Carlosturanite, a New Asbestiform Rock-Forming Silicate from Val Yaraita, Italy
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AmericanMineralogist, Volwp 70,pages 767-772, 1985 Carlosturanite,a new asbestiformrock-forming silicate from Val Yaraita, Italy Rornnro Colap,lcNoNr Dipartimento di Scienze ilella Terra Uniuersitd della Calabria, Castiglione scalo,87030 Cosenza,Italy Grov,tNNt FnnnlRts Dipartimento di Scienze della Terra Uniuersitd di Torino, uia S. Massimo 22, 10123 Torino, Italy ,C,NDMARCSLIO MELLINI C.N.R., C.S. Geolo gia Strutturale Dinamic a dell Appennino uia S. Maria 53,56100 Pisa,Italy Abctract Carlosturaniteis a new rock-forming silicate occurring in a network of veins crosscutting the antigorite serpentiniteof Sampeyrein the Monviso ophiolite, Italy. It is light-brown, asbestiform,and the [010] fibers are paralleled by fibrous diopside and chrysotile. The mineral is monoclinicCm, with a : 36.70,b :9.41, c :7.2914, f : 101.1".The strongest lines in the X-ray powder diffraction pattern are: 18.02(25X200),7.1(100X001,201), 3.595(45X10.00,W2), 3.397(55t202), 256440X8O2),2.280(35X14.01,16.01). Similar refractive indicesare measuredalong (1.605)and across(1.600) the fiber axis. Carlosturanitedehydrates upon heating, forming chrysotile and hematite (,100.C)and finally forsterite (770"C).The infrared pattern shows absorption bands due to hydroxyl anions and silicate tetrahedra. Chemicaldata lead to the empiricalchemical formula (Mgrr.rrFe2r+rrTif.lnMnfr.!rCr3.12h41.54(Si22.e2Alo.81h2t.rcHrz.srorzu (D""r" : 2.606, Dob" : 2.63glcmt) or, ideally and according to a structural model, MztffrzOze(OH)41(OH)3o'}{2O(Z:2). Carlosturaniteor carlosturanite-likephases may be expectedto develop in serpentinitecompositions under low grade matamorphic con- ditions. Introduction Names. The holotype material is kept at the Museo Re- Specimensof the new mineral were fust collected on the gionaledelle Scienze(Torino). working face of a chrysotile-asbestosmining prospect,lo- catedsome 5 km westof Sampeyre(Val Varait4 Piedmont, Occurrence Italy). The mine is situated in a metamorphic serpentinite Carlosturaniteis quite common in the Sampeyreserpen- belongingto a southernportion of the Monviso ophiolite, tinite, where it occursover an area of a few squarekilome- which is part of the internal Piemonte-Liguria Mesozoic ters. The zone underwenta polyphasemetamorphic evolu- ophiolitic belt, in the pennidicdomain o[ the WesternAlps. tion characterizedby an Early Alpine event under greens- The new mineral strongly resemblesfibrous serpentine, chist facies conditions (Hunziker, 1974). Structural and for which it can be mistakenin the field; it has commonly mineralogical relics indicate that the antigoritic serpen- been called metaxite or metaxitic serpentine (Zucchetti, tinite derivesfrom an upper-mantlespinel lherzolite partly 1968)or xylotile, i.e.,the samenames used for balangeroite, re-equilibratedunder the plagioclaseperidotite facies,like another macroscopicallysimilar fibrous silicate from Ba- most serpentinitesof the ophiolitic belt of the WcsternAlps langero (Lanzo Valley, Piedmont) described by Com- (Compagnoniet al. 1985). pagnoniet al. (1983). Carlosturanitedevelops in a closenetwork ofveins (from The name carlosturanite is after Carlo Sturani (1933- 0.1 mm to severalcentimeters thick) randomly crosscutting 1976),Professor of Geology at the University of Torino, the serpentinite(Fig. 1). Usually it occurs together with whose untimely death occurred in a fatal accident during chrysotile, diopside, and opaque ore minerals (magnetite field work. The name and the specieshave been approved and native NiFe alloys); locally, clinohumite, perovskite, by the I.M.A. Commissionon New Minerals and Mineral and a green uvarovite garnet are also found. Frequently, 0003-{04x/8s/0708-o767$02.00 767 768 COMPAGNONI ET AL.: CARLOSTURANITE ed determination of the hardness.Parallel intergrowth be- tween [010] carlosturanite, t0011 diopside (Fig. 2), and [100] chrysotilefibers occursin extremelyvariable degtees from macroscopicto TEM scale;at the latter scalebrucite occurs in the presenceof abundant chrysotile (Fig. 3). Single fibers of carlosturanite are seldom larger than 0.2 pm. Very good {001} cleavageand an approximate {010} fracture ate observed.A densityof 2.63(2)g/cm3 was deter- mined by the heavyliquid method. Optical measurementsare very dillicult and imprecise.In thin section carlosturanite is transparent and pleochroic with orange brown and pale orange brown parallel and perpendicular to [010], respectively.The extinction is parallel and the elongationpositive. Because ofthe texture, only an average refractive index, 1.600(5),has becn ob- servedperpendicular to [101]; 1.605(5)has beenmeasured along [010], Birefringenceis low (first order orange) to very low (grey), though greyish-blue anomalous inter- fsrenc€colors are locally shown by sectionscut parallel to the fiber axis. Interferencefigures, performed on fiber bun- dles cut perpendicularto the fiber axis, are definitely posi- tive, but may appeareither pseudo-uniaxialor biaxial with small to moderateoptic axial angle.Microscopically, carlo- sturanite is very similar to balangeroite,from which it may be distinguished by lower refractive indices (n : 1.68 in balangeroite)and by weakerpleochroism (a : pale yellow- ish red-brown and,y: reddishbrown in balangeroite). Only the powder diffraction pattern and the repetition period along the fiber axis could be obtained by X-ray diffraction. The b-rotation photographs(CuKa radiation) show strong zero and third layer lines, all the other layer lines being definitely weak (Fig. 4); such a pattern can be Fig. 1. Deformedvein (upper) and bundles(lower) of carlostu- compared with that of antigorite around b (Ar$a, L946) ranitecut paralleland perpendicular to the fiberaxis, respectively c (Compagnoni et al', 1983). (planepolarized light). and of balangeoitearound Superpositionof 5.2A layer lines of chrysotile occurs with some samples.Only continuous lines are observedon the Weissenbergphotographs, indicating complete rotational the carlosturanite-bearingveins appear to have been de- disorder around the elongation direction. Selectedarea formed andlor reactivatedseveral times, resulting in a com- electron diffraction patterns indicated C2fm, Cm, ot C2 plex structural and mineralogicaltexture. On the one hand symmetry(Mellini et al., 1985)and supplementedthe a, c, these veins appear to be crosscut by the later mono- and p starting valuesused to index the powder diffraction mineralic veins of antigorite, chrysotile,brucite, and mag- pattern (Table 1); a least-squaresunit cell refinementbased nesite;on the other hand they seemto reactivateor cross- on the powder data gave a:36.7V3), b:9.41(2), c: cut earlier veins of antigorite, of chrysotile and of olivine 7.2916)4,and B :101.1(1f. Becauseof the fibrousnature + clinohumite + diopside+ opaques.Microscopic obser- of the material,preferred orientation may affectthe powder vations indicate that carlosturanite,as well as the other diffraction patterns. In particular, most reflections with serpentine minerals, is frequently replaced by brucite. k # 0 have been observed only with Guinier-Lenn6 TransmissionElectron Microscope(TEM) images,showing camera; thesereflections are starred in Table 1. Undoubt- brucite crystalsapparently pseudomorphic after singlestu- edly, the unit cell dimensionsand the distribution of the ranite fibers and embayedby abundant chrysotile,suggest strongestreflections recall the serpentineminerals (Whitta- the following breakdown reaction: carlosturanite+ bru- ker and Zussman,1956), but the overall powder pattern is cite * chrysotile. definitely characteristic for carlosturanite. In particular, supplementarylower angle reflections (18.02 and 9'01A) Physical and crystallographic properties occur in the new mineral. Carlosturanite is light-brown, flexible, and develops Thermal and infrared study [010] fibers severalcentimeters long, commonly gathered in folded bundles.The streak is whitish. and the luster is The weight loss at 1000oC,as determinedby Thermogra- vitreous-pearly.The fibrous nature of the mineral prevent- vimetric Analysis (TGA) (Fie. 5), is 16.85%.Comparison COM P AGNON I ET AL.: C ARLOSTURAN I'r E Fig. 2. TEM imageof parallelintergrowth of diopside(di) and carlosturanite(cst). The opaqueinclusion (m) is chromianmagnetite. with Differential Thermogravimetric Analysis (DTG) re- just above room temperature,suggests that H2O may be veals that the weight loss begins at rtO'C and continues presentas well. quite smoothly with minor flexes at 75"C (1%), 105'C (1.8%) and 195"C (2.5%). After further loss to 5.6Voat 380'C, a smoother slope begins,and then a step (600'C) Compositional data brings the weight loss to l3Vo at 750'C; the last part of the Table 2 reports the average results of fifteen electron curve has a flex at 900'C- microprobe analyses obtained by wavelength dispersive A continuousX-ray powder pattern recordedfrom 20 to analysis on a fully automated ARL-SEMQ instrument, 1100'C (20'C/hour) showsa practically constantpattern of using olivine (Si, Mg, Fe), jadeite (Al), ilmenite (Ti), Mn- carlosturaniteup to about 400'C whereit vanishes,leaving hortonolite (Mn), and Cr-garnet (Cr) as standards;H2O is at least two other phases:a serpentinephase, which disap- from TGA. No other element with Z > 11 was shown by pearsat about 500"C,and hematite,which is still presentat TEM/EDS analyses.Some variation, particularly for the 1100"C. The latter is the only crystalline phase evident minor constituents,was detectedwith the microprobe; this from 5fi) to 700'C, when forsteriteappears. Minor shifts of variation