond), (b) vitreous (e.g . glass), (c) resinous (e.g., opal), (d)greasy (e.g greasy glass), (e) pearly (e.g., PElarl), ( f ) silky (e.g., asbestos).(e) Play at colors shown by a succes'mon of prismatic colors when the mi ..eral is turned (e.g., in diamond andopal).Further serIes of calor effects are ob~ E F V a b l e when thin secrlor.s of a miueral are examined unaer polarised hg.lt .To do this an I'xt!"f'lIleiy thin section is('ut (l/50()o to 1/1'),00') of a mm) andexamined under a special type of microsi;0pe which divides the rays ot light tIn-tI l only certain rays with waves movingin certain fixed direc:tions are used. Bythis means a great vaciety or character.!are ascertc..med dependant on the illterference I)f the 'itomic structure to OH!passage of the light waves. This is thellH'S! u ~ e f u ! meth')d of determmin;; ,heminet'al specie. By it chemical composition and c r y s t a l o g r a ~ h i c form canbe d i ~ < ! ( , v e r e d . No twc minerals allowthe ligJ.t thl'ough in the same way. Somesplit the rays t;.p into different colors;others only allow them through whenthe mineral is in a certain position, andas there are a great number of variationsin these their diffucnce can be accurateIy ceaRured; (}thers again givIl distinctive figures of light and shade. Howel'er, ad expensive appal'YGnd the scope of this work.4. Characters Depending on ChemicalCompOSition.Chemista-y divides matter into !l3 elebents, whicl are Bubstances that cannotbe decomposed or subdivided by anyknown method of analysis. MineralsconsiEi; either of these elements in afree sta,te or of definite compounds ofthem formed in accordance with chemi

C'.l.l laws. Of all the known elementsonly twelve play an important part in'the composition of the rocks of the globe.This chemical composition is the ultimate test In determining the species of.the mineral, and forms the scheme underwhich minerals are classified. As h',s been.explained, chemical composition isgoverned hy atomic structure which alsoimparts the characters r e r ~ r r e d to beforethere a definite relation between composition and physical c h a r a ~ t a ' i s t j C l ,

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either ot whiCh having been determinedthe mineral can be identified and theother can be ascertained from a recogmsed description; but ror safety al lmethods of determination must be usedto check each other.Composition is determined by analysis,but this usually I'equires much equipment and a high technical skill. Further, i t the mineral occurs in a rock, itis ditl1cult to separate it from otherminerals. Thus a rock may consist ofminerals A, B. and C. Say A consIstsof elements z, y and x in proportions 6,:! and 1., B consists of elements x andw in proportions 4 to 5, and C consistsof elements z, wand v in proportiom;1, 2 and 4. An analysis of the rock willonly give 7z, 3y, 5x, 7w, ar,d 4v. Thismay b ~ arranged to give a great varietyof mineral species. The separation orminerals for analysis is oft-en very difficult. Again, materials of similar' compooition often have st;.ch different characters rlepending UBually on mode of origin thee they must be grouped as differen,; minerals. To give an example: -Pure carbon-isometric, cleavage highIy pk'rfect, fractuce conchoidal; brittle H-10, 0'3.5, luster adamantine, colOt., white,transparent-diamond.Pure carbon-rhombo:thedral, Bcalycleavage perfect. flexible, greasy, H-l-2 ,G-2, luster metallic. color black, opaque,a conde-eta'r of electrIcity-graphite("J"ad" of lead pencils).

(Note the W3.y a mineral Is usually.lcscribed.)IllinCf2.IC'asses.

Millel'al species are divided into eigh tclasses, depending on compo!lition, andthese are subdivided into group,g. Theclasaes are:-1. Native elements-thosefew elements whi


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THE TASMANIAN NATURALISTlaws from other groups. 8. Hydro-carbon compoullds--these are not strictlyminerals. but highly complex chemicalmlxtwres; they include many impurtantsubstances such as eoal, oil shale, andoil.Within each class minerals lIlay besubdivided into metals-those compounded with elements possessing the characteristics of a perfect metal sl:ch as malleability. metallic luster conductivity.Semi-metals-those compounded withelements possesing these characteris,ticsin a less perfect degree, and non-metals.F'or OUIt" purpolle we will classify minerals into A. Comerciallty valuable minerals; B. lmpvrtant rockforming minerals; and C. Rarer minerals.

"A" COMMERCIALLY VALUABLEMINERALS.Native Elements.

Carbon (C), sulphur (S.), and selenium(Se) are the only non-metal elements tnatoccur free in nature. Carbon occurs asdiamond and graphite. Spveral small buttrue diamonds were found in some trioutaries of the Pieman and three of theseform specimen No. 1 of the Petterd collection in the Tasmanian Mnseum. Graphite, a form usually as'sociated with metamorphic rocks. and usually formed frommetamorphism of carbonaceous depositsis found in the mines of Zeehan and Dundas, but not in commercial quantities. Sulphur is rare in Tasmania, and hithertohas only been found in the Bischoff mine.Of the semi-metals several free elementsoccur. Arsenic (As) has been found in thelower levels ut Bi,schoff. Antimony :Sb),in radiating patches lino in diameter atthe British-Zeehan mine, bismuth (Bi) isplentiful at Mt. Ramsay and has beenfound in the Mt. Read and other tinmines. Tellurium (Te), an important alloy for steel, has not yet been found inTasmania. nor has tantalum (Ta).The metals form the most importantt:roup of this dass. They are all impoftan t ores when fonnd in commercial quantities. The elements so found are : -Gold (An), a widely distributed roekfound in quartz veins at Heaconsfield. Lefroy, Mathinna, and other places throughout Western and Northern Tasmania inmetasomatic lodee. with other ores at Mt.Lyell and in alkali igneous rocks at Cygfield, and the New Golden Gate Mine at

~ 1 : a t h i n n a were amongst the most famousgold producers in Australia. Silver (Ag),a rare mineI'll1 in Tasmania, found in thintreads in some of the silver-lead lodes of?;eehan and Dundas. The Magnet andHercules mines have yielded l o m ~ fin" i I ~ -(9 )

mens. Copper (,u), also rare, found atMt. Lyell in patches up to 801bs. Lead(Pb), very rare, two s p e c i i l l e n ~ beingfound at l:louth .Nevada and COlllet llllnes,Dunda,s. I t has also been reported atCygnet. Platinum (Pt), has not been reported in 'l'asmania, but its allied demcnt,oSl1liridiUIIl (Os 11') occurs throughout the

~ e r l ' e n " t i n e ro


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'rHlt l 'ASMANIANoccurring in many igneous rocks, also common in coal. It OCCUl'l8 in mostmineral veins, and was largely worked atLyell for fiuxes for pyritic smelting,. theresidues of which are manufactu'red intosulphuric a.cid and superphosphates at theLyell works in Melbourne.

3: Sulp,ho Salts.The only minerals of this class sufficiently important to warrant notice hereare l!:inkenite (sulphantimoniate of lead,orthorhombic form Pb S-b2 84), found inthe Magnet mine; J amesonite (sulphan.timoniate of lead, monoclinj,c form Pb28M 85), found through BischofI, Dundas,and Zeehan fields, both useful leadand antimony ores; and Tetrahedrite (suI.

phantimoniate of copper or fahl or greyeo,pper ore Cu 8 Sb2 87), found at CurtainDa\-is, ~ I t . Read, and Mt. Lyell mines.4: Haloids.This class contains a few minerals, andthese are rare and unimportant. Thebest-known is Halite (or rock salt, sodiumchloride Na Cl), found in Tasmania inthe salt pans of Mona Vale and otherplaces in the Hoss_Tunbridge area, whichyield a pure salt which would be commercially valuable if salt were not so cheapto import. Fluritc (calcium fiouride Ca1


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THE TASMANIAN NATURAL1STWolframite (Fe, Mn), W04) and scheelite (Ca W04), important tungsten oresfound on King Island, and on Ben Lomond, and uset'nl as alloys, also came 111this class.7.-Salts of Organic Acids do not y i ~ l dimportant mineraIs.8-Hydrocarbon compounds, a l t h o u ~ hyielding the commercially valuable coaiand oil, will be considered rock-formin!;minemls.

"B" IMPORTANT ROCK FORMINGMINERALS.The minerals occurring in the classes-1 , native elements; 2, sulphide class;3 sulpho-salts; and 4, haloids, althoughIllany of them are amongst our greatsources of natural wealth, de, not occurin great masses. They are essentiallyrare. The minerals which go to makeup th e rocks oJ' the crust of the ear th-the cOlllmon minerals are mostly included under the heading (6) Oxygen-Salts.There nne important rock constituent in the class of 5, oxides, that is theoxide of silicon. The mineral form isknown as qnartz, which is a grnup ofdifferent forms with similar composition

1;,;1 ( 1 ) - (a ) Phenocrystalline or vitreousvarieties. Quartz or rock crystal, thewell known glassy hexagonal pyramidcrystals. Amethyst, a clear purple orviolet colored variety. Cairngorm stone,smoky yellow to dark brown in color.- (b) Cryptocrystalline varieties: Chalce-dony, a white, gray or blue waxy colo.red Rtone; cornelian, a red chalcedony;agate, a variegated chalcedony; o.nyx, resembling agate, but in layers of differentcolors; flint, allied to chalcedo.ny, but ofa darker color. Of these (Z) varietiesquartz is the only important rock-fo.rmingmineral, and enters into the composition of acid igneous rocks. I t is an important constituent of granite. also ofBorne sandstones and mudstones.

6. Oxygen Salts.A. Carbonates.-l Calcite (calcium carbonate, Ca C03). This is the great Iimeproducing rock. I t o.ccurs in crystals inveins, which are used for o p t i c ~ l workbut its most important occurrencesin beds as limestone, lithograhphic stone,

marble. and chalk. The great limestonedeposits oJ' Chudleigh, Mole Creek Florpnt:ne. Ida Bay, Gordon River, ' MariaIsland, Granton, etc., largely consist ofthis mineral.2. Dolomite (carbonate of calcium and~ a g n e s i l 1 m (Ca. M;9 C03-magnesiumhmestone). Thlfl IS rare in Tasmania.

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I t has been reported from Mo.unt elaude,Mount PeIion, and Dnndae.B. SiJicates.-l. Felspar Group: Themost important rock-building mineraland a nearly universal constitutent ofrocks. (Note, felspar, strictly speaking,is not a mineral, but a group or family).(a) Monoclinic felspars.Orthoclase (silicate of aluminium andpotassium X: Al 8;3 OQ). (b)Triclinicfelsp8J's.Microliue (similarly K Al .S13 OS).Anorthoclase (soda-potash silicate o.faluminium (Nu K) (AI Se3 08). (c)Albite-Anorthite series or plagioclas('felspar5l.

,\ I ) l t e (silicate of ,1Iuminium and w-dium Na Al Si3 08 or Ab).01:g10clase (Ab3 Ani).Andesine (Ab An).Labradol'ite (Abl .\n3).Anorthite (silicate of aluminium andearcinm, Ca 312 S12 08 or An).Rock classificatio.n depe:lds on the felspar present. Therefore it will be bettel'to reserve details of occurrence untilwe reach the section on petrology. Or.thoclase is the base o.f granite rocks. I to c ~ r s massive on the Mersey, near Gad'sHIll, and as the form sanidine in thealkaline rocks at Port Cygnet. Microcline is frequently found in Tasmaniangranite, and is abundant in the graniteat. St. Mary's Pass. Anorthoclase oc.curs in the Port Cygnet alkali series. AIbite occurs in compact masses on theHeazlewood River, and as crystals inthe Port Cygnet rocks. Oligoclase iscommon in the dolel'ites (di,abase) andbasalts which cover a great part of Tasmania. Andesine also occurs throughthe basaltic group oJ' rocks, and has been

r e p o r t ~ d from the Cygnet alkalis. Labradorlte iR the base o.f the dolerites andb a s a ~ t s , the commonest rocks in Tasmama. Anorthite is also common inthe dolerites and basalts.2. pyroxene group.(a) Orlhorhombic section.Enstatite (silIcate of magnesium M9Si 03).Hypersthene (silicate of magnu-

"Ium and iron (Fe M9) Si 03.(h ) Monodinic section.Hypersthene (silica,te of magn'

sium. calcium, etc.)(i) Non-alumlnouB.Diopside (calcium-ma.gnes.ium py.. roxene Ca M9 (Si (0 ) 2.(u ) A luminous.Augite (Aluminous pyroxenE' CaM9 06.~ h e . s e are only. the more importantvarIetIes. Elli-ta.trte OCC1:,Te a.bundantly


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THE TASMANIAN011 thp. Huek!sson River, and atParsons Hood and -Magnet Range.Hyphesthene occurs with enstathe rut those localities, also ill graniiteat st. MafCY's Pass and on the ForthRiver and at Dundas. Pyroxines, particula.rly augite are second constituent otthe dolerites and felspars, and probablythe second most important r'Ock formin2'mineral.

B. Amphibole group.The most importa.nt rod. forming mineral in this group is Hornblende (Ca (M9Fe) 3 Si 0'3) 4 with Na and al). I toccurs in many places through the West'Coast mining fields. The Cygnet rocksyif'ld a rare soda variety.

Hydrous Silicates.4. Zeolite group.A group of rocks anulogous toO thefelspars and are often a hydrated felspar. They are found as re-crYstallisedp r o d u c ~ s of de'composed felspars fCunningas white veins through the basalts anddolerites. Of this large group chabazite Ca N a2) A12 Si4 012 6H20) ha;;frequently been recorded. A vein of thi.mineral caused great trouble a t theGreat L,ake Dam.

Mica Group.(i) Muscovite (potaseium mica H2 K a l ~(8i 04) 3).(ii) Paragonite (soOdium mica H2 N ~(Si 04) 3) .(iii) LapidoJi.te (lithium mica).(iv) ZinnW'aldite (lithium 11'.>11 mica.)(vi) P'hloganite (magnesium mica).vi) Phloganite (magncsium mica.)Muscovite or common white mica iscomnwn in the granite rocks particularl yon Flinders Island. Paragonite hasnot been repoOrted in Tasmania. A smalldyke of lepedolite oocurs at Mt. Ramsay. Zinnwaldite is the c h a ' r a c t e r i s t i ~white mica of the tin bearing granitesand is abundant throughout the NorthE&"t tin fields and in the HeemskifCkgranites. Biotite is the thirdconstituent of the ordinary gra-nite, and is a common roclt formingmineral in some other rOCks. Phlogophite occurs in granite of HampshireHills and Heemskirk. The mineral seri

cite is a f,)rm of muscovte with a highe:'proportion of water. I t is common inmany metamorphic rocks and 'Occurs atMr. Lyell, Mr. Read, Hamilton on Forthand many other localities.6. Serpentine and talc group.Serpentine is a hydrated silioa.te ofmagnesia (H4 Mg3 Si2 09"). I t occursin large rock m a . s s e ~ formed from peri-(12)

NATURALISTdotites. I t is common in many pla."esthrough the western portion of Tasmania.Talc (acid metacllicate of magnesium H2M93 (Si 03) 4) 'OCc\:.r8 in places throughout the wcstern mining fields especiallynear the junctions of the Arthur andHellyer Rivers, and at Bischoff, Beaco11l'l'Held and other places.

7. Kaolin division.KaolinHe (hydra,ted si1ica,te of aluminium H4 AI2 Si2 09) occu.rs on FlindersIsland. at Middlesex and at seveml localities on the North IDast Coost.The remaining impoOrtant rock constituents fall in clas,s:8. Hydroca,rbon Compounds.(i.) Amber-Fossil ve'getable resin. Not

hitherto fOl:nd in Tasmania.(ii.) Tasmanite - Latrobe oil shale. Itconsists of micro'acopic foO,a,sil space casesof a highly resinous nature. These yi>8ldoil on d'lstillation. The mineral is uniquein that sulphur ha,s repla()ed the usualoxygen oompound of this cla,ss. I t isonly found in Ta,smanla.(iii.) Petroleum - mineral oil - nothitherto found in Tasmania.(Iiv ,) A s : p h a l t l u m ~ m J l n e r t l . l pitch-alsonoOt found here.

(v.) Coal.(a) Anthracite (93-95 per eent. carbon.(b) Sub-antbracite (90-93 per cent.carbon.(c) Humites. (a) Non-caking humic(70-90 per cent. cairbon); (b) caking humic (75-90 p.eT cent. carbon); (c) gasmaldng humic (78-80 per cent. carbon).(d) Humic KCl"oginitc,; (60_80 pel"

cent. carh-on).(e) Keroginites (vola,tile hydro-c'al'bons in excess of fixed carbon).( f ) Sub humic (60'75 pl.' cent. ca.I'bon).(vI.) Lignites-Brown coal."C" RARER MINERAL TYPES.

Dana's "Telct; Book ot Mineralogy" describes over 3,200 mineral species. Thelarge majority of these are very ra.re anddo not contribute materia.lly to the consitruction ot the earth. I t Is clearlyimpassible toO refer to these here and itwill be readily seen that few have theIiesure to assimila,te ttltis mass of detailNo one can work at mineralogy w1thoutag-uiding r e f e ~ n c e book at his elbo'w.

(The writer acknowledges his indebtedneBS to W. F. Petterd's "Minerals of Tasmania" for much of the information illthis artiele). A. N. Lewis

TasNat 1926 Vol1 No5 Pp7-12 Lewis OutlinesGeology

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Transcript of TasNat 1926 Vol1 No5 Pp7-12 Lewis OutlinesGeology