Eyselite, Fe (super 3+) Ge (super 4+) (sub 3) O (sub 7) (OH), a new mineral species from Tsumeb, Namibia

Eyselite, ideally Fe (super 3+) Ge (super 4+) (sub 3) O (sub 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) Aa, V 365,2(3) Aa (super 3) , a:b:c 0.8543:1:0.4658, Z = 2. The strongest eight lines of the X-ray powder-diffraction pattern [d in Aa (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 mu m in maximum length. Typical crystals are 20X14X1 mu m 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 (calc.) is 3.639 g/cm (super 3) 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, Fe (sub 2) O (sub 3) 18.54, Ga (sub 2) O (sub 3) 1.01 GeO (sub 2) 77.75, H (sub 2) O [2.64] (by difference), total [100.00] wt.%. The empirical formula, on the basis of O = 8, is (Fe (super 3+) (sub 0.93) Ga (super 3+) (sub 0.04) ) (sub Sigma 0.97) Ge (super 4+) (sub 2.98) O (sub 6.90) (OH) (sub 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-Universitat, Heidelberg, Germany, for his contributions to the study of germanates and for his numerous contributions to the Powder Diffraction File.