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Scandiobabingtonite, a New Mineral from the Baveno Pegmatite

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Scandiobabingtonite, a New Mineral from the Baveno Pegmatite American Mineralogist, Volume 83, pages 1330-1334, 1998 Scandiobabingtonite,a new mineral from the Bavenopegmatite, Piedmont, Italy Ploro ORl.tNnIrr'* Ma,nco PasERorr and GrovlNNa Vn,zzllrNr2 rDipartimento di Scienzedella Terra, Universitd di Pisa, Via S. Maria 53,1-56126 Pisa, Italy '?Dipartimento di Scienzedella Terra, Universitd di Modena, Via S Eufemia 19, I-41 100 Modena, Italy Ansrntcr Scandiobabingtonite,ideally Ca,(Fe,*,Mn)ScSi,O,o(OH) is the scandium analogue of babingtonite; it was found in a pegmatitic cavity of the Baveno granite associatedwith orthoclase, albite, muscovite, stilbite, and fluorite. Its optics are biaxial (+) with 2V : : ^v: 64(2)",ct 1.686(2),P: 1.694(3), 1.709(2).D-"." : 3.24(5)slcm3, D.",.:3.24 sl cm3, and Z : 2. Scandiobabingtoniteis colorless or pale gray-green, transparent,with vitreous luster. It occurs as submillimeter sized, short, tabular crystals, slightly elongated on [001],and characterizedby the associationof forms {010}, {001}, {110}, {110}, and {101}. It occurs also as a thin rim encrustingsmall crystals of babingtonite.The strongest lines in the X-ray powder pauern are at2.969 (S), 2.895 (S), 3.14 (mS), and 2.755 (mS) 4. fn" mineralis triclinic, ipu.. g.oup PT, with a : 7.536(2),b : 1L734(2),c : 6.t48(Z) A, : 91.10(2), : 93.86(2), : lO+.S:(2)'. Scandiobabingtoniteis isostructural " B r with babingtonite, with Sc replacing Fe3* in sixfold coordination, but no substitution of Fer* by Sc takes place. Due to the lack of a suitably large crystal of the new species, such a replacementhas been confirmed by refining the crystal structure of a Sc-rich babingtonite (final R : O.O47)using single-crystal X-ray diffraction (XRD) data. INrnolucrroN mica (probably zinnwaldite) on which scandiobabington- This paper describes the new mineral species scan- ite crystals were emplanted (Fig. 1). Platy crystals of diobabingtonite.Both the mineral and its name were ap- scandiobabingtonitewere also found in epitaxial inter- proved by the IMA Commission on New Minerals and growth as a thin rim around green-blackprismatic crys- Mineral Names. Scandiobabingtoniteis the sixth scandi- tals of babingtonite (Fig. 2). The few crystals of scan- um mineral found in nature after bazzite, cascandite,jer- diobabingtonite studied here came from one sample. visite, kolbeckite, and thortveitite. Two other Sc minerals Scandiobabingtoniteis transparent,with vitreous luster were recently discovered: the scandium-dominant ana- and pale gray-greencolor. Scandiobabingtoniteoccurs as logue of xenotime-(Y) (IMA no. 96-024) and the scan- submillimeter-sized, short, prismatic crystals, slightly dium-dominant analogue of overite and segelerite(IMA elongated on [001] and characterizedby the association no. 96-060) (Mandarino and Grice 1997). of theforms {010}, {001}, {110}, {110}, and{101}, as Baveno is the type locality for bazzite (Artini 1915), shown in Figure 3. The faces were indexed using a two- cascandite,and jervisite (Mellini et al. 1982). Recently circle optical goniometer,in conjunction with X-ray sin- thortveitite (Orlandi 1990) was found in this classic lo- gle-crystal study, to overcome problems associatedwith cality; therefore, the only scandiummineral not found to a slight distortion and opacity of the faces from a thin date at Baveno is kolbeckite. Due to this peculiarity, the crust of mica. pegmatiticgranite of Bavenois unique. Two small crystals used in the optical and chemical The type material is preserved in the Museo di Storia study showed a very small black core and a large gray- Naturale e del Territorio, University of Pisa.A small crys- green transparentrim. It appearsthat the two crystals of tal is also stored in the Royal Ontario Museum, Univer- scandiobabingtonitegrew starting from a green seedcrys- sity of Toronto. tal of babingtonite (Fig. 4). Babingtonite, normally as- sociated with minute lamellar crystals of hematite, is a OccunnnNcE, pHysICAL, AND oprrcAI, pRopERTTES common mineral in the pegmatitic cavities of the Baveno Scandiobabingtonitewas found in the "Montecatini" granite. The color of scandiobabingtonite(light gray- granite quarry near Baveno (Novara, Piedmont, Italy), green) is however easily distinguishable from that of within a 10 x 10 x 20 cm cavity, associatedwith snow- babingtonite. like orthoclasecrystals, smoky qvafiz, light sky-blue al- Scandiobabingtoniteis brittle and shows {001} and bite, stilbite, fluorite, and light green-yellow rosettes of {110} perfect cleavage;its hardnessis 6. The measured density, obtained by the heavy liquid method on a zoned * E-mail: [email protected] crystal, is 3.24(5) g/cm3;the calculatedvalue for the pure 0003-004x/98/1I l2-1330$05 00 | 1330 ORLANDI ET AL.: SCANDIOBABINGTONITE 1331 aa,/ o Frcunt 1. SEM photo of scandiobabingtonitecrystals on l- : micafrom Baveno,Piedmont, Italy. Scalebar 0 1 mrn. F Sc and Fe3* end-membersis 3.22 and 3.29 glcm., respectively. Optically, scandiobabingtoniteis biaxial positive, shows a 2V(meas) of 64(2)' and a strong dispersion (r > Frcunn3. Drawingof a singlecrystal of scandiobabingtonite v); pleochroism is strong with colors from pink (f') to green (a'). Refractive indices for Na light are: ct : r.686(2);p : r.694(3);1 : 1.7o9(2). voltage 15 kV, sample current 20 nA, beam size 15 pm. An evaluation of the optical orientation was done on a The elementsmeasured were Si, Sn, Sc, Fe, Ca, Mn, and thin plate cut perpendicularto [001] with a crystal section Na. All other elements,including REE and Y, were below delimitedby (110),(010), (110), (110), (010), and (110) detection limit. Spessartine(for Fe and Mn), kaersutite faces,where an evidentcleavage (110) was also present; (for Ca, Na, Si), metallic Sn (for Sn), and synthetic an extinction angle of approximately 6'was measuredon NaScSirOu(for Sc) were used as standards. (Il0). Z:$ : -250, p = 47"; Y:$ : 146, p = 751'X:6 The crystal examined shows a sudden transition from : 42", p : 47'. Figure 5 shows the relationship between a core with a composition essentiallyequal to scandium- optical and crystallographic axes in scandiobabingtonite free babingtonite to a rim with a high and homogeneous from Baveno. scandium content and a composition typical of scandio- babingtonite. These data show that whenever the Fe con- CnnlrrcAr, nar.l tent is high the Sc content is low and vice-versa,whereas Electron microprobe analyses(Table 1) were collected the Mn content remains constant acrossthe two zones. using a wavelength dispersive ARL-SEMQ instrument The HrO content could not be directly determinedbe- under the following operating conditions: accelerating cause of the scarcity of material, but it was inferred to Frcunn 2. Epitaxial growth of tabular scandiobabingtonite FrcunB 4. Thin section approximatelyon (001) of a submil- crystals on a shorl prismatic crystal of babingtonite SEM photo limeter sized crystal showing a gray core of babingtonite and a Scalebar : 0.1 mm. light rim of scandiobabingtonite. | -)-)L ORLANDI ET AL : SCANDIOBABINGTONITE -t'lo Taele 1. Chemicaldata for scandiobabingtonite(average of 7'io,-' three analyses) ,.7 Weightpercent Range sio, 55 26 54.61-55.62 SnO, 12'l 1 09-1.34 xX Sc,O" 1132 1101-11 51 FeO. 893 7.74-9 91 , ,/, CaO 1733 17 14-17 69 t MnO 411 3 43-5 18 , I l0lO Naro 140 1 37-1 43 -l 2\ X H,Ot too 010 I xI I : 101 24 t l, " Total Fe: This was recalculatedas FeO 8.48 and Fe,O30.50. l, t :,:i I t Constrainedto give t hydroxylpfu i'-.- -_--1- I y 1 is. \i Sc-rich crystals were unsuitable for a structural study. \l Therefore, a zoned crystal was used. ilj, ro Operating conditions were: 48 kV 28 mA, graphite- rioX monochromatedMoKa radiation (I : 0.71069A;. Ze-* Frcunn 5. Stereographicprojection of the optical(X, Y, D with respect to the crystallographic(x, y, z) axes tn Taele 2. X-raypowder diffraction pattern for scandiobabingtonite. scandiobabingtonite -t1 possessone (OH) group per 14 O, as in the chemical m 37 1134 010 formula of babingtonite. Fe was recalculatedas FeO and MW 696 698 110 666 672 001 FerO. basedon stoichiometry. On these grounds,the em- 509 515 120 pirical formula of scandiobabingtonite,recalculated on 514 101 the basis of a total of 15 O, is (Ca,,,Naorr)r,nu(Fefr{, 474 4 to 101 474 111 Mnor)ro r, (Scon,Sno ooFefij.)>o nrsi, .nO,o -(OH), oo.The sim- 444 445 111 plified formula is: Ca.(Fe,*,Mn)ScSi,O,"(OH),and thus MW 411 411 121 scandiobabingtonite is the scandium analogue 374 375 210 of m 348 349 220 babingtonite. 329 3.30 131 mS 3.14 J. tc 102 X-nay cRysrALLocRApHy m 3.03 3.04 221 S 2.969 2.974 112 Preliminary X-ray single-crystalWeissenberg measure- J 2 895 2.903 140 ments indicate that scandiobabingtonite,similarly to ba- mS 2.755 2.760 220 2 680 2 684 141 bingtonite, is triclinic with space group Pl or pl. The 2.679 122 unit-cell parameters, refined at the four-circle z 056 2 648 141 diffractometer,are'. a:7.536(2) A, U : 11.734(2),c : 2 579 I CAI 212 : 2 483 2 484 122 6.148(2),a 9r.70(2)", I : 93.86(2),1 : 104.53(2). 2 458 2 467 320 The X-ray powder pattern (Table 2), collected with Gan- 2 286 2296 311 2 244 2 247 141 dolfi camera and FeKct radiation on a small frasment se- 2 216 2.219 013 lected from the rim of a zoned crystal, is almosiidentical 2 184 2 191 250 to that of babingtonite. 2 190 103 z tJo 2.158 231 Chemical dishomogeneityand rhe evident distortion of 2.120 2.120 023 some crystals, displayed also by a light curvature of the mw 2.O84 2.089 311 faces of the crystals, caused 2.083 251 all diffracted reflections of 2.040 2 046 322 the single crystal to be rather broad.
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