IMA 2014-083 = agmantinite

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Marco E. Ciriotti
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IMA 2014-083 = agmantinite

Messaggio da Marco E. Ciriotti » mer 04 lug, 2018 10:46

Prossima pubblicazione.

▪ Keutsch, F.N., Topa, D., Takagi Fredrickson, R., Makovicky, E., Paar, W.H. (2018): Agmantinite, Ag2MnSnS4, a new mineral with a wurtzite derivative structure from the Uchucchacua polymetallic deposit, Lima Department, Peru. Mineralogical Magazine, 82, (in press).

Agmantinite, ideally Ag2MnSnS4, is a new mineral from the Uchucchacua polymetallic deposit, Oyon district, Catajambo, Lima Department, Peru. It occurs as orange-red crystals to 100 μm across. Agmantinite is translucent with adamantine luster and possesses a red streak. It is brittle and neither fracture nor cleavage was observed. Based on the empirical formula the calculated density is 4.574 g/cm3. On the basis of chemically similar compounds the Mohs hardness is estimated between 2 to 2 ½. In plane-polarized light agmantinite is white with red internal reflections. It is weakly bireflectant with no observable pleochroism with red internal reflections. Between crossed polars, agmantinite is weakly anisotropic with reddish brown to greenish grey rotation tints. The reflectances (Rmin and Rmax) for the four standard wavelengths are 19.7, 22.0 (470 nm), 20.5, 23.2 (546 nm), 21.7, 2.49 (589 nm), and 20.6, 23.6 (650 nm), respectively. Agmantinite is orthorhombic, space group P21nm, with unit-cell parameters: a 6.632(2), b 6.922(2), c 8.156(2) Å, V 374.41(17) Å3, a : b : c 0.958:1:1.178, Z 2. The crystal structure was refined to R = 0.0575 for 519 reflections with I > 2σ(I). Agmantinite is the first known mineral of MI2MIIMIVS4 type that is derived from wurtzite rather than sphalerite by ordered substitution of Zn, analogous to the substitution pattern for deriving stannite from sphalerite. The six strongest X-ray powder-diffraction lines derived from single-crystal XRD data [d in Å (Int.)] are: 3.51 (s), 3.32 (w), 3.11 (vs), 2.42 (w), 2.04 (m), and 1.88 (m). The empirical formula (based on 8 apfu) is (Ag1.94Cu0.03)Σ1.97(Mn0.98Zn0.05)Σ1.03Sn0.97S4.03.The crystal structure-derived formula is Ag2(Mn0.69Zn0.31)Σ1.00SnS4 and the simplified formula is Ag2MnSnS4. The name is for the composition and the new mineral and mineral name have been approved by the Commission on New Minerals, Nomenclature and Classification, IMA (2014-083).
Marco E. Ciriotti

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