American Mineralogist, Volume 71, pages 1198-1200, 1986

Structural intergrowth of in anthophyllite

G. N. Surn.c.NNA.,T.R.N. Kurrv Materials ResearchLaboratory, Indian Institute of Science,Bangalore 560012, India G. V. AN.qsrHn Ivrn Department of Inorganic and Physical Chemistry, Indian Institute of Science,Bangalore 560012, India

Ansrucr "Mountain wood," a morphological variant ofanthophyllite, occursin the Hol6narasipur asbestosmines. High-resolution transmission-electron microscope (Hnrertr) images of mountain wood show intimate brucite-anthophyllite intergrowths. Partial dissolution in dilute acids indicates that the intergrown may be Fe-containing brucite. Coex- isting anthophyllite asbestosdoes not show the brucite intergrowths, whereas both the anthophyllites have chain-width disorder.

During the course of a mineralogical investigation on tallites giving rise to a subparallel-orientedtexture (Fig. asbestosfrom Hol6narasipur, HassanDistrict, 1a).The tip of the samespecimen is seento split into fine Karnataka,South India (lat I 2"38-49'N, long 76' | 4-22' E), fibers (Fig. lb). An X-ray pattern, recordedwith a Philips we noticed a morphological variant of anthophyllite, which PWl700 powder diffractometer (FeKa radiation), could the miners call "mountain wood." The occurs in be indexed on the basis of an orthoamphibole unit cell the anthophyllite asbestosmines and has a schistosetex- (spacegroup Pnma) with a: 18.48,,: 18.09and c: ture, partly fibrous but more brittle than ;it re- 5.30A. semblessandalwood in appearance.Because the physical Chemical analysesby a combination of wet-chemical featuresof mountain wood specimensare different from methods (complexometric and redox titrations, as well as those of the coexisting massive and asbestiform antho- colorimetry and atomic absorption) of mountain wood phyllites, we studied the chemistry and electron micro- scopic characteristicsof this material. Anthophyllite asbestosoccurs within the metamor- Table1. Representativecompositions of phosedultramafic rocks that are part ofthe complex anthophyllites belt nearHol6narasipur. The geologyofthis Archean com- "Mountain plex within the regionally metamorphosed - wood" Asbestos facies quartzofeldspathic grreisseshas been reported by sio, 55.05 56.60 (1978), nil nil Chadwick et al. Naqvi and Hussain(1979), and Tio, 't.22 Kutty et al. (1984). The ultramafic rocks have been com- Alro3 0.76 Fe2O3 0.48 0.28 pletely metamorphosed to anthophyllite * actinolite + FeO' 19.32 18.09 * .Petrographic studies indicate that MnO 0.08 0.19 (Raghavendra Mgo 21.88 21.38 anthophyllite is replacing antigorite et al., CaO 0.56 0.40 1976).Anthophyllite asbestosoccurs in restricted locali- Naro 0.02 0.12 ties as veins and pods. Asbestosfibers in different stages K.O 0.01 0.01 Hr9*.- 2.70 2.05 of development and with varying aspect ratios are ob- Hro-'* 0.02 0.08 servable. The fibers are usually arranged parallel to the Total 100.88 100.42 walls of the veins; random seamswith cross-fibers are also No.of cationsper 24 O atoms present. Si 7.754 7.985 Mountain wood anthophyllite has a hardness in the Alrv 0.126 0.015 rangeof 5-5.5, D = 3.19, and the hand specimensare Alrv 0 188 Fe.. 0.051 0 030 pale brown. Under the polarizing microscope, polished Fe'- 2.273 2.131 thin sectionsreveal aggregates oflong, fine crystalliteswith Mg 4.588 4.486 subparallel orientation. The thin sections are colorless, Mn 0.010 0.023 Ca 0.085 0.060 optically cloudy (translucent),with very weak pleochro- Na 0.006 0.034 ism. Mean refractive index is 1.66.Directional variations K 0.002 0.002 (oH) 2.54 1.93 in refractive index could not be measured,owing to the texture ofthe rggregatesand small crystal size.spv images * Determinedby redox titrations. '* gas (Cambridge StereoscanS- I 50) show the stacking of crys- Determinedby microgravimetric analysis. 0003404x/86/09r 0-1 l 98$02.00 I 198 SUBBANNAET AL.: BRUCITE-ANTHOPHYLLITEINTERGROWTHS I 199 I { BRUCITE { ji{ i

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)' 75A .',"11t il Fig.2. A nnrnr"rimage of mountainwood anthophyllite con- tainingthe structuralintergrowths of brucite.

is probably brucite. To our knowledge, this is the first report of the intimate intergrowth of brucite in antho- phyllite. Coherent intergrowths of serpentineand brucite type layersare described by Mackinnon and Buseck(1979) from an altered chondrite meteorite. Veblen (1980) has reported brucitelike sheetsin talc (5-A spacings)that fill the grain boundariesbetween two anthophyllite crystals. Since dissolution in dilute acids liberates both Mg'z+and Fe2+,the intergrown mineral is presumably Fe-rich bruc- ite. The observedwidths ofbrucite lamellae vary from 20 to 200 A. ttre coexisting asbestiform anthophyllite does not show such brucite intergrowths. The only structural defectsobserved are apparently random intergrown triple chains (Veblen et al., 1977).Similar chain-width disorder is also observedin the mountain wood anthophyllite. The differencein physical propertiesbetween mountain wood Fig. 1. sErramicrographs of mountainwood showing(a) the and the massiveas well as asbestiformanthophyllite may orientedcrystallites and (b) tip of the specimensplitting into arise from the structural intergrowth of brucite in moun- smallerfibers. tain wood. Since this material exists as relics within the asbestosmines, with the fully developed asbestosfibers and the coexistingasbestiform anthophyllite are given in apparently growing out of it, mountain wood can be con- Table l. The mountain wood has more Fe and Mg cations sidered as a precursor to the asbestiform anthophyllite. and hydroxyl ions and less Si per formula unit than the AcrNowr,nncMENT asbestiform variety. When placed in 0. I M hydrochloric gratefully acid, the mountain wood givesrise to solubleFe2+ * Mgz+ Thekind encouragementof Professor C.N.R. Rao is (up to 8.9 wto/oFeO + MgO) (Raghavendraet al., 1978). acknowledged. < The asbestiformanthophyllite givesrise to l.3oloFeO + RrrnnnNcns MgO in the same medium. Ramakrishnan,M., Viswanatha,M.N., andMur- Becauseofthe higher octahedralcation contents in the Chadwick,B.. thy, V.S.(1978) Structural studies in the ArcheanSargur and mountain wood, nnreu analysisof this material was car- Dharwarsupracrustal rocks of the Karnatakacraton' Geolog- ried out. The investigationswere made in a rBor-rErtl 200 cx electron microscope operating at 200 kV with a rsc2 objective stage.The sampleswere preparedby fine grind- ing in "specpwe" acetone. Figure 2 shows a typical intergrowth of a mineral with 4.9 A fringe spacingsin anthophyllite. The basal spacings of brucite, Mg(OH), (00/), and gibbsite, A(OH)3 (0k0), are 4.8 and 5.1 A, respectively(Deer et al., 1962; Veblen 220. Naqvi,S.M., and Hussain, S.M. (1979) Geochemistry of meta- and Buseck, 1979).The presenceof gibbsite can be ruled anorthositesfrom a greenstonebelt in Karnataka,India. Ca- out due to the low Al-content of the mountain wood. nadianJournal of EarthScience, 16, 1254-1264. Therefore, the mineral intergrown with the anthophyllite Raghavendra,R.V., Ananthalyer, G.V., andKutty' T.R'N. (1976) 1200 SUBBANNA ET AL.: BRUCITE-ANTHOPHYLLITE INTERGROWTHS

Mineral chemistry of asbestosbearing ultramafics from Hol- tergrowths and new combination structwes. Science,206, 1398- EnarasipurSchist belt. Indian Institute ofScience Journal, 58, I 400. 460-49r. Veblel, D.R., Buseck,P.R., and Burnham, C.W. (1977) Asbes- -(1978) Acid resistancecharacteristics of amphibole as- tiform chain silicates: New and structural gfoups. bestos.Indian Journal of Technology, 16, 317-322. Science,198,359-365. Veblen,D.R. (1980)Anthophyllite asbestos:Microstructures, in- tergrown sheet silicates and mechanisms of fiber formation. American Mineralogist,65, 1075-1086. MeNuscnrrr REcETvEDOcrosrn 23, 1984 Veblen, D.R., and Buseck,P.R. (1979) Serpentineminerals: In- MnNuscnrpr AccEPTEDMav 16. 1986