Eyselite, Fe3+Ge4+ 3O7(OH), a new mineral species from Tsumeb, Namibia

Eyselite, ideally Fe³⁺Ge⁴⁺ ₃O ₇(OH), is orthorhombic with a P lattice, and unit-cell parameters refined from powder data: a 8.302(4), b 9.718(4), c 4.527(2) Å, V 365.2(3) ų, a:b:c 0.8543:1:0.4658, Z = 2. The strongest eight lines of the X-ray powder-diffraction pattern [d in Å (I)(hkl)] are: 4.105(40)(011), 3.681(100)(111), 3.121(60b (220,121), 2.921(100)(211), 2.512(40)(131), 2.403(90)(320), 1.646(80)(322 and 1.624(50)(142). The mineral occurs on a single specimen originating from the Tsumeb mine, Tsumeb, Namibia, as very fine-grained aggregates in vugs of massive renierite - germanite - tennantite ore. There are no other associated secondary phases. Individual subhedral to rarely euhedral crystals are platy to very thin prismatic, elongate [001], with a length-to-width ratio of approximately 3:1 and not exceeding 20 μm in maximum length. Typical crystals are 20 × 14 × 1 μm in size. Forms are 100 major, very thin 010, rounded 011, and very thin rounded 001 minor. Crystals show growth steps on 100 and are partly hollow. The mineral is dirty brown-yellow (aggregates) to yellow-tan (crystals), opaque (aggregates) to transparent (crystals), with a brownish yellow streak, and vitreous luster. Eyselite is brittle, with an uneven fracture, and is nonfluorescent. The aggregates are soft; D (ca1c.) is 3.639 g/cm³ for the empirical formula. It is biaxial positive, and two indices of refraction exceed 1.80; 2V (meas.) is large; pale yellow in plane-polarized light, with no pleochroism, low birefringence, moderate anisotropy, and length-fast. Averaged results of electron-microprobe analyses yield CaO 0.06, Fe₂O₃ 18.54, Ga₂O₃ 1.01, GeO₂ 77.75, H ₂O [2.64] (by difference), total [100.00] wt.%. The empirical formula, on the basis of O = 8, is (Fe³⁺ _0.93Ga³⁺ ₀.₀₄) _∑0.97Ge⁴⁺ _2.98O_6.90 (OH)_{1. 17}. The infrared-absorption spectrum shows bands for structural (OH). A micro-XAS study shows that all the Fe is trivalent, and the Ge atoms are probably in octahedral coordination. The mineral's name honors the late Walter H. Eysel, Professor of Crystallography, Ruprecht-Karls-Universität, Heidelberg, Germany, for his contributions to the study of germanates and for his numerous contributions to the Powder Diffraction File.