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The Raman band related to the stretching mode of hydroxyl (OH Ϫ ) centered at about 3650 cm Ϫ1 is investigated in several natural specimens of topaz single crystals with a composition Al 2 SiO 4 (OH x F (1Ϫx) ) 2 within a wide range of OH mole fractions (x). The analysis of the OH band shape shows that its asymmetry is due to an unresolved splitting in two peaks, centered at 3639 and 3647 cm Ϫ1 , labeled as OH A and OH B , respectively. Although the asymmetry is drastically different for OH-rich and OH-poor topaz, we show that this is only caused by the change of the OH A /OH B intensity ratio with the total OH concentration. In order to explain the splitting of the OH band, we suggest a model which involves a reduction of the local symmetry from D 2h 16 to C 2v 9 , resulting in two types of physically nonequivalent F sites (A and B) where the OH/F substitution can occur. Both sites have C 1 site symmetries and a multiplicity of 4.

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) 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 P4 3 32 symmetry, instead of Fd 3m. 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 P4 3 32 or lower-symmetry space group can be discovered and characterized.

The structure of equiatomic iron-cobalt alloys containing 1, 2 and 3 wt% niobium has been studied using a variety of techniques, including X-ray diffraction, scanning and transmission electron microscopy and differential thermal analysis. It has been found that niobium is effective in reducing the kinetics of ordering to the B2 structure and of anti-phase domain growth, even though the solubility is only 0.3-0.5 wt %. The magnetic properties (coercive force and saturation magnetization) and the electrical resistivity of the three alloys have also been determined.