Interpretation of the Metamorphic Processes in Various Rock Types Using the Chemistry of Garnets (Selecka Mountain, Macedonia)

The garnets from the complex of Selecka Mountain could be divided in two groups on the basis of their chemical composition and zoning. The garnets from muscovite-schist and amphibole-schist show characteristics typical for a growth in prograde metamorphic conditions up to the medium metamorphic grade. Characteristics of the other garnets (from aplite granite, aplite veins and metabasite) suggest their formation in anatectic or highly metasomatic conditions. The inversion in chemical zoning, observed in some of them, is discussed and related to possible processes of growth

The crystal structure of balićžunićite, Bi2O(SO4)2, a new natural bismuth oxide sulphate

The crystal structure of balićžunićite, Bi2O(SO4)2, a new mineral species from "La Fossa" crater of Vulcano (Aeolian Islands, Italy), was solved from single-crystal X-ray-diffraction data and refined to R = 0.0507. The structure is triclinic, space group P-1, with lattice parameters a 6.7386(3), b 11.1844(5), c 14.1754(7) Å, α 80.082(2){degree sign}, β 88.462(2){degree sign}, γ 89.517(2){degree sign}, V 1052.01(8) Å3 and Z = 6. The crystal structure consists of 6 independent Bi sites, 6 S sites and 27 O sites of which 3 are oxo oxygen atoms not bonded to sulphur. Bi and S atoms are arranged close to an eutectic pattern parallel to the (100) crystal planes. The planes are stacked atom on atom such that Bi always overlays S and vice versa. This structural feature is shared with the known structure of the high temperature polymorph of the same compound, stable over 535oC. However, the sequences of Bi and S atoms in the two structures are different and so are the arrangements of oxygen atoms. Characteristic building blocks in the structure of balićžunićite are clusters of five Bi atoms which form nearly planar trapezoidal Bi5 groups with oxo oxygens located in the centres of the three Bi3 triangles which form the trapezoids. The trapezoidal Bi5O39+ ions are joined along [100] with SO42- groups by means of strong bismuth-sulphate oxygen bonds, forming infinite [100] rods with composition Bi5O3(SO4)51-. One sixths of Bi atoms do not participate in trapezoids, but form with additional SO42- groups rows of composition BiSO41+, also parallel to [100]. [Bi5O3(SO4)51-] rods form infinite layers parallel to the crystal plane (010) with [BiSO41+] rows located on the irregular surface of contact between adjacent layers. Bi atoms occur in four different coordination types, all showing the stereochemical influence of the Bi3+ lone electron pair. In this respect the crystal structure of balićžunićite shows greater variability than its high temperature polymorph which has only two types of Bi coordinations present in balićžunićite

Jukonite, a questionable hydrated Ca-Fe arsenate mineral from Grotta della Monaca (Sant'Agata d'Esaro, Cosenza, Italy)

We report the first Italian occurrence of yukonite, a rare hydrated arsenate of calcium and ferric iron, from Grotta della Monaca cave, S. Agata di Esaro, Cosenza, Italy. We have studied samples of cotype yukonite from the Daulton mine, Yukon, Canada, for comparison. At Grotta della Monaca, yukonite occurs in compact masses, dark reddish brown to brownish yellow in color. At the TEM scale, it consists of a chaotic assemblage of small grains (maximum dimension <20 nm) embedded in an amorphous matrix, occasionally admixed with scorodite. SAED patterns of these grains consist of weak and diffuse diffraction rings, indicating low crystallinity, and preventing any definition of symmetry or lattice parameters. Results of a combination of microanalytical, spectroscopic and thermogravimetric techniques provide chemical compositions: Ca1.76Fe2+0.09Fe3+3.12 [(As0.81Si0.10P0.09)O4]3(OH)3.76•4H2O for yukonite from Grotta della Monaca and Ca1.76Fe2+0.10Fe3+3.56[(As0.89Si0.08P0.03) O4]3(OH)5.16•3H2O for yukonite from the Daulton mine, close to the stoichiometry Ca2Fe3(AsO4)3(OH)4•4H2O. The thermogravimetric data indicate a total H2O content of ~17 wt%; FTIR data show that most of this occurs as H2O, but some OH could be present as well. The FTIR data show, in addition, the presence of ammonium and organic matter, suggesting that poorly crystalline organic matter is trapped in the material examined. Both chemical and XRD data point to a close relation with arseniosiderite [Ca2Fe3(AsO4)3O2•3H2O]. Both occur as oxidation products of As-bearing sulfides and seem to be composed of basically the same type of layers; arseniosiderite represents a fully crystalline substance, whereas yukonite is a semicrystalline material with a large variation in composition due to the admixture of amorphous material