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STRONTIOMELAN E, Srmn4*6Mn3*Zor S, A

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STRONTIOMELAN E, Srmn4*6Mn3*Zor S, A 673 The Canadian Mineralo gist Vol. 37, pp 673-6'78(1999) STRONTIOMELANE, SrMn4*6Mn3*zOrs, A NEW MINERALSPECIES OF THE CRYPTOMELANEGROUP FROMST. MARCEL- PRABORNA,AOSTA VALLEY, ITALY NICOLAS MEISSER'K Musdede gdologieet Institutde mindralogie,BFSH-2, Universitd, CH-1015 Lausanne, Switzerland ELENA-ADRIANA PERSEIL Laboratoirede mindralogie,Musdum National d'Histoire Naturelle, 61, rue de Buffon,F-75005 Paris, France JOEL BRUCCER$ Mineralogischund Petrographisches Institut, Universitiit, Bernoullistasse 30, CH-4056BaseL, Switzerland PIERRE-JACQUES CHIAPPERO Galerie de mindralogie, Mus6um National d'Histoire Naturelle, 36, rue Geoffroy-Saint-Hilaire, F-75005 Paris Cedex, France Assrnncr Strontiomelane is a new mineral speciesfrom the Praboma mine near Saint-Marcel, Aosta Valley, Italian Alps It occurs as small anhedral inclusions (about 100-200 pm) and veinlets associatedwith braunite, cryptomelane, hollandite, quaftz, fluorapa- iite, titanite,jadeite and strontiopiemontite. The new speciesis opaque, black with a submetallic luster and a strong grey-white pleochroism; in plane-polarized light in oil immersion, it presents a medium anisotropy with grey-blue to white tints. Strontiomelane, whose ideal formula is SrMn4+6Mn3+2O16,crystallizes in space group P21ln (14) with a 10.00(1), b 5.158(1), c 9.83(1) A, p 90.64(6)' andZ = 2. The srrongesrlines in the X-ray powder pauern td in A(D(htl)l are: 3.15(100)(103), 3.13(80)(103),2.409(80)(122),2.17060)(320)and1.556(50;i225)Thecalculateddensityis466(1)g.cm3.Strontiomelaneis the Sr end-member of the cryptomelane group The proposed name is derived from the chemical element strontium and the color black, melas,- anos in Greek. Keywords: strontiomelane, strontium, manganeseoxide, cryptomelane group, Praborna,Alps, Italy. Souvatne La strontiom6lane est une nouvelle espdcemin6rale d6couverte d la mine de Praborna,prbs de Saint-Marcel, vall6e d'Aoste dans les Alpes italiennes. Cette espdcese prdsenteen petites inclusions x6nomorphes de 100-200 pm ainsi qu'en veinules. Les min6raux associdsh la strontiom6lane sont: la braunite, la cr1ptom6lane, la hollandite, la fluorapatite, la titanite, le quartz, la jad6ite et la strontiopi6montite. La strontiom6lane est noire, opaque, avec un 6clat subm6tallique et un fort pldochrolsme gris blanc; en nicols crois6s, elle pr6sente une anisotropie d'intensit6 moyenne avec des teintes gris bleudtre d blanc. La formule chimique iddale de la strontiomd^laneest SrMna+6Mn3*2O16.Cette espdcecristallise dans le groupe d'espace P21ln (14) avec a 10.00(1),b 5.758(7),c 9.88(1) A. P 90.64(6)' et Z =2. Les raies principalesdu diagrammede poudre [d en A(IXftLl)] sonr: 3.15(100)(103),3 13(80X103),2.409(80)022),2t70(60)(320) et 1.556(50X225).La densitdcalcul6e est de 466(1). La strontiom6lane est le terme sfontifdre du groupe de la cryptom6lane. L'6tymologie du nom d6rive de l'6l6ment strontium et du grecmelas,- anos. signifiant noir. Mots-cl6s: strontiom6lane, strontium, oxyde de mangandse,groupe de la cryptom6lane, Praboma, Alpes, Italie a E-mail address: [email protected] $ Present address:Off,rce 258, VIEPS, Department of Earth Sciences,Monash University, Clayton, Victoria 3168, Australia. 674 THE CANADIAN MINERALOGIST INrnooucrroN contentup to 1.54 wt.Vo;T = 350'C, P < 7 kbar); Cabella et al.1994). Cryptomelane - hollandite-type compounds are tet- Preliminary studies of the ore from the Praborna ragonal or monoclinic (pseudotetragonal)complex mine by electron microprobe has suggestedthe emer- oxides, of general formula A1BgO16.nH2O(0 < x < 2, gence of a new Sr end-member in the cryptomelane 0 < n < 0.16) (Bums & Burns 1979,Kudo et al. 1990, group, associatedwith strontiopiemontite in the same Waychunas1991). Only memberswithA = Ba, K, Na, sample (Perseil 1988). The mineral is named strontio- Pb, Sr and B = Cf+, Fe3*, Mg, Mn4*, Ti, V3*, Zn, Zr melane,from the element strontium and from the Greek (Perseil 1988, 1990, Fleischer & Mandarino 1995) are melas,- anos,black. The new specieswas approved by actually known as mineral species. In this paper, we the Commission on New Minerals and Mineral Names describe the properties of a newly establishedmember of the International Mineralogical Association prior to of the series,strontiomelane, discovered at the Praborna publication. The holotype material is deposited at the mine, in Aosta Valley, Italian Alps. Geological Museum of Lausanne, Switzerland (#MGL 58770). Cotype material is also preserved at the BacrcnouNo h{ponlrerroNr Laboratoire et Galerie de Mindralogie, Mus6um National d'Histoire Naturelle, Paris, France (#SM Natural examples of this solid-solution series are 7090.96). typically manganian (ideally with B = Mna*): crypto- melane (A = K), hollandite sensu stricto (A = Ba), OccunruNcs manjiroite (A = Na) and coronadite (A = Pb). Small amounts of Ce, La, Nd, Y, P and Nb have also been The Praborna manganesedeposit is situated about observedby Fodor et al. (1994) and Mitchell & Meyer 5 km south of the village of Saint-Marcel, in a lateral (1989) in natural hollandites. An ever wider variety of valley of Val d'Aosta, Italy. Manganese-enrichedhori- synthetic phaseswith the hollandite-type structurehave zons are embeddedin a 4- to 8-m thick quartzitic layer been reported, with A = Li, Na, K, Rb, Cs, Mg, Ca, Sr, that representsthe basal part of the sedimentary cover Ba, Ag, Tl and Pb, andB = Sc, Ti, Cr, Mn4*, Fe, Co, Ni, of an ophiolitic sequence, tectonically related to the Cu, Zn, A1, Ga, In, Si, Ge, and Sn, defining 30 end- Piemont Nappe. During the Alpine orogenesis,this ore member compositions (Pentinghaus 1978, Zhang & concentrationwas metamorphosedin the eclogite facies Burnham 1994). in Cretaceous times and then retrograded to the In the group of manganianphases, the crystal struc- greenschist facies in Tertiary times (Mottana 1986). ture of pure synthetic hollandite, BaMn6O16,was first During the late stages of the polyphase Alpine meta- studied by Bystrcim & Bystrdm (1950): large cations, morphism, in the greenschistfacies, the manganese-rich like Ba, occupy the A site within the tunnel formed by horizons were subsequently cross-cut by networks of corner-sharing of columns of edge-sharing (Mna+O6) veins and late fractures responsiblefor the mobilization octahedra along [001]. These authors showed that the of severalelements like K, Ba, Sr, Sb, W and As (Mar- ideal structure possessesa tetragonal symmetry, I4/m. tin-Yernizzi 1982, Mottana & Griffin 1986, Perseil Two monoclinic structuresarerecognized in this group: 1985, 1988, 1991,Perseil & Smith 1995).Strontio- Mukherjee (1960) found evidenceof a monoclinic sym- melane crystallized during this late stage of veining. metry, space grotryP21ln, for natural ferrian hollandite, The mineralogical association of Praborna, with a BaFeMn7Ol6, and Miura (1987) investigated a natural long and complex metamorphic story, has already pro- potassium- and iron-rich hollandite from India and duced two new mineral species:piemontite, a manga- found a monoclinic structure,I2lm.ln contrast with the neseend-member of the epidote group (Kenngott I 853) observationsof Bystnim & Bystrrim (1950), Mukherjee and rom6ite, a calcium-antimony oxide, structurally (1960), Post et al. (1982) and Mrtra et al. (1987) re- relatedto pyrochlore(Damour 1841,Brugger et al.1991). ported that with increasingBa2+ content, with respectto K*, the symmetry of cryptomelane changes from tet- Hesrr aNo PanacsNpsrs ragonal to monoclinic. For several years, significant amounts of Sr have Strontiomelane occurs as thin (100-200 pm), sub- been found in minerals of this group. Occurrences of metallic, black, elongate xenomorphic inclusions com- Sr-bearing hollandite have been reported in several monly aggregated with braunite in quartz and, more localities: 1) the Falotta and Parsettens mines, rarely, in violet pyroxene (variety "violan" of diopside- Oberhalbstein, Graubiinden, Switzerland, as a super- aegirine-jadeite) and in strontiopiemontite. Thin late- gene mineral (with 24 wt.% SrO) resulting from the stage veinlets of the new species were also observed oxidation of Sr-bearing sursassiteand barite (Perseil associatedwith braunite, hollandite, cryptomelane,ja- l99O),2) the Cerchiara mine, eastern Liguria, Italy deite, strontiopiemontite, Sb-bearing titanite, As-bear- (1.93-3.72 wt.VoSrO;' 250 < T < 300'C, 2 < P < 3 kbar), ing fluorapatite and quafiz. On polished surfaces, (Cabella et al.1994), and 3) the Upper Jurassicquartz- strontiomelanediffers slightly from braunite by a lower itic meta-arenitesof the Maritime Alps, Italy (SrO hardnessand a poorer polish. STRONTIOMELANE FROM THE PRABORNA MINE. ITALY 675 TABLE I REFI.ECTANCE VAIIJES OF STRONTIOMELANE of hollandite; between crossedpolars, its anisotropy is moderate,with a grey-blue to white tint. No internal re- Refl€ctil@ Reflectance Wavelengths flections are noted, and the color of the streak is sepia. |]lamm nmum (coM) Reflectance values of the new speciesare presented [R%w] tR%minl [l,m] in Table l. The observed values of reflectivity of strontiomelane lie in the range of hollandite and 342 260 4',70 317 244 543 cryptomelane (Perseil & Pinet 1976, Picot & Johan 306 ,14 547 1982). )o1 223 657 The Vickers hardness,measured by micro-indenta- tion using a load of 100 g, gives a mean of 398 and a Objtrtive: LEITZ 50/0 85; stedtrd LEITZ WTiC; meeued in air range of 309-450 (number of measurements:16). Strontiomelane is brittle, with a rarely observed cleav- age. The size of the new mineral precluded a direct mea- Pnystcal Pnoppnrres surementof the density. It was calculatedfrom the com- position richest in Sr (Table 2, #6) and the refined In polished sections, microscopically, strontio- unit-cell parameters:4.66( | | g.cm-3. melane is an opaque mineral. Observed in oil immer- sion, in plane-polarized reflected light, the mineral is X-ray-dffiaction study grey with a strong grey-white pleochroism similar to that As all the available grains of the new mineral spe- cies are quite small, the crystallographicstudy was made X-ray-diffraction data and ana- TABLE 2 CIIEMICAL COMPOSITIONOF STRONTIOMELANE, with a Gandolfr camera. ST MARCEL - PRABORNA AOSTA VALLEY. ITALY lytical conditions are presentedin Table 3.
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