Pyrophanite pseudomorphs after perovskite in Perkupa serpentinites (Hungary): a microtextural study and geological implications

• Published in: Physics and chemistry of minerals 2013-09, Vol.40 (8), p.611-623
• Main Authors: Zajzon, N., Váczi, T., Fehér, B., Takács, Á., Szakáll, S., Weiszburg, T. G.
• Format: Article
• Language: English
• Subjects: Anatase; Brookite; Calcite; Calcium titanate; Cores; Crystal structure; Crystallography; Crystallography and Scattering Methods; Crystals; Domains; Earth and Environmental Science; Earth Sciences; Electron backscatter diffraction; Geochemistry; Mapping; Mineral Resources; Mineralogy; Original Paper; Perovskites; Precursors; Raman spectra; Rims; Rocks; Single crystals; Titanium dioxide
• ISSN: 0342-1791; 1432-2021
• DOI: 10.1007/s00269-013-0596-2

Pyrophanite in serpentinite at Perkupa (Hungary) is described in detail for the first time as a replacement product of perovskite. It occurs as a 20- to 30-μm-wide rim, mantling a remnant core composed of perovskite or its alteration products. The pyrophanite rim consists of an inner zone, representing a pseudomorph after perovskite, and an outer overgrowth zone. Raman mapping and electron backscatter diffraction data show that the pyrophanite rims typically represent single crystals rather than being composed of multiple domains in different crystallographic orientations. Perovskite occurs exclusively in the core of pyrophanite and was identified as the orthorhombic CaTiO 3 phase, based on Raman spectra. Heterogeneous, polyphase mineral cores, consisting of calcite, anatase and/or brookite, kassite, and Mn-bearing kassite, in some cases in association with relict perovskite, are typical in the larger pyrophanite-rimmed grains. The crystallographically coherent pyrophanite rims could have formed through a process where the precursor perovskite crystal acted as a structural template for the newly forming phase, that is, by interface-coupled dissolution reprecipitation during serpentinization of the precursor rock. This alteration of perovskite to pyrophanite was not complete, resulting in the presence of perovskite fragments enclosed in pyrophanite. During the metamorphic evolution of the rock, some of the remnant perovskite cores further altered to TiO 2 polymorphs (anatase and brookite) and calcite, via transitional alteration products.