SHORT COMMUNICATIONS 133
(Me75-83), aluminous diopside, green amphibole, colourless spinel, large poikilitic blue tourmaline, and a little calcite. Tourmaline includes everal minerals, especially clinopyroxene, spinel, and a few small crystals of serendibite. Scattered grains of the latter in one crystal of tourmaline are i...
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| Language: | English |
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| Online Access: | http://citeseerx.ist.psu.edu/viewdoc/summary?doi=10.1.1.631.4842 http://www.minersoc.org/pages/Archive-MM/Volume_54/54-374-133.pdf |
| Summary: | (Me75-83), aluminous diopside, green amphibole, colourless spinel, large poikilitic blue tourmaline, and a little calcite. Tourmaline includes everal minerals, especially clinopyroxene, spinel, and a few small crystals of serendibite. Scattered grains of the latter in one crystal of tourmaline are in optical continuity, suggesting that tourmaline is a breakdown product after serendibite. The ser-endibite is colourless to very light green. The mineral is much less coloured than the prussian blue crystals in a calcsilicate rock associated with clintonite clinopyroxenites from Ianapera, in SW Madagascar (Nicollet, 1988, 1990). The Ihosy ser-endibite has low birefringence and fine polysyn-thetic twinning. It may be mistaken for sapphirine, but it is distinguishable from the latter by a larger extinction angle and by its occurrence in calcic rocks. The ferromagnesian minerals in the rock are magnesium rich. Tourmaline (Table 1) is a magnesian uvite and the XMg ratio of the spinel is greater than 0.9. Hornblende and clino-pyroxene are close to their Mg end-members (XMg~0.97). The Fe-Mg partitioning between tourmaline and serendibite (Kd) is ~0.4 as in the more iron rich pair from Ianapera; it is lower than the Ka of coexisting tourmaline and serendibite from Melville Peninsula, Canada (Kd=0.61: Hutcheon et al., 1977). The P-T conditions of the crystallization f this rock are similar to those estimated for the neigh-bouring seven-phase anatectic gneisses (Nicollet |
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