Windhoekite, Ca _2Fe _(3-x)~(3+)(Si _8O _(20))(OH) _4 10H _2O, a new palygorskite-group mineral from the Aris phonolite, Namibia

摘要

The new mineral windhoekite was discovered in a specimen from the Ariskop Quarry, near Windhoek, Namibia. Associated minerals are fluorapophyllite, aegirine, microcline, arisite-(Ce), arisite-(La). Windhoekite is yellow-brown, translucent; streak is beige. It forms isolated long-prismatic crystals up to 0.15 × 4 mm and their radial aggregates in cavities in phonolite. D _(calc) is 2.630 g/cm ~3, D _(meas) is 2.62 (2) g/cm _3. The new mineral is biaxial (-), α = 1.610(3), β = 1.662(3), γ = 1.671(3), 2V (meas.) = 50(10)°, 2V (calc.) = 44°. Dispersion is not observed, pleochroism is strong (Y ≈ Z > X, brown to dark brown), orientation: X ≈ α; Z = c. The IR spectrum is given. The chemical composition is (electron microprobe, mean of 5 analyses, wt%): CaO 9.24, MnO 0.85, Fe _2O _3 23.14, Al _2O _3 0.41, SiO _2 46.32, H _2O (by gas chromatography) 21.0; total 100.96. The empirical formula based on 34 O atoms is: (Ca _(1.68)Mn _(0.12))Fe_(2.96) ~(3+) (Si _(7.87)Al _(0.08)O _(20))(OH) _4 10H _(1.98)O. The simplified formula is: Ca _2Fe _(3-x)~(3+) [(Si,Al) _8O _(20)](OH) _4 10H _2O. The crystal structure was solved using single-crystal diffraction data. Windhoekite is monoclinic, space group C2/m, α = 14.319(5), β = 17.825(4), c = 5.242(1) ?, β = 103.5(2), V = 1301.0(6) ? _3, Z = 2. Ca and Fe ~(3+) occupy two large octahedra and two smaller octahedra, respectively. The strongest lines of the powder diffraction pattern [d, ? (I, %) (hkl)] are: 11.04 (100) (110), 4.432 (10) (021), 4.133 (6) (22-1), 3.754 (4) (240), 3.486 (11) (400), 2.636 (8) (35-1), 2.551 (4) (002), 2.505 (6) (26-1). Type material is deposited in the collections of the Fersman Mineralogical Museum of the Russian Academy of Sciences, Moscow, Russia, under registration number 4018/1.