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The relationship between magnetic properties and chemical composition of 19 natural carbonate single crystals of the calcite, aragonite, and dolomite groups and of azurite were investigated. Magnetic susceptibility was determined in low fields, and magnetic anisotropy was measured at room temperature and at 77 K using a high-field torque magnetometer. The chemical composition was analyzed using LA-ICP MS. Planar arrangement of the C[O.sub.3]-groups generates an oblate diamagnetic anisotropy with the minimum susceptibility [k.sub.3] along the crystallographic c-axis in all investigated anhydrous carbonate minerals. A prolate paramagnetic anisotropy with the maximum susceptibility [k.sub.1] along the c-axis is produced by [Fe.sup.2+] in the trigonal carbonate lattice, which can lead to a transition from oblate to prolate shape of the total anisotropy. The transition occurs in the calcite structure above an Fe concentration of 400 ppm at room temperature and 150 ppm at 77 K. The susceptibility difference [k.sub.1]-[k.sub.3] for pure calcite is 4.06 x [10.sup.-10] [m.sup.3]/kg and increases to 9.4 x [10.sup.-9] [m.sup.3]/kg for 10800 ppm Fe. In the hexagonal aragonite structure, no paramagnetic anisotropy due to [Fe.sup.2+] was detected. Azurite shows a strong anisotropy with [k.sub.1]-[k.sub.3] = 1.5 x [10.sup.-8] [m.sup.3]/kg, which is assigned to [Cu.sup.2+]. The paramagnetic anisotropy due to [Fe.sup.2+] increases at 77 K in minerals of the calcite and dolomite group and in azurite, but not in minerals of the aragonite group. Upon cooling from room temperature to 77 K, [k.sub.1]-[k.sub.3] increases 13.3 times for 500 ppm to 100 000 ppm [Fe.sup.2+] in the trigonal lattice and 7.2 times for siderite. Keywords: Carbonate minerals, magnetic properties, anisotropy, calcite, siderite, rhodochrosite, LA ICP-MS

The crystal structure of the rare hexagonal mineral painite [a = 8.724(1), c = 8.464(2) [Angstrom]] from Mogok (Myanmar), with the ideal composition CaZrB[[Al.sub.9][O.sub.18]], was re-determined by single-crystal X-ray diffraction. Structure refinements were performed in space groups P[6.sub.3]/m and P[6.sub.3]. The centrosymmetric P[6.sub.3]/m model yielded excellent agreement ([R.sub.1] = 1.44%, 1189 reflections > 2[alpha] [I.sub.obs], 54 parameters) with the observed diffraction data without any unusual atomic displacement parameters, thus the acentric P[6.sub.3] model was rejected. A previous structural study claimed that painite was noncentrosymmetric and differed from the related structures of jeremejevite [B.sub.5][[cube root of (term)].sub.3][Al.sub.6][(OH).sub.3][O.sub.15]] and fluoborite [B.sub.3][[Mg.sub.9][(F,OH).sub.9][O.sub.9]] in having lower symmetry. The structure of painite comprises a framework of Al[O.sub.6] octahedra that features two types of channels parallel to the c axis. One channel has a trigonal cross-section and is occupied by threefold coordinated B and Zr in sixfold prismatic coordination. The other channel has a hexagonal cross-section and is occupied by Ca. Chemical analysis by laser-ablation inductively-coupled plasma-mass spectrometry indicated that the crystal studied has significant substitution of Na for Ca (ca. 20%) charge-balanced by [Ti.sup.4+] replacing octahedral Al leading to the formula [Ca.sub.0.77] [Na.sub.0.19][Al.sub.8.80][Ti.sub.0.19][Cr.sub.0.03][V.sub.0.01] [Zr.sub.0.94][Hf.sub.0.01][B.sub.1.06][O.sub.18].

Experimental investigations in the system albite-water have been carried out to evaluate the accuracy of a new procedure to determine major-element solubilities in fluid phases and water solubilities in melts. The system albite-water had been examined previously and some consensus has been achieved with respect to phase relations, solidi, and water solubilities in the melts. Above 15 kbar the system approaches a second critical end point, where a distinction between fluid and melt no longer can be made and the term solidus has to be reconsidered. All experimental runs were carried out at near-solidus conditions at 5-17 kbar and 625-775 [degrees] C. Experimental charges contained albite, water, and a layer of diamond crystals (grain size 50 [micro]m). The pore space between the diamonds is preserved during the run indicating that the fluid was able to circulate throughout the entire capsule. During quenching, the material dissolved in the fluid precipitated between the diamond crystals and, as a result, could so be separated from the solid residue. The recovered capsules were directly embedded in epoxy and the diamond layers were analysed by laser ablation microprobe (LAM-ICP-MS). The new method allows the determination of a broad range of silicate/water ratios in aqueous fluids and hydrous melts. In some experimental runs at 15 and 17 kbar, fluids with approximately 40 wt% solute could be trapped leading to the conclusion that supercritical conditions were reached. Results furthermore indicate that albite dissolves non-stoichiometrically at 5 kbar producing a higher Na/Al and Si/Al in the solute than in the melt. Dissolution and melting at 10 kbar and higher pressures appears to be congruent within the limits of this method.