ALBERT

Description: Hydroxycalciomicrolite, Ca 1.5 Ta 2 O 6 (OH) is a new microlite-group mineral found in the Volta Grande pegmatite, Nazareno, Minas Gerais, Brazil. It occurs as isolated octahedral and as a combination of octahedral and rhombic dodecahedral crystals, up to 1.5 mm in size. The crystals are yellow and translucent, with a white streak and vitreous to resinous lustre. The mineral is brittle, with a Mohs hardness of 5–6. Cleavage is not observed and fracture is conchoidal. The calculated density is 6.176 g cm –3 . Hydroxycalciomicrolite is isotropic, n calc. = 2.010. The infrared and Raman spectra exhibit bands due to O–H stretching vibrations. The chemical composition determined from electron microprobe analysis ( n = 13) is (wt.%): Na 2 O 0.36(8), CaO 15.64(13), SnO 2 0.26(3), Nb 2 O 5 2.82(30), Ta 2 O 5 78.39(22), MnO 0.12(2), F 0.72(12) and H 2 O 1.30 (from the crystal structure data), O = F –0.30, total 99.31(32), yielding an empirical formula, (Ca 1.48 Na 0.06 Mn 0.01 ) 1.55 (Ta 1.88 Nb 0.11 Sn 0.01 ) 2.00 O 6.00 [(OH) 0.76 F 0.20 O 0.04 ]. Hydroxycalciomicrolite is cubic, with unit-cell parameters a = 10.4205(1) Å, V = 1131.53(2) Å 3 and Z = 8. It represents a pyrochlore supergroup, microlite-group mineral exhibiting P 4 3 32 symmetry, instead of Fd m . The reduction in symmetry is due to long-range ordering of Ca and vacancies on the A sites. This is the first example of such ordering in a natural pyrochlore, although it is known from synthetic compounds. This result is promising because it suggests that other species with P 4 3 32 or lower-symmetry space group can be discovered and characterized.

Description: Fluorcalciomicrolite, (Ca,Na,) 2 Ta 2 O 6 F, is a new microlite-group, pyrochlore supergroup mineral approved by the CNMNC (IMA 2012-036). It occurs as an accessory mineral in the Volta Grande pegmatite, Nazareno, Minas Gerais, Brazil. Associated minerals include: microcline, albite, quartz, muscovite, spodumene, "lepidolite", cassiterite, tantalite-(Mn), monazite-(Ce), fluorite, "apatite", beryl, "garnet", epidote, magnetite, gahnite, zircon, "tourmaline", bityite, hydrokenomicrolite, and other microlite-group minerals under study. Fluorcalciomicrolite occurs as euhedral, untwinned, octahedral crystals 0.1–1.5 mm in size, occasionally modified by rhombododecahedral faces. The crystals are colourless and translucent; the streak is white, and the lustre is adamantine to resinous. It does not fluoresce under ultraviolet light. Mohs' hardness is 41/2–5, tenacity is brittle. Cleavage is not observed; fracture is conchoidal. The calculated density is 6.160 g/cm 3 . The mineral is isotropic, n calc. = 1.992. The Raman spectrum is dominated by bands of B–X octahedral bond stretching and X–B–X bending modes. The chemical composition (n = 6) is (by wavelength dispersive spectroscopy, H 2 O calculated to obtain charge balance, wt.%): Na 2 O 4.68, CaO 11.24, MnO 0.01, SrO 0.04, BaO 0.02, SnO 2 0.63, UO 2 0.02, Nb 2 O 5 3.47, Ta 2 O 5 76.02, F 2.80, H 2 O 0.48, O=F –1.18, total 98.23. The empirical formula, based on 2 cations at the B site, is (Ca 1.07 Na 0.81 0.12 ) 2.00 (Ta 1.84 Nb 0.14 Sn 0.02 ) 2.00 [O 5.93 (OH) 0.07 ] 6.00 [F 0.79 (OH) 0.21 ]. The strongest eight X-ray powder-diffraction lines [ d in Å ( I )( hkl )] are: 5.997(59)(111), 3.138(83)(311), 3.005(100)(222), 2.602(29)(400), 2.004(23)(511), 1.841(23)(440), 1.589(25)(533), and 1.504(24)(444). The crystal structure refinement ( R 1 = 0.0132) gave the following data: cubic, Fd 3I m , a = 10.4191(6) Å, V = 1131.07(11) Å 3 , Z = 8.

Description: Hydroxycalciomicrolite, Ca1.5Ta2O6(OH) is a new microlite-group mineral found in the Volta Grande pegmatite, Nazareno, Minas Gerais, Brazil. It occurs as isolated octahedral and as a combination of octahedral and rhombic dodecahedral crystals, up to1.5 mm in size. The crystals are yellow and translucent, with a white streak and vitreous to resinous lustre. The mineral is brittle, with a Mohs hardness of 5–6. Cleavage is not observed and fracture is conchoidal. The calculated density is 6.176 g cm–3. Hydroxycalciomicroliteis isotropic,ncalc.= 2.010. The infrared and Raman spectra exhibit bands due to O–H stretching vibrations. The chemical composition determined from electron microprobe analysis (n= 13) is (wt.%): Na2O 0.36(8), CaO 15.64(13), SnO20.26(3),Nb2O52.82(30), Ta2O578.39(22), MnO 0.12(2), F 0.72(12) and H2O 1.30 (from the crystal structure data), O = F –0.30, total 99.31(32), yielding an empirical formula, (Ca1.48Na0.06Mn0.01)∑1.55(Ta1.88Nb0.11Sn0.01)∑2.00O6.00[(OH)0.76F0.20O0.04].Hydroxycalciomicrolite is cubic, with unit-cell parametersa= 10.4205(1) Å,V= 1131.53(2) Å3andZ= 8. It represents a pyrochlore supergroup, microlite-group mineral exhibitingP4332 symmetry, instead ofFd3m. Thereduction in symmetry is due to long-range ordering of Ca and vacancies on theAsites. This is the first example of such ordering in a natural pyrochlore, although it is known from synthetic compounds. This result is promising because it suggests that other species withP4332or lower-symmetry space group can be discovered and characterized.

Description: Fluorcalciomicrolite, (Ca, Na, ☐)2Ta2O6F, is a new microlite-group, pyrochlore supergroup mineral approved by the CNMNC (IMA 2012-036). It occurs as an accessory mineral in the Volta Grande pegmatite, Nazareno, Minas Gerais, Brazil. Associated minerals include: microcline, albite, quartz, muscovite, spodumene, “lepidolite”, cassiterite, tantalite-(Mn), monazite-(Ce), fluorite, “apatite”, beryl, “garnet” , epidote, magnetite, gahnite, zircon, “tourmaline” , bityite, hydrokenomicrolite, and other microlite-group minerals under study. Fluorcalciomicrolite occurs as euhedral, untwinned, octahedral crystals 0.1–1.5 mm in size, occasionally modified by rhombododecahedral faces. The crystals are colourless and translucent; the streak is white, and the lustre is adamantine to resinous. It does not fluoresce under ultraviolet light. Mohs' hardness is 4½–5, tenacity is brittle. Cleavage is not observed; fracture is conchoidal. The calculated density is 6.160 g/cm3. The mineral is isotropic, ncalc. = 1.992. The Raman spectrum is dominated by bands of B–X octahedral bond stretching and X–B–X bending modes. The chemical composition (n = 6) is (by wavelength dispersive spectroscopy, H2O calculated to obtain charge balance, wt.%): Na2O 4.68, CaO 11.24, MnO 0.01, SrO 0.04, BaO 0.02, SnO20.63, UO20.02, Nb2O53.47, Ta2O576.02, F 2.80, H2O 0.48, O=F–1.18, total 98.23. The empirical formula, based on 2 cations at the B site, is (Ca1.07Na0.81☐0.12)Σ2.00(Ta1.84Nb0.14Sn0.02)Σ2.00[O5.93(OH)0.07]6.00[F0.79(OH)0.21]. The strongest eight X-ray powder-diffraction lines [d in Å (I)(hkl)] are: 5.997(59)(111), 3.138(83)(311), 3.005(100)(222), 2.602(29)(400), 2.004(23)(511), 1.841(23)(440), 1.589(25)(533), and 1.504(24)(444). The crystal structure refinement (R1 = 0.0132) gave the following data: cubic, Fdm, a = 10.4191(6) Å, V = 1131.07(11) Å3, Z = 8.