Balangeroite, a New Fibrous Silicate Related to Gageite from Balangero

American Mineralogist, Volume 6E, pages 214-219, I9E3

Balangeroite,a new fibrous silicate related to gageitefrom Balangero,Italy

Ronpnro CoupecNoxr Dipartimento di Scienzqdella Terra Universitd della Calabria Castiglione scalo, 87030Cosenza, Italy

GtoveNNr Fr,nnents Istituto di Mineralogia, Cristallografia e Geochimica "G. Spezia", Universitddi Torino via S. Massimo 22. 10123Torino. Itqlv

eNo LRune Flone Istituto di Petrografia, Universitd di Torino and Centro di Studio per i Problemi dell'Orogeno delle Alpi Occidentalidel C.N.R., via S. Massimo22, 10123Torino, Italy

Abstract

Balangeroiteoccurs as brown asbestiformfibers in a paragenesiswith long-fiber chrysotile,magnetite, and native Fe-Ni in the Balangeroserpentinite (Lanzo Valley, Piedmont,Italy). It is orthorhombicwith a 13.85(4),b 13.58(3), and c 9.65(3)A;a sub-cell with c' : c/3 is stronglyevident. The X-ray powderpattern demonstrates an isostructural relationshipof the newmineral with gageite;the strongestreflections are:9.59(40)(ll0), 6.77(80)(020),3.378(45)(410), 3.278(40Xr40), 2.714(100)(050,510), 2.674(75)(150,223), and 2.516(40)(250).The fibers are elongated parallel to [fi)l], showone or more{/r/<0} cleavages, and are usuallyintergrown with chrysotile.The refractiveindexes are aboutthe same, 1.680(5),both parallel and perpendicular to [fi)l]; coloris darkbrown and yellow brown in thesetwo directions,respectively. Chemical analysis gives a unit-cellcontent of (Mgzs.zoFez2.tqFe].t3Mn?:5Ab.rzCao.ozCro.orTio.or)>rae3Si15.3sO53oe(OH)ls.sz, andthe presenceof a sub-cellwith c' = c/3 suggeststhe formula (Mg,Fe2*,Fe3*,Mn2*,!;o2sir5(O,OH)eo with contentsdivisible by 3; a similarformula is probablyalso correct for the Mn-analogue, gageite.DTA, TGA, and high-temperatureX-ray powder analysesshow structural breakdownwith a total weightloss of 9.4Voand the appearanceof olivineat 8fi)'C. The infrared spectrumshows strong absorptionscorresponding to vibrationsin OH- and (SiO4)4-groups.

Introduction Balangero(or San Vittore) mine was brought to our attention by the mineral collector Enrico Beccuti in Europe's most important chrysotile asbestos 1977, and other fresh samples were subsequently mine occurs in the Balangero serpentinite. This is a collected. [t was eventually discovered that a large satellite body of the Lanzo Massif (Lanzo Valley, and relatively pure specimen had existed in the Piedmont, Italy), which is the largest ultramafic Turin University Mineralogy Institute's museum body of the Western Alps- A good referencelist for since 1925with the inventory no. 14873and label the geology and petrology of the area is given by "fibrous serpentine(asbestos!-San Vittore, Balan- Compagnoniet al. (1980). gero." The type material is now depositedin the A recent specimen of a new mineral from the museum. 0003-004x83/0102-0214$02.00 214 COMPAGNONI ET AL.: BALANGEROITE 215

To the workers of the mine the mineral was ble 1) and chemical compositions (Table 2) of known as xylotile or metaxite.Stragiotti (1954) and xylotile and metaxite (Cailldre, 1936),however, do Peretti and Zucchetti (1968)applied the term xylo- not correspond to those of the new mineral. Metax- tile to a mineral from Balangerowith "fibers which ite is usually describedas green and is regardedas a are severalcentimeters long, rigid, xyloid, yellow- variety of antigorite or "serpentine," while xylotile brown and pleochroic." X-ray powder spectra (Ta- is yellow-brown and is classed as a species by

Table powder l. X-ray data. For balangeroite,data were obtainedby ditrractometry with CuKa radiation; indexeswith / I 3n are not consideredbecause thev are verv weak.

Balangero ite cage i Ee Metaxiteb Xylotileb

I d(A) hk1 d (A) I d(A) I d(E) r d(A) oo c oo o ooo 40 9.59 110 9.70 3 9.70 vs 8.16 u 4.31

80 6.77 020 6.79 10 6.87 s 6.92 w 3.81 30 3.405 o40 3.39s 2 6.14 4.72 w 3.38 45 3.378 410 3.355 6 3.44 4.31 w 3.12 40 3.278 140 3.297 4 3.34 3.8r v 2.7O 35 3. 198 oo3 3.277 6 3.25 s 3.s8 w 2,53

30 3.o30 240 3.048 4 3.O8 2.96 w 1.45

too 2.714 05O;510 2.716;2.7L4 8 2.758 VS 2.53 w 2,3O

75 2.674 223tL50 2.680;2.665 8 2.707 s 2.43 w 2.O8

5 2.613 o3.3 2.622 3 2.6s9 w 2.O9 la 1.94

5 2.57I 133 2.576 5 2.614 |.97 w 1.73

30 2.536 520 2.565 6 2.556 t.73 w 1.53

40 2.516 250 2.528 2 2.48L s 1.53 w 1.35

5 2.442 233:440 2.452;2.424 | 2.365 1.31 w 1.29

25 2.324 4L3 2.322 3 2.307 1.05 w l.2Q

10 2.274 610 z.zto 3 2.244 t.oo w 1.02

5 2.243 423 2.226 3 2.116 w o.91 20 2.138 450 2.r37 4 2.151 o.89 25 2.123 503 2.O99 3 2.LO3 10 2.023 360i523 2.O32t2.Oo5 3 2.O81 15 L.927 443iI7O 1.936;I.921 r 2.056 15 1.863 270 I .868 3 2.OI5 a Moonz(1968) 5 1.688 7o3;180 1.685;1.685 3 1.952 b cni,LLanu(trsr) s 1.656 073;553 1.661;1.661 2 t.894 vA = venq ALtLong 10 1.595 106;016;380 1.598;1.597;1.593 3 1.863 ^- == ) L. )ZJ 803;480 L.524;1.524 3 1.802 ls t.494 473;183 1.498;1.493 6 r.674 -*"u* 5 I.475 236 1.480 2 r.627 5 I.443 146; 833; 66 3 1.446;I.445;1.444 2 t.6r7

5 1.358 526;256;763 1.363;l.357 3l ,352 4 r.587 3 1.554 6 1.518 216 COMPAGNONI ET AL.: BALANGEROITE

Table 2. Chemical analyses of balangeroite and related Long fiber asbestosveins containing balangeroite compounds developed early in the Balangero serpentinite evo- lution, as indicated by the presence of chrysotile corroded by metamorphic olivine which, in turn, has been partially transformed into antigorite by t{CO 30.35 - 34.15 31.81 13.93 37.68 later shear movements along the original veins FeO 2L.43 - 27.95 15.95t 0.60 r.ov (Compagnoniet al., 1980).Balangeroite also has Ftzo3 8.89" 1g.3o 2.94 been identified in thin sections of rocks from the lIno 2.93 - 3.92 3.59 0.17 Lanzo Massif as a pseudomorph after orthopyrox- ct203 o - 0.13 0.03 enes. It is probably an uncommon, but not rare' CaO o - 0,16 o,13 0.76 2.09 serpentinite mineral. TiO2 o - o.o5 0.03 0.04 sio2 24.4L - 29.36 28.37 46.65 42.L5 Physicaland crystallographicproperties - 0.56 o.79 Atzol o 1.5o o.27 Balangeroite is found as brown, rigid and brittle 9.9:b rg.r7 13.04 "zo xyloid fibers that are either loose, or compact when Total 100.00 100.28 100.38 in large quantities, and may run severalcentimeters in the [001] direction. They have a vitreous-greasy |. fulfunge^nae - Mittlnax E we,ighfa 6otL 16 elec.tn-onnLctu- luster and are only transparentin thin section. Even ptabe o.wl4te.r, StanlatlAt oLLvine, gd.,Lneldnd hM fibers with a very small cross-sectionare composite pqnoxenL'. (Fig. 1) and usually intergrown with chrysotile; one 2. Ba,LlvtgeJwi.te- Ave.u4e 2 weigh.t. or more {hlc0}cleavages are very good. The physical - (19361 3. Xr/loLi.te Avutge 06 4 araLgsu bg Coil2dne . aspect of the material makes optical measurements 4. Mel*xi..te - Avena4e 06 4 eMIqLu bq Ca.illdtte 11936l; difficult and imprecise. The mineral is definitely thi.t aathatt't anoltlti.t No. 12 i.t excLuded beutue o( i.il lange Mo0^ content. anisotropic and shows distinct pleochroism, being o dark brown and yellow brown parallel and perpen- A no.Lin ruz') iul' = z.tt ha,r been de.duee.d[a.on we.t wa,trtAu. dicular to [001] respectively.By comparisonwith b oi56*n*o tt ,OOq $ee tertl . gageite(see below), balangeroiteshould be biaxial, but, because of its texture, only one refractive index value, 1.680(5),has been observed perpendic-

Cailldre (1936). Both display optical properties close to those of the serpentine minerals and a xyloid aspect(Cailldre, 1936). The new mineral, balangeroite,is namedafter the locality where it was discovered. The name and the specieshave been approved by the LM.A. Commis- sion on New Minerals and Mineral Names.

Paragenesis Balangeroite occurs as brown asbestiform fibers in a paragenesis with long-fiber slip chrysotile, magnetite, and native Fe-Ni; it is relatively abun- dant in the schistoseserpentinite at the contact with the unproductive Balangero serpentinite. Other as- sociated minerals are metamorphic olivine, chlo- rite, Ti-clinohumite, diopside, antigorite, and opaqueores; calcite, aragonite,opal, chalcedony, and clays are usually found in the serpentinite fractures. Fig. |. SEM view ofa typical fiber of balangeroite. COMPAGNONI ET AL,: BALANGEROITE 217 ular to [001]; practically the same value has been of water is proved by the large IR absorption found in the elongation direction. A density of around 35CI cm-r (Fig. 3). The high temperatureof 2.98(3)g/cm3 was measuredwith a torsion balance. dehydration suggeststhat there exists more OH- The [001] rotation X-ray photographs give c than H2O in the structure. The minor thermal 9.65(3)A (CuKa radiation) und r"u"ul the presence effects are not so easily interpreted. Shrinkage of of a pseudocell with c' : cl3; in fact, the first layer the cell and other TGA and DTA reactions noted line is very weak and the second absent. Only before breakdown are probably due to minor struc- continuous lines appear on the Weissenbergphoto- tural changes(including some earlier loss of water) graphs, showing that there is rotational disorder related to the atomic disorder that can be deduced about the elongation direction. The X-ray powder for balangeroite from Moore's (1969) structural pattern (Table 1) indicates that balangeroiteis iso- model for isostructural gageite. structural with gageite (Moore, 1968); by least- squaresrefinement the following cell was obtained: Compositionaldata a 13.85(4),b 13.58(3),and c 9.65(3)A.fne valueof c In addition to wet chemical and X-ray fluores- is trebled with respect to gageite,for which, howev- cence analysis, electron microprobe er, weak streaks on c-axis rotation photographs analysis was performed; the common intergrowth with chrysotile were ignored (Moore, 1968).The unit cell has been suggestedthat the latter would provide confirmed by electron diffraction. better chemical resolution(Table 2). Theseresults also showed some variability due to submicroscopic inter- Thermal and infrared study growths or zoning. A ratio Fe2*/Fe3* : 2.12 was The continuous X-ray powder pattern between20 deduced from wet chemical analyses; average and 900'C recorded for balangeroite with a high weight loss after calcination at 1000'C was 9.5Vo. temperature Guinier-Lennd camera (CuKa radia- This was attributed to H2O, though total H2O was tion) displays a sudden shrinkage(about l%) of the taken as the difference from l00Vo of the micro- cell parametersat 300"C; structural collapse occurs probe results due to the more probable presenceof at 650'C, and at 800'C olivine appears. impurities in analysesinvolving larger quantities of The TGA curve (Fig. 2) showsa stepbetween 580 material and the possible oxidation of Fe2+ under and 750'C corresponding to 6.4% weight loss; heating. The following empirical formula for the slopesbetween 50 and 580"Cand between750 and unit cell was obtained on the basis of the known 1000'Caccount for 1.6 and l.4Voweight loss respec- volume and density: tively. The DTA curve is characterized by endo- +6rpe3 (Mg25.7sF e] 1, nan ? i5Als. 17C ao. 67 Crs. s 1 and exothermic reactions at 620 and 700"C, in that order. Tie.s1)133.e3Sir5. 38053.66(0H)ss.sz. The major effects thus revealedare interpreted as breakdown of the crystal structure with the contem- After normalization to 15 Si in the unit cell, as : poraneousloss of the water, followed by the crys- discussedbelow, D..rc 2.904glcm3 was obtained : tallization of at least one new phase. The presence with M.W. 3174.24.The measuredspecific refractive energy is 0.228, and 0.223 and 0.220 are calculatedwith Larsen and Berman's (1934)and Mandarino's (1976) refractivitie s, respectively. Thi s agreementis "excellent" on Mandarino's (1981) scale. o zo 1 o I Crystal chemistry

Moore's (1969) structural model for gageite is a basedon a unit cell with c' : 3.279(3)Aand space 8 gtoup Pnnm,' streaks requiring a trebled c were axl :m soo Trto g6o rer ignored. The chemical formula proposed was Fig. 2. DTA (top) and TGA (bottom) curves in Ar atmosphere; M?+O(OH)8(Si2O6)(z : 2) insread of M?* l0 mg of balangeroitewith 20 and l0"C/min speed,respectively. (OH)6(Si3Or6)given previously by the same author 2lE COMPAGNONI ET AL.: BALANGEROITE

Fig. 3. Infrared spectrum of balangeroite(KBr disk).

(Moore, 1968).New chemical analyseshave sug- Acknowledgments gested (Mn4Mg16ZntxsSi15O5o(OH)+0,with one grateful to E. Beccuti for drawing their unit formula in the trebled cell (Dunn, 1979). The writers are attention to the Balangerosamples. Their thanks are particularly WhereasMoore's (1969)model surely needsad- extended to Dr. A. Kato (Tokyo) for his criticism and valuable justments in the silicon tetrahedra, the octahedral suggestionsas regards the crystal chemistry. The microprobe framework seemsreasonable: therefore, 42 octahe- and TGA analyseswere performed at the Istituto di Mineralogia dral cations should be present in the real cell. To e Petrologia of Modena University. Information obtained with has been kindly suppliedby Drs. A. Marconi account for the sub-cell with c' : cl3 it seems electron diffraction and C. Paoletti of the Istituto Superioredi Saniti. The research reasonable, even if not strictly necessary for all was supportedby the ConsiglioNazionale delle Ricerche,Rome' atoms, to postulateany atomic multiplicity in the cell as a multiple of 3 and write the corresponding chemical formula (Mg,Fe2+,Fe3*,Mn2*,n)lzSirs References (O,OH)e6,where fl representsvacancies. Such a formula with vacancies and an indefinite O/OH CaillCre,S. (1936) Contribution d l'€tude des mineraux des Bulletin de la Soci€t€ Frangaisede Min€ralogie, pres- serpentines. ratio would reconcile Moore's model with the 59, t63-f26. ence of trivalent cations and some variation in the Compagnoni,R., Sandrone,R., and Zucchetti,S. (1980)Some water content. Some HzO could be presentin the remarks on the asbestosoccurrences in the WesternAlps with structural "pipes" instead of the disordered Si- special reference to the chrysotile asbestosdeposit of Balan- tetrahedra and account for loss of weight at low gero (Lanzo Valley, Piedmont, Italy). Fourth International Conferenceon Asbestos(Torino, 26-3OMay 1980),Preprints, temperature. Different occupation of the cation t,49Jt. sites,partial order of the tetrahedra,and the possi- Dunn, P. J. (1979) The chemical composition of gageite: an ble presenceof H2O in the pipes could causethe empirical formula. American Mineralogist, 64, 1056-1058. breakdown of the higher symmetry with c' : cl3 Larsen, E. S. and Berman, H. (1934)The microscopic determi- which is required to a first approximation by the nation of the non-opaque minerals. U.S. Geological Survey Bulletin 848. octahedral framework. Mandarino, J. A. (1976)The Gladstone-Dalerelationship-Part Balangeroite,with the abundancesof its cations I: Derivation of new constants. Canadian Mineralogist, 14, decreasingin the order shownabove, represents the 498-s02. Mg-dominant analog of gageite, its Mn2+-dominant Mandarino, J. A. (1981)The Gladstone-Dale relationship: Part member.Nearly pure Mn2* membershave recently IV. The compatibility concept and its application. Canadian 19, 4,r'.1450. parts (A. Mineralogist, been found in various of Japan Kato, Moore, P. B. (l%8) Relations of the manganese-calciumsili- personal communication),and other members of cates, gageiteand harstigite. American Mineralogist, 53, 309- the series are probably present in Nature. 315. COMPAGNONI ET AL.: BALANGEROITE 2r9

Moore, P. B. (l%9) A novel octahedral framework structure: Stragiotti, L. (1954) Su alcune ricerche intese ad ottenere un gageite. American Mineralogist, 54, 1005-10 I 7. maggiore recupero di fibra dalla serpentina amiantifera di Peretti, L. and Zucchetti, S. (1963)Mineralizzazioni a solfuri nel Balangero.La Ricerca Scientifica, 24, 355:365. giacimento asbestiferodi Balangero (Torino). proceedingsof the International Symposium on the Mineral Deposits of the Manuscript received,March 11, 1982; Afps (Trento, ll-lE September1966), j,929-937 acceptedfor publication, June 21, 19E2.