Alpeite (IMA2016-072), Ca4Mn3+2Al2(Mn3+Mg)(SiO4)2(Si3O10)(V5+O4)(OH)6, is a new member of the ardennite group from the Monte Alpe mine, Liguria, Italy. It crystallized from V- and Mn-rich hydrothermal fluids in an oxidizing environment. Alpeite occurs as intergrowths of plates, flattened on {1 0 0} and exhibiting the forms {1 0 0}, {0 0 1}, {0 1 2} and {1 0 2}. The mineral is brownish red and transparent with a vitreous to silky lustre and beige streak. Crystals are brittle with a curved and stepped fracture, and a perfect cleavage on {1 0 0} and good cleavages on {0 1 0 and {0 0 1}. The Mohs' hardness is 5½ to 6. The calculated density is 3.374 g cm−3. The mineral is inert in concentrated HCl at room temperature. Optically, alpeite crystals are biaxial (−), with α = 1.747(3), β = 1.785(3) and γ = 1.808(3) (white light); 2V = 73(2); dispersion r > v, strong; the optical orientation X = a, Y = b, Z = c. Pleochroism is Y > Z > X in shades of reddish brown. Electron microprobe analyses gave the empirical formula (Ca3.84Mn2+0.16)Σ4.00(Mn3+1.33Al0.67)Σ2.00(Al1.29Mn3+0.60V5+0.10)Σ1.99(Mn3+0.70Mg0.65Al0.42Co2+0.20)Σ1.97(SiO4)2(Si3O10)[(V5+0.75Si0.25)O4](OH)6, based on 28O atoms per formula unit. Alpeite is orthorhombic, Pnmm, with the unit-cell parameters: a = 8.9421(11), b = 6.0534(6), c = 18.9781(18) Å, V = 1027.29(19) Å3 and Z = 2. The four strongest lines in the X-ray powder diffraction pattern are [dobs/Å (I) (hkl)]: 3.022 (93) (1 1 5), 2.673 (100) (1 1 6), 2.572 (69) (2 0 6) and 1.5112 (83) (0 4 0,4 2 7). The crystal structure was refined to R1 = 0.044 for 834 observed reflections [Fo > 4σF]. The structure is the same as that of other members of the ardennite group. It contains chains of edge-sharing Mn3+ and Al octahedra extending along the b axis, which are linked into a framework by sharing corners with SiO4 tetrahedra and with Si3O10 tetrahedral groups. Within the cavities in this framework are 7-coordinated Ca sites. In the structure of alpeite, Mn3+ is the dominant cation in the M1 site and is the dominant trivalent cation in the M3 site, while in all other members of the ardennite group [ardennite-(As), ardennite-(V) and kannanite], Al is the dominant cation in the M1 site and is the dominant trivalent cation in the M3 site.
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Research Article|
April 05, 2017
Alpeite, Ca4Mn3+2Al2(Mn3+Mg)(SiO4)2(Si3O10)(V5+O4)(OH)6, a new ardennite-group mineral from Italy
Anthony R. Kampf;
1
Mineral Sciences Department, Natural History Museum of Los Angeles County
, 900 Exposition Boulevard
, Los Angeles
, CA
90007, USA
*
Corresponding author, e-mail: akampf@nhm.org
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Cristina Carbone;
Cristina Carbone
2
Dipartimento di Scienze della Terra, dell'Ambiente e della Vita (DISTAV), Università degli Studi di Genova
, Corso Europa 26
, 16132 Genova
, Italy
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Donato Belmonte;
Donato Belmonte
2
Dipartimento di Scienze della Terra, dell'Ambiente e della Vita (DISTAV), Università degli Studi di Genova
, Corso Europa 26
, 16132 Genova
, Italy
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Barbara P. Nash;
Barbara P. Nash
3
Department of Geology and Geophysics, University of Utah
, Salt Lake City
, UT
84112, USA
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Luigi Chiappino;
Luigi Chiappino
4
via Palmanova 67
, 20132 Milano
, Italy
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Fabrizio Castellaro
Fabrizio Castellaro
5
via XXV Aprile 28
, 16046 Mezzanego
, Italy
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European Journal of Mineralogy (2017) 29 (5): 907–914.
Article history
received:
13 Jan 2017
rev-recd:
08 Feb 2017
accepted:
16 Feb 2017
first online:
15 Dec 2017
Citation
Anthony R. Kampf, Cristina Carbone, Donato Belmonte, Barbara P. Nash, Luigi Chiappino, Fabrizio Castellaro; Alpeite, Ca4Mn3+2Al2(Mn3+Mg)(SiO4)2(Si3O10)(V5+O4)(OH)6, a new ardennite-group mineral from Italy. European Journal of Mineralogy 2017;; 29 (5): 907–914. doi: https://doi.org/10.1127/ejm/2017/0029-2636
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