Milanriederite, (Ca,REE)₁₉Fe³⁺Al₄(Mg,Al,Fe³⁺)₈Si₁₈O₆₈(OH,O)₁₀, a new vesuvianite-group mineral from the Kombat Mine, Namibia

Milanriederite, a new vesuvianite-group mineral, was discovered in the Kombat deposit, Grootfontein district, Otjozondjupa region, Namibia, in association with baryte, calcite, jacobsite, hausmannite, glaucochroite and native copper. The new mineral forms dark brownish-red dipyramidal crystals up to 3 mm across. The major crystal form is {111} and the minor forms are {001}, {110}, and {100}. Milanriederite is brittle, has a Mohs’ hardness of six and an uneven fracture. Neither cleavage nor parting is observed. D_meas = 3.53(2) g/cm³, D_calc = 3.547 g/cm³. The new mineral is optically uniaxial (–), ω = 1.744(3), ε = 1.737(3). The infrared (IR) spectrum of milanriederite significantly differs from IR spectra of other vesuvianite-group minerals. According to Mössbauer data, all iron in the mineral is trivalent. The chemical composition is (electron microprobe, H₂O determined by gas chromatography of ignition products, wt%): Na₂O 0.47, MgO 5.49, CaO 29.86, Mn₂O₃ 1.40, Al₂O₃ 9.75, Fe₂O₃ 6.03, Y₂O₃ 2.44, La₂O₃ 4.43, Ce₂O₃ 0.86, Pr₂O₃ 0.32, Nd₂O₃ 1.31, SiO₂ 34.84, H₂O 2.9, total 100.10. The empirical formula based on 78 O atoms is [Ca_16.48(La_0.84Y_0.67Ce_0.16Nd_0.24Pr_0.06)Na_0.47]_Σ18.92(Al_5.92Mg_4.22$Fe2.343+Mn0.553+$⁠)_Σ13.03Si_17.95O_68.04(OH)_9.96. The simplified formula is (Ca,REE)₁₉Fe³⁺Al₄(Mg,Al,Fe³⁺)₈Si₁₈O₆₈(OH,O)₁₀. The crystal structure was refined to R = 0.027 for 1684 unique observed reflections with |F_o| ≥ 4σ_F. The mineral is tetragonal, P4/nnc, a = 15.6578(4), c = 11.8597(5) Å, V = 2907.6(2) Å³, Z = 2. Milanriederite is the analogue of vesuvianite with Mg predominant at the Y3 site. The strongest lines of the powder X-ray diffraction pattern are [d, Å (I, %) (hkl)]: 2.970 (50) (511, 004), 2.774 (100) (204, 432, 440), 2.617 (87) (423, 531, 224, 522), 2.481 (30) (620), 2.143 (19) (315, 641), 1.676 (17) (921, 842, 436), 1.638 (38) (931, 526, 804, 922). The mineral is named in honour of Czech mineralogist Professor Milan Rieder (b. 1940), in recognition to his contributions to mineralogy and his service to the international mineralogical community.