American Mineralogist, Volume 71, pages 1198-1200, 1986
Structural intergrowth of brucite in anthophyllite
G. N. Surn.c.NNA.,
T.R.N. Kurrv
Materials ResearchLaboratory, Indian Institute of Science,Bangalore560012, India
G. V. AN.qsrHn Ivrn
Department of Inorganic and Physical Chemistry, Indian Institute of Science,Bangalore560012, 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 hydroxide may be Fe-containing brucite. Coexisting anthophyllite asbestosdoes not show the brucite intergrowths, whereas both the
anthophyllites have chain-width disorder.
During the course of a mineralogical investigation on
amphibole asbestosfrom Hol6narasipur, HassanDistrict,
Karnataka,South India (lat I 2"38-49'N, long 76' | 4-22' E),
we noticed a morphological variant of anthophyllite, which
the miners call "mountain wood." The mineral occurs in
the anthophyllite asbestosmines and has a schistosetexture, partly fibrous but more brittle than asbestos;it resemblessandalwoodin appearance.Becausethe physical
featuresof mountain wood specimensare different from
those of the coexisting massive and asbestiform anthophyllites, we studied the chemistry and electron microscopic characteristicsof this material.
Anthophyllite asbestosoccurs within the metamorphosedultramafic rocks that are part ofthe complex schist
belt nearHol6narasipur.The geologyofthis Archean complex within the regionally metamorphosedamphibolitefacies quartzofeldspathic grreisseshas been reported by
Chadwick et al. (1978),Naqvi and Hussain (1979), and
Kutty et al. (1984). The ultramafic rocks have been completely metamorphosed to anthophyllite * actinolite +
talc * antigorite schists.Petrographicstudiesindicate that
anthophyllite is replacing antigorite (Raghavendraet al.,
1976).Anthophyllite asbestosoccurs in restricted localities as veins and pods. Asbestosfibers in different stages
of development and with varying aspect ratios are observable. The fibers are usually arranged parallel to the
walls of the veins; random seamswith cross-fibersare also
present.
Mountain wood anthophyllite has a hardness in the
rangeof 5-5.5, D = 3.19, and the hand specimensare
pale brown. Under the polarizing microscope, polished
thin sectionsrevealaggregates
oflong, fine crystalliteswith
subparallel orientation. The thin sections are colorless,
optically cloudy (translucent),with very weak pleochroism. Mean refractive index is 1.66.Directional variations
in refractive index could not be measured,owing to the
texture ofthe rggregatesand small crystal size.spv images
(Cambridge StereoscanS-I 50) show the stacking of crys0003404x/86/09r 0-1l 98$02.00
tallites giving rise to a subparallel-orientedtexture (Fig.
1a).The tip of the samespecimenis seento split into fine
fibers (Fig. lb). An X-ray pattern, recordedwith a Philips
PWl700 powder diffractometer (FeKa radiation), could
be indexed on the basis of an orthoamphibole unit cell
(spacegroup Pnma) with a: 18.48,,: 18.09and c:
5.30A.
Chemical analysesby a combination of wet-chemical
methods (complexometric and redox titrations, as well as
colorimetry and atomic absorption) of mountain wood
I 198
of
compositions
Table1. Representative
anthophyllites
sio,
Tio,
Alro3
Fe2O3
FeO'
MnO
Mgo
CaO
Naro
K.O
Hr9*.Hro-'*
Total
"Mountain
wood"
Asbestos
55.05
nil
0.76
0.48
19.32
0.08
21.88
0.56
0.02
0.01
2.70
0.02
100.88
56.60
nil
't.22
0.28
18.09
0.19
21.38
0.40
0.12
0.01
2.05
0.08
100.42
No.of cationsper24 O atoms
7.985
7.754
0.015
0.126
0 188
0.051
0 030
2.131
2.273
4.486
4.588
0.023
0.010
0.060
0.085
0.034
0.006
0.002
0.002
1.93
(oH)
2.54
Si
Alrv
Alrv
Fe..
Fe'Mg
Mn
Ca
Na
K
* Determinedby redox titrations.
'* Determinedby microgravimetricgas analysis.
I 199
SUBBANNAET AL.: BRUCITE-ANTHOPHYLLITEINTERGROWTHS
I{
B R U C IE
T
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75A
.',"11
t il
Fig.2. A nnrnr"rimageof mountainwoodanthophylliteconof brucite.
tainingthe structuralintergrowths
is probably brucite. To our knowledge, this is the first
report of the intimate intergrowth of brucite in anthophyllite. Coherent intergrowths of serpentineand brucite
type layersaredescribedby 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 brucite. The observedwidths ofbrucite lamellae vary from 20
to 200 A. ttre coexisting asbestiform anthophyllite does
not show such brucite intergrowths. The only structural
defectsobservedare apparently random intergrown triple
chains (Veblen et al., 1977).Similar chain-width disorder
is also observedin the mountain wood anthophyllite. The
differencein physical propertiesbetweenmountain wood
micrographs
of mountainwood showing(a) the
Fig. 1. sErra
and the massiveas well as asbestiformanthophyllite may
orientedcrystallitesand (b) tip of the specimensplittinginto
arise from the structural intergrowth of brucite in mounsmallerfibers.
tain wood. Since this material exists as relics within the
asbestosmines, with the fully developed asbestosfibers
given
in apparently growing out of it, mountain wood can be conand the coexistingasbestiform anthophyllite are
sidered as a precursor to the asbestiform anthophyllite.
Fe
Mg
cations
wood
has
more
and
mountain
Table l. The
and hydroxyl ions and less Si per formula unit than the
AcrNowr,nncMENT
asbestiform variety. When placed in 0. I M hydrochloric
C.N.R.Raois gratefully
of Professor
encouragement
The
kind
gives
rise to solubleFe2+* Mgz+
acid, the mountain wood
(up to 8.9 wto/oFeO + MgO) (Raghavendraet al., 1978). acknowledged.
The asbestiformanthophyllite givesrise to < l.3oloFeO +
RrrnnnNcns
MgO in the same medium.
M.N., andMurM., Viswanatha,
Ramakrishnan,
Becauseofthe higher octahedralcation contents in the Chadwick,B..
thy, V.S.(1978)Structuralstudiesin the ArcheanSargurand
mountain wood, nnreu analysisof this material was carrocksof the Karnatakacraton'GeologDharwarsupracrustal
ried out. The investigationswere made in a rBor-rErtl200
cx electron microscope operating at 200 kV with a rsc2
objective stage.The sampleswere preparedby fine grinding 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),
220.
are 4.8 and 5.1 A, respectively(Deer et al., 1962; Veblen
of metaS.M.(1979)Geochemistry
Naqvi,S.M.,andHussain,
and Buseck, 1979).The presenceof gibbsite can be ruled
belt in Karnataka,India. Cafrom a greenstone
anorthosites
out due to the low Al-content of the mountain wood.
nadianJournalof EarthScience,16, 1254-1264.
R.V.,Ananthalyer,G.V.,andKutty'T.R'N.(1976)
Therefore,the mineral intergrown with the anthophyllite Raghavendra,
1200
SUBBANNA ET AL.: BRUCITE-ANTHOPHYLLITE
Mineral chemistry of asbestosbearing ultramafics from HolEnarasipurSchist belt. Indian Institute ofScience Journal, 58,
460-49r.
-(1978)
Acid resistancecharacteristicsof amphibole asbestos.Indian Journal of Technology, 16, 317-322.
Veblen,D.R. (1980)Anthophyllite asbestos:Microstructures,intergrown sheet silicates and mechanisms of fiber formation.
American Mineralogist,65, 1075-1086.
Veblen, D.R., and Buseck,P.R. (1979) Serpentineminerals: In-
INTERGROWTHS
tergrowths and new combination structwes. Science,206, 1398I 400.
Veblel, D.R., Buseck,P.R., and Burnham, C.W. (1977) Asbestiform chain silicates: New minerals and structural gfoups.
198,359-365.
Science,
MeNuscnrrr REcETvED
Ocrosrn 23, 1984
MnNuscnrpr AccEPTED
Mav 16. 1986