Geologic, petrographic and tectonic background

A series of different rocks is produced by the alteration of these rocks by metasomatic events (i.e. chemical and mineralogical alteration of the rocks due to the infiltration of fluids). The marbles and calcsilicates are altered to various skarnoids.

Sapphire (5 cm in diameter) from the Andranondambo mine in the host rock

These skarns are mainly composed of impure calcitic marbles, diopside- pyroxenites with variable amounts of scapolite, spinel, pargasite-rich types and peraluminous rocks made up of plagioclase and/or scapolite with spinel and/or corundum, as well as hibonite and phlogopite.