CESIUM REMOBILIZATION AND THE FORMATION OF PEZZOTTAITE AT THE SAKAVALANA PEGMATITE, MADAGASCAR

The Sakavalana pegmatite, located in the northern part of the famous Ampandramaika-Malakialina pegmatite district in Central Madagascar, is hosted by impure marbles of the Vohimena Group. It is a “mixed feature” granitic pegmatite, i.e. one that has characteristics of both the LCT and NYF families of the Rare-Element and Miarolitic pegmatite classes. The outer portion of the pegmatite consists mostly of K-feldspar, quartz, plagioclase, and black mica. The core zone consists of amazonite, zoned crystals of black mica and purple lithian muscovite, smoky quartz, and traces of cleavelandite, danburite, beryl, spessartine, spodumene, zircon, monazite, Nb-Ta oxides, and chiavennite. Crystal-lined cavities present in the core zone, contain the core-zone minerals and locally the new mineral, pezzottaite.

Pezzottaite, Cs(Be₂Li)Al₂ Si₆O₁₈, is a purplish pink Cs-, Li-rich member of the beryl group that was first discovered in November 2002 in this pegmatite. Cs₂O contents range from 11 to 18 wt. % and Li₂O is about 2.16 wt.%. Pezzottaite occurs as tabular or short prismatic crystals up to several centimeters in maximum dimension, exhibiting prism, pinacoid, and dipyramids, and as stacked aggregates.

Pezzottaite apparently formed from late-stage Be-bearing hydrothermal fluids that remobilized Cs by the corrosion of pollucite. The presence of Cs-analcime suggests that pollucite was originally present. The limited occurrence of pezzottaite in the pegmatite, as well as the nearby presence of secondary voids with a morphology consistent with that of pollucite, provide additional evidence for pezzottaite formation via Cs remobilization. Extensive etching and corrosion of the pezzottaite itself also reflects the prevalence of late-stage hydrothermal fluids in this strongly Cs-mineralized area of the pegmatite.