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American Mineralogist, Volume 72, pages 1204-1210, 1987 Incorporation of Al, Mg, and water in opal-A: Evidencefrom speleothems JonN A. Wrsn Department of Geology, La Trobe University, Bundoora, Victoria 3083, Australia Bnr.q.NL. FrNr,.q.vsoN Department of Geography,University of Melbourne, Parkville, Victoria 3052, Australia Ansrru.cr Opal-A speleothemsfrom a variety of caves show a wide range of composition, containing up to 50/oAlrO, and MgO. The infrared spectraof the samplesshow that the Al is substituting for Si, with OH groups compensatingfor the resultant chargeimbalance. The Mg occursas interstitial ions to which OH groups are bonded to balancethe charge.Thus the hydroxyls in opal are presentnot only as surfacesilanol groups,but also as OH groups within the opal structure to balance the charge of the substituted Al and as OH groups bonded to interstitial elementslike Mg. present opal speleothemswere obtained Iu'nonucrroN For the study, oparisa naruraily occurring hydrous "amorphous" sil- 11"3,il:#[l::il*'i ;Ti'fi J#ffiH"11'f1"fi: ica mineral that forms in a variety of surface and near- Ou"."rfu"d. Severalspeleothems from a small area (Frondel, 1962; Wilding al., surface environments et "ur,oi.uJi, cave were combined to give a single sample, and 1977). Opal from many of these environments has been rn two caves,samples were collected at two different lo- paid studied intensively, but little attention has been to ca[ties within the cave. occurrencesof opal in cavesas secondarymineral O"O.g:- In addition speleothemswere collected from a granite (speleothems). "speleothem" is its In this study, the term cave in southern California, U.S.A.; these samplescon- general precipitated used in a senseto encompassopals tain interlayers of poorly crystallized sepiolite (a hydrous in cavesfrom both high- and low-temperature solutions, -agnesi.rm silicati clay mineral), which is useful for gel i.e., to cover both hyalites and opals as defined by coriparison with the opal speleothems. Langer and Fliirke (1974). Opal speleothemshave been frequently recorded (Hill Basaltcaves and Forti, 1986),but little detailed work has been carried Muchof westernVictoria is occupiedby a largevolcanic prov- out on them. However, the analytical data available show ince,containing basalts collectively known as the NewerVol- a wider range of composition than otherwise known for canics(Joyce, 1975). The main periodof volcanicityprobably Holo- opal (Cody, 1980; Webb and Finlayson, 1984), so opal^for commencedin the late Plioceneand continuedinto the physiographicfeatures, such speleothemsappear to have considerable potential cene(McDougall et al., 1966),so lavacav,es, are well preserved'More than 50 lavacaves have throwing new light on the way in which variou, -ino. 1s elementsareincorporated intothe opal *',,",,,i". wla- nf,l*:lt:Li:jTil"JilTjiTfiJi l"J,'Jj'ji,?.::T:; ing et al. (1977) noted that impurities in opal may- be ;;;; ; Jntui' opat speleothems:Skipton Cave and Mt. occluded, chemisorbed,or in solid solution, but to date Hu,,'it,o' cu.r.. little work has been done on the relative contributions of The exactage ofthe lavassurrounding these two cavesis un- these three processesto the minor-element composition known,but in eachcase it is likely to be 2.5 Ma or less(Webb, of opal. 1985;J. A. Webb,E. B. Joyce,and N. C. Stevens,unpub. manu- occunnrNcEsoFopAL spELEorHEM" ::lft'ifff'l'}o."ffil'::1"1':fttX'i."i,:T.llt".,'.";:'fi:TJ; areabun- Opal speleothemshave beenfrequently recorded in lava of the southernmostchamber in Mt. Hamilton Cave crystals;these are composed ofopal and cavesand are alSOOCCaSionally present in limestoneCaveS, dantrosettes ofacicular representpseudomorphs after a fibrouszeolite (webb' granite caves, and sandstoneoverhangs (Fith;t;;;;; ?Poffit"ot^-il Webb, 1985). Small irregular coralloids or botryoidat GreatDividing Rangein southeasteueensland is made coatingsare the most common form taken by the speleo- up of 900m of nearlyhorizontal basalts and minor trachytesof thems, although they also occur as stalactites,stalagmites. laleOligocene to earlyMiocene age (Webb et al., 1967).A small and flowstone (Hill and Forti, 1986). Maximum dimen- cave(Holy Jump Lava Cave) is presentin basalttoward the top sionsrarely exceed5 cm. ofthesequence.Thewallsofthecaveareencrustedbysecondary 0003-o04x/87/rrr2-r204s02.00 1204 WEBB AND FINLAYSON: COMPOSITION OF OPAL-A SPELEOTHEMS r205 silica minerals: glass-clearbotryoidal layers of hyalite (opal-A) and milky stalactitesand flowstone of opal-CT and chalcedony (Webb, 1979).Only the opal-A crustswere examined during the courseofthe presentstudy. Granite caves The "Granite Belt" in southeastQueensland is a large area of granite terrane that contains a number of caves and sinking streams(Finlayson, 1982).Most of the caveshave formed where a stream has developed an underground passagealong a joint, and two (South Bald Rock Cave and River Cave) have opaline coralloids coating the undersidesof boulders roofing the caves (Webb and Finlayson, 1984). South Bald Rock Cave also contains allophane flowstone on the walls and floor. Extensive areasof granite terrane occur in California, and in one of these, about 100 km north-northeast of San Diego, is Cahuilla Creek Cave (Finlayson, 1985). This cave consists of granite boulders roofing a stream. Coralloids occur on the walls of the cave and flowstone on the floor; both speleothemtypes consist largely of banded calcite, although one of the coralloid samplescontains very thin interlayers of poorly crystallized sepiolite. MrcnosrnucruRE Freshly broken surfacesof all the opaline speleothems were examined under the scanning-electronmicroscope. A variety of microstructuresare evident, including glassy conchoidal fracture surfaces(Fig. lc), irregular colloidal aggregates(Fig. 1a), and in one instancecolloform or botryoidal surfaces(Fig. lb). Similar microstructures have been documented previously for opal-A (e.g., Segnit et al., 1970, 1973). ANalyttcl.L METHoDS AND RESULTS Chemical analysesof the speleothemswere carried out using X-ray fluorescenceand an electron microprobe, and showedthat severaldifferent minor elementsare present (Table l). Apart from Al and Mg, which will be discussed further below, these elements are present in very small amounts comparable to, or less than, those recorded in opal from other environments (Frondel, 1962; Wilding et al., 1977;Segnit and Jones,pers. comm.). The X-ray diffraction patterns of all the opal speleothems are typical of opal-A (Jonesand Segnit, l97l). Langerand Flcirke(1974, p. 18) subdividedopal-A into two varieties:opal-AN (hyalite),with "no remarkablelow Fig. l. Surfacemicrostructures of opal speleothemsunder angle X-ray scattering," and opal-AG, distinguished by the sEM.(a) Colloidalaggregate (8a16-12); coralloid from South "low angle scattering typical for gelJike structures of BaldRock Cave. (b) Colloform surface (8316-10); zeolite pseu- monocrystalline SiOr." The speleothemsamples all have domorphfrom Mt. HamiltonCave. (c) Conchoidal fracture sur- (8516-3); xRD patterns very similar to the latter type, including the face coralloidfrom Mt. HamiltonCave. hyalite specimenfrom Holy Jump Lava Cave. The infrared (rn) absorption spectraofthe opal speleothems (Fig. 2) were obtained using pressed KBr discs of any Christianseneffect (seePrice and Tetlow, 1948) in (preparedin the standard manner, e.g., Russell, 1974) the spectra(Fig. 2) shows that the grain size of the sam- and a JascoA302 spectrometer.Each disc contained 0.2 ples was sufrciently small for high-quality spectra to be mg of sample and 200 mg of KBr, both accurately obtained. Since the specimensdid not contain Cl, anion weighed. The samples were dry ground; grinding in al- exchangewith the KBr of the disc could not occur. To cohol is preferablefor crystalline materials (Tuddenham ensure maximum consistency between the spectra, all and Lyon, 1960),but may lead to water loss in amor- samples were run in the one batch, immediately after phous samples(C. Barraclough, pers. comm.). The lack weighing. t206 WEBB AND FINLAYSON: COMPOSITION OF OPAL-A SPELEOTHEMS 3 16-38 h Bald Rock Cave 8416-12 South Bald Rock Cave SkiptonCave uJ o 2 f = U)z (r Mt. HamiltonCave F )u I si-o-A Ar)-oHJ Fig. 2. Infrared spectraof speleothemsamples. Cahuilla Creek specimenis poorly crystallized sepiolite; all others are opal-A. Assignmentsofabsorption peaks to bond vibrations applicableto all spectra. WEBB AND FINLAYSON: COMPOSITION OF OPAL-A SPELEOTHEMS r207 TABLE1. Chemicalanalyses of speleothems(in wt%) Loss Si/(Si on +AD Cave Sampleno and type SiO, TiO, AlrO3 Feroo MnO MgO CaO NarO KrO PrOsignition Total (molar) CahuillaCreek' 8316-38; coralloidinterlayers 62.18 0.07 0.32 0.04 0.o5 24.62 1.80 0.02 0.02 0.00 n.d. 89.12 South Bald Rock 8416-12;coralloid 80.71 0.05 5.96 0.37 0.00 0.17 0.09 0.19 0.26 0.40 10.93 99.13 0.93-. South Bald Rock 8316-8;coralloid 81.44 0.04 5.55 0.40 0.00 0.10 o.12 0.16 0,28 0.67 10.64 99.47 South Bald Rock- 8316-8;coralloid 86.64 0.00 4.41 0.02 0.00 0.03 0.13 0.09 0.50 O.12 n.d. 91.94 0.94 River 8316-7;coralloid 69.70 0.08 9.70 2.54 0.01 0.04 0.07 0.23 0.27 1.66 15.61 99.91 River. 8316-7;coralloid 82.42 0.02 8.41 0.06 0.06 0.04 0.06 0.01 0.05 1.2'l n.d. 92.34 0.93" Skipton 8516-4;coralloid 89.740.11 2.30 0.65 0.00 0.10 o.21 0.2s 0.10 0.21 6.18 99.85 0.971 HolyJump Lava 8616-19;botryoidal crust 93.08 0.01 1.02 0.53 0.00 0.28 0.06 0.16 0.06 0.11 3.78 99.09 0.99 Mt Hamilton 8516-3;coralloid 81.79 0.05 0.64 0.35 0.00 4.71 329 0.13 0.07 0.30 8.02 99.35 0.99 Mt Hamilton- 8316-10; zeolitepseudomorph 92.51 0.09 0.00 0.00 0.05 0.07 0.04 0.01 0.o2 0.00 n.d.

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