Crystal Chemistry of Ca/Sr Metasilicates and Metagermanates

Charles T Prewitt (prewitt@gl.ciw.edu), Hexiong Yang & Stephen Gramsch

Geophysical Laboratory, Carnegie Institution of Washington, 5251 Broad Branch Road, NW, Washington, DC 20015, USA

Metasilicates and metagermanates containing the cations Ca and Sr have been of interest to mineralogists and petrologists for many years, but substantial confusion remains with respect to their chemistries, structures, and relative stabilities. Among the calcium metasilicates, wollastonite is the most important phase, occurring primarily in metamorphic environments. Wollastonite occurs in several polytypic forms, one having a triclinic space group (wo-1A), another with a monoclinic space group (wo-2 M), plus several metastable polytypes. At higher temperatures, wollastonite transforms to a pseudowollastonite polytype, a phase not recognized as a mineral by IMA, but which occurs naturally in fire-metamorphosed marls in Iran (McLintock, 1932). Although SrSiO₃ and SrGeO₃ samples have also been reported to have the pseudowollastonite structure, a number of questions remain about the exact nature of these phases.

A high-pressure form of CaSiO₃ was first synthesized by Ringwood and Major (1967) and its structure containing three-membered silicate rings was determined by Trojer (1969) to have the walstromite structure (Ca₂BaSi₃O₉). Recently, a CaSiO₃ crystal with this structure was discovered as an inclusion in a diamond (Joswig et al. 1999). CaSiO₃ also adopts the perovskite structure at high pressures, but its symmetry at all pressures has not been confirmed.

In our effort to complete a review of the crystal chemistry of these phases, we pose the following questions and will attempt to provide answers as a result of our current experimental and theoretical research:

1. Why does CaSiO₃ occur in wo-1A and wo-2 M forms, whereas CaGeO₃ is only found with the 1A structure?

2. How many pseudowollastonite polytypes have been identified?

3. Does the walstromite structure occur in other compositions?

4. What is the symmetry of the perovskite form of CaSiO₃?

5. What structures are known for compositions SrSiO₃ and SrGeO₃?

Joswig W, Stachel T, Harris JW, Baur WH, & Brey GP, Earth. Planet. Sci. Lett., 173, 1-6, (1999).

McLintock WFP, Min. Mag, 23, 207-226, (1932).

Ringwood AE, Major A, Earth. Planet. Sci. Lett, 2, 106-110, (1967).

Trojer FJ, Zeit. Krist, 130, 185-206, (1969).

The Mineralogical Aspect of Metamorphic Complexes History

Alexander Pystina (reg@geo.komi.ru)

Institute of Geology, Komi Sci. Cent., Uralian Div. of RAS 54, Pervomaiskaya, st., Syktyvkar, Russia

Strongly metamorphosed rock complexes have, as a rule, a long and multistage history of formation. These are polymetamorphic complexes almost without exception. However, parageneses of only the latest metamorphic episode are readily observed in such natural objects, hence reconstruction of polymetamorphism is a complicated problem. Of major importance for reconstruction of the metamorphic history of such complexes are: a) relict minerals and mineral parageneses, b) optical and chemical zonality in rock-forming and accessory minerals, and c) geochronologic reference dates in the history of mineral metamorphism. Based on these features we have reconstructed the main stages in formation of polymetamorphic complexes in the north of the Urals. The following sequence of metamorphic events has been established: 1. Aureole metamorphism of the granulite facies (mineral parageneses of the granulite facies, ball-like zircon crystals of the "granulite type", dated at more than 2.2. bln years). 2. Diaphthoresis of the amphibolite facies (mineral parageneses of the amphibolite facies, garnets with regressive zonality, prismatic zonal zircon crystals of the "amphibolite type", dated at ca. 2.0-1.7 bln years). 3. Zonal metamorphism reaching the grade of the epidote-amphibolite facies (mineral parageneses of the epidote-amphibolite and greenschist facies, garnets with progressive zonality, metamict zircon crystals dated at ca. 0.9-0.6 bln years). 4. Metamorphism of the greenschist facies (mineral parageneses of the greenschist facies, low-temperature pseudomorphs after garnet and hornblende dated at ca. 0.4 bln years).

Zircon as an Indicator of Polymetamorphic Processes

Julia Pystina (reg@geo.komi.ru)

Institute of Geology, Komi Sci. Cent., Uralian Div. of RAS 54, Pervomaiskaya, st., Syktyvkar, Russia

The work deals with early Precambrian metamorphic evolution in the Uralian segment of the lithosphere. Three main stages in the metamorphic alteration of rocks have been established. At the first two stages dated at 2.6-2.3 and 2.1-1.9 bln years on the basis of single grains of accessory zircons, the temperature conditions reached the level of the granulite facies. The third stage dated at 2.0-1.7 bln years by the same method occurred under the temperature corresponding to the amphibolite facies. Distinctions in the dynamic conditions were presumably observed as early as at the initial stage of the metamorphism (in the late Archean) and became most pronounced in the end of early Proterozoic, when tectonic conditions favoured formation of eclogite parageneses. It should be noted that accessory zircons from the rocks under consideration differ not only in dates but in crystal habit, colour, internal structure, and impurity content, which makes them suitable for metamorphic reconstructions.