Dmitryvarlamovite, Ti (sub 2) (Fe (super 3+) Nb)O (sub 8) , a new columbite-supergroup mineral related to the wolframite group

The new columbite-supergroup mineral dmitryvarlamovite, ideally Ti (sub 2) (Fe (super 3+) Nb)O (sub 8) , was discovered in weathered alkaline metasomatic assemblages formed after Late Riphaean sedimentary carbonate rocks of the Verkhne-Shchugorskoe deposit, Middle Timan Mts., Russia. The associated minerals are columbite-(Fe), pyrochlore-group minerals, monazite-(Ce), xenotime-(Y), baryte, pyrite, drugmanite and plumbogummite. Dmitryvarlamovite occurs as isolated anhedral equant grains up to 0.5 mm across. The colour of dmitryvarlamovite is black, the streak is black and the lustre is submetallic. The new mineral is brittle, with the mean VHN hardness of 753 kg mm (super -2) corresponding to the Mohs' hardness of 6. No cleavage is observed. The fracture is conchoidal. The calculated density is 4.891 g.cm (super -3) . In reflected light, dmitryvarlamovite is light grey; no pleochroism is observed. The reflectance values (R (sub min) , % / R (sub max) , % / lambda , nm) are: 19.8/20.3/470, 18.3/18.9/546, 17.8/18.5/589 and 17.3/17.8/650. The chemical composition is (electron microprobe data, with iron divided into Fe (sub 2) O (sub 3) and FeO based on the charge balance, wt.%): MnO 0.11, FeO 1.51, V (sub 2) O (sub 3) 0.89, Cr (sub 2) O (sub 3) 0.28, Fe (sub 2) O (sub 3) 19.26, TiO (sub 2) 37.72, Nb (sub 2) O (sub 5) 40.08, total 99.85. The IR and Raman spectra indicate the absence of H-, C- and N-bearing groups. The empirical formula is (Fe (super 2+) (sub 0.08) V (super 3+) (sub 0.05) Cr (super 3+) (sub 0.01) Fe (super 3+) (sub 0.92) Ti (sub 1.79) Nb (sub 1.15) ) (sub Sigma 4.00) O (sub 8) . The crystal structure was determined using single-crystal X-ray diffraction data and refined to R = 0.048. Dmitryvarlamovite is orthorhombic, space group P2 (sub 1) 2 (sub 1) 2, a = 4.9825(6), b = 4.6268(4), c = 5.5952(6) Aa and V = 5.5952(6) Aa (super 3) (Z = 1). The structure is related to those of wolframite-group minerals but differs in the scheme of cation ordering. The crystal-chemical formula derived based on the structural data is (Ti (sub 0.57) Nb (sub 0.21) Fe (super 3+) (sub 0.15) Fe (super 2+) (sub 0.04) V (sub 0.02) Cr (sub 0.01) ) (sub 2) (Nb (sub 0.36) Ti (sub 0.33) Fe (super 3+) (sub 0.31 ) ) (sub 2) O (sub 8) . The strongest lines of the powder X-ray diffraction pattern [d, Aa (I, %) (hkl)] are: 3.58 (40) (011), 2.911 (100) (111), 2.809 (40) (002), 2.497 (38) (020), 2.447 (29) (103), 1.7363 (32) (103) and 1.7047 (29) (220). Dmitryvarlamovite is named after Dmitry Anatol'evich Varlamov (b. 1965).