Alexandrite and colour-change chrysoberyl from the Lake Manyara alexandrite-emerald deposit in northern Tanzania

摘要

Mineralogical and gemmological properties of alexandrite from the Mayoka deposit, Lake Manyara mining area, Tanzania, are described. The alexandrite crystals are located in a phlogopite-bearing schist which is part of a metamorphic-metasomatic association of mafic or ultramafic rocks in contact with pegmatitic intrusions. Emeralds are associated with the pegmatites. Single crystals, contact twins, penetration twins and cyclic twins (trillings) are all present in this area and are developed in two major habits, one tabular parallel to the a pinacoid and the other columnar along the a-axis. Microscopic features of faceted and rough alexandrites are characterized by internal growth structures reflecting the external morphology of the samples. Common-mineral inclusions are phlogopite, apatite and zircon. Associated with series of channels parallel to the a-axis three groups of elongated or tabular negative crystals oriented in a plane parallel to the a pinacoid form milky white zones in some samples. The complexity of alexandrite colours is clarified to some extent by correlation of colorimetric parameters with trace element contents and with the orientation of the alexandrites. The designation of samples as alexandrites and a possible distinction from colour-change chrysoberyl is discussed. Alexandrite and green chrysoberyl from Lake Manyara is compared with samples from other occurrences in phlogopite-bearing schist such as the 'classical' alexandrite deposits in the Urals, Russia, and from Novello, Zimbabwe.