Occurrence and mineralogy of the margarite- and muscovite-bearing pseudomorphs after topaz in the Juurakko pegmatite, Orivesi, southern Finland

• Main Author: S.I. Lahti
• Format: Article
• Language: English
• Published: Geological Society of Finland 1988-06-01
• Series: Bulletin of the Geological Society of Finland
• Subjects: margarite; muscovite; topaz; pseudomorphism; chemical composition; pegmatite; Juurakko; Eräjärvi; Orivesi; Finland
• Online Access: http://www.geologinenseura.fi/bulletin/Volume60/sgs_bt_060_1_pages_027_043.pdf

Margarite- and muscovite-bearing pseudomorphs after topaz are described from the Juurakko pegmatite dyke, Orivesi, southern Finland. A supercritical vapour phase rich in calcium and alkalies caused alteration of topaz and some other silicates during final phase of crystallization of the dyke. The original columnar form of topaz crystals is characteristic in the pseudomorphs, although roundish or irregular mica aggregates are also common. The pseudomorphs are composed of fine-scaled, light-brown muscovite, but they may have a topaz-margarite or margarite core. Coarse-scaled pink, lilac or yellow muscovite forms a rim around the pseudomorphs. The muscovites are nearly ideal dioctahedral. The amount of paragonite and phengite substitution is minute. The pink muscovite is slightly enriched in Mn, but the mica is poor in Fe. Margarite is fibrous or massive, fine-scaled and white in colour. The fibre axis is either a crystallographic a or b axis. Microprobe analyses show that the composition of margarite varies largely from one crystal to the other. The mineral has appreciable paragonite and ephesite as solid solution. Fine-scaled muscovite is also a main mineral in the pseudomorphs after schorl and garnet. The pseudomorphs after topaz and tourmaline may be similar. The prismatic form and the hexagonal cross-section is, however, often well-preserved in the pseudomorphs after tourmaline and the muscovite is richer in Fe, Mg, Mn, and Ti. The muscovite in the pseudomorphs after garnet has also appreciable phengite component. Being composed of bertrandite or fine-grained bertrandite, chlorite and muscovite mass, the pseudomorphs after beryl differ from the pseudomorphs after topaz and tourmaline in mineralogy.