Willem B. Stern

Regional distribution of white K-mica polymorphs and their phengite content in the Central Alps

Some 150 white K-micas from the Central Alps were analysed for their polymorph and phengite content.Pre-Alpine white K-micas and those belonging to the Meso-Alpine Lepontine Metamorphic “High” show exclusively the 2M1 polymorph. The 3T structural form, on the other hand, has been found in one third of the white K-micas formed during the Alpine regional metamorphism. In most cases this trigonal structure coexists with varying amounts of the 2M1 form. The 3T distribution pattern suggests that this polymorph originated during the Eo-Alpine high-pressure/“low temperature” metamorphism. Provided this interpretation is correct, the sporadic occurrence of this polymorph within the Meso-Alpine staurolite zone may be used as a tracer for the Eo-Alpine metamorphism.The following improved correlation between the (060, 331) reflections of 2M1 white K-micas and the RM-content (= 2Fe2O3+FeO+MgO in molar proportions), based on 24 micas from granitoid rocks, is presented: d(060, 331)= 1.498+0.082 RM.The phengite content of Alpine white K-micas belonging to the assemblage muscovite-biotite-K-feldspar-quartz was estimated from RM values or derived from chemical analyses and was found to be clearly related to metamorphic grade. Phengite-rich micas were formed during the Eo-Alpine high-P/“ low-T” metamorphism while aluminous muscovite was found within the Meso-Alpine thermal high of the Lepontine gneiss area. White K-micas from areas which underwent both the Eo-Alpine and the Meso-Alpine metamorphism display variable phengite contents. Although these micas show Tertiary Rb-Sr and K-Ar ages, the variable phengite content presumably reflects conditions during the Eo-Alpine high-P/“low-T” metamorphism. This interpretation implies that the cations occupying the interlayer positions are more easily equilibrated than those in octahedral and tetrahedral structural sites.A compilation of 3T white K-mica occurrences described in the literature is given in the appendix.

Diagenesis and metamorphism of clay minerals in the Helvetic Alps of eastern Switzerland

Helvetic sediments from the northern margin of the Alps in eastern Switzerland were studied by clay mineralogical methods. Based on illite “crystallinity” (Kübier index), the study area is divided into diagenetic zone, anchizone and epizone. Data on the regional distribution of the following index minerals are presented: smectite, kaolinite/smectite mixed-layer phase, kaolinite, pyrophyllite, paragonite, chloritoid, glauconite and stilpnomelane. Isograds for kaolinite/pyrophyllite and glauconite/stilpnomelane are consistent with illite “crystallinity” zones. Using the ordering of mixed-layer illite/smectite, the diagenetic zone is subdivided into three zones. The illite domain size distribution was analyzed using the Warren-Averbach technique. The average illite domain size does not change much within the diagenetic zone, but shows a large increase within the anchizone and epizone. The average illite b0 value indicates conditions of an intermediate-pressure facies series.The Helvetic nappes show a general increase in diagenetic/metamorphic grade from north to south, and within the Helvetic nappe pile, grade increases from tectonically higher to lower units. However, a discontinuous inverse diagenetic/metamorphic zonation was observed along the Glarus thrust, indicating 5–10 km of offset after metamorphism. In the study area, incipient metamorphism was a late syn- to post-nappe-forming event.

Size driven phase transition and thermodynamic properties of nanocrystalline BaTiO3

Abstract Submicron powders of BaTiO3 with narrow size distribution were prepared by a modified sol-gel method. X-ray diffraction, Raman spectroscopy, calorimetry and Kurtz powder tests on nonlinear optical properties were applied to study size effects on ferroelectricity. The different methods show consistently that the ferroelectric properties disappear below a critical size of about 50 nm. The size driven phase transformation was found to be an order- to disorder transition similar to the well-known temperature driven transformation. From shifts of phase transformation temperatures with varying particle sizes, differences of surface free energies of different phases of BaTiO3, were calculated.

Geochemistry of muscovite from pegmatites of the Eastern Brazilian pegmatite province: a clue to petrogenesis and mineralization potential

Single crystals of muscovite ( N = 143) from different zones of selected granitic pegmatites belonging to two well-known gem-producing districts of the Eastern Brazilian Pegmatite Province (northern Aracuai and western Governador Valadares in Minas Gerais) were analyzed for major, minor and trace elements. Structural formulae display considerable (Fe+Mg) variation within the octahedral site, coupled with tetrahedral Si-Al substitution (Tschermak substitution – phengite component). Tetrahedral Si varies from 3.06 to 3.21 and Al VI from 1.71 to 1.90 apfu. Trace elements in muscovite are significant indicators for the economic potential of pegmatites as well as for the differentiation degree and origin of the magma. Muscovite with higher Li and B contents is characteristic for gem-tourmaline-bearing pegmatites. Lower concentrations in K/Rb, Ti and Mg and higher F contents are found in muscovites from higher differentiated zones within the pegmatite. In comparison with anatectic pegmatites, relatively low K/Rb ratios of muscovite from the studied pegmatites are indicative of derivation by fractional crystallization of granite magma. Zn, Ga and Y, elements rarely analyzed in micas, tend to increase with decreasing K/Rb ratios, showing them to be good indicators of the fractionation degree of magmas.

Chemical zoning of muscovite megacrystal from the Brazilian Pegmatite Province

Macroscopically homogenous muscovite plate from the Cruzeiro pegmatite, located in the Eastern Pegmatite Province in Minas Gerais, may show complex distribution patterns of some trace elements. In geochronological and petrological studies, as for example in the distinction of magmatic and post-magmatic mica, the cause of zoning could be taken into consideration. The complex chemical zoning in the studied mica plate can be best explained by growth in an evolving magma followed by alteration due to percolation of hydrothermal fluids. Enrichment of Rb towards the border is interpreted as resulting from the chemical evolution of the residual magma during crystal growth. The depletion in ( IV Al+ VI Al) as well as the increase in (Fe+Mg) and Si along a fracture could be due to the hydrothermal celadonitic substitution of muscovite. This alteration also caused depletion in the contents of Rb, Ga, Y, Nb, Sn, and Zn and residual concentration of Ti. Elements such as Ga, Y, Nb, Sn, and Zn, rarely considered in the discussion of differentiation or alteration processes in micas, have been shown to be as significant as the alkali-elements.