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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.

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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.

The structure and magnetic properties of equiatomic FeeCo alloys with ternary additions of u p t o 1.92 atyo n'b and 6 atyo V are studied by means of Mossbauer spectroscopy, complemented \+ith light and electron microscopy and the determination of magnetisation curves, as function of heat treatment. The main features of the Mossbauer spertra, namely a Zeeman hyperfine pattern and a paramagnetic peak are discussed in terms of the number and the position of the ternary solute atoms in the ferromagnetic matrix, and the volume fraction of second phase, respectively. It is shown that the solubility of niobium in the matrix is considerably less than that of vanadium, and that vanadium occupies the iron sublattice of the B2 ordered matrix. The saturation magnetisation of the alloys is correlated with the volume fraction of the second phase and the average hyperfine field.Struktur und magnetische Eigenschaften aquiatomarer Fe-Co-Legierungen mit ternaren Zusatzen von bis zu 1,92 Ato/o Nb und 6 Atyo V werden mittels MoBbauerspektroskopie, optischer und Elektronenmikroskopie und der Aufnahme von Magnetisierungskurven als Funktion der J\'armebehandlung untersucht. Die wesentlichen Merkmale der MoBbauerspektren (Zeeman-Hyperfeinaufspaltung und eine paramagnetische Linie) werden mittels der Anzahl und Anordnung der in der ferromagnetischen Matrix gelosten ternaren Atoine und im Hinblick auf den Volumenanteil der Sekundarphase diskutiert. Es wird gezeigt, daB die Loslichkeit von Niob in der Matrix wesentlich geringer als die von Vanadium ist und da5 Vanadium das Eisenuntergitter der B2-geordneten Matrix besetzt. Die Sattigungsmagnetisierung der Legiernngen wird mit dem Volumenanteil der Sekundarphase und dem mittleren Hyperfeinfeld korreliert.