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Yazganite, NaFe³⁺ ₂(Mg,Mn)(AsO₄)₃.H₂O, a new mineral

Halil SARP¹ and Radovan ERN²

¹ Département de Minéralogie du Museum Histoire naturelle de Genève, 1 route de Malagnou, CH-1208 Genève, Suisse Corresponding author
² Laboratoire de Cristallographie, Université de Genève, 24 quai Ernest-Ansermet, CH-1211 Genève 4, Suisse

e-mail: halil.sarp{at}mhn.ville-ge.ch

Yazganite, NaFe³⁺ ₂(Mg,Mn)(AsO₄)₃· H₂O, occurs in the andesitic rocks of the volcanic complex of Erciyes (prefecture of Kayseri, Turkey), associated with hematite, tridymite, cassiterite, magnetite, orpiment and realgar. It forms isolated prismatic crystals or aggregates formed by intergrown perfect crystals (10 x 3 x 2 mm maximum size). The crystals are brown to brown black color, showing a metallic luster. They are elongated along c and slightly flattened on {010}. Twinning was not observed. The mineral is monoclinic, C 2/c, a = 12.181(1), b = 12.807(1), c = 6.6391(5) Å, β = 112.441(9), V = 957.2(2) ų, Z = 4, D_meas = 4.18(2) and D_calc = 4.19 g/cm³ based on the crystal structure refinement. The strongest lines in the X-ray powder diffraction pattern (d_obs in Å, (hkl), I_obs) are: 6.40, (020), 20; 5.630, (200), 20; 3.575, (31), 30; 3.202, (040)(12), 40; 2.917, (12)(22), 35; 2.780, (240), 100; 2.611, (32), 40. The Mohs' hardness is about 5. The crystals are translucent and have adamantine luster, streak is bright brown to yellow gray; crystals are biaxial (-) with = 1.870(2), β = 1.897(2), = 1.900(2) at 589 nm; 2V_obs = 35(2), 2V_calc = 36.5(3). Pleochroism is moderate with = orange brown, yellow; , β = grayish yellow, colorless. The crystal structure was solved by direct methods (MoK radiation) and refined using 1528 observed unique reflections to Rw(F²) = 0.073, R(F) = 0.029. The structure is a variant of the johillerite structure, which belongs to the alluaudite group. The mineral is the first example of a hydrated mineral from this group.

Key-words: yazganite, new mineral, X-ray powder pattern, crystal structure, arsenate, alluaudite group, hydrogen bonding, Erciyes (Turkey).

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