feldspar, gems

Bridges, Graziani and Gübelin describe a newly recognized gem-quality feldspar from Kioo Hill, Kenya, occurring in the central portions of larger feldspar crystals in a kyanite-bearing pegmatite associated with vermiculite and quartz. Mineralogical and chemical investigation identifies the material as a low-temperature albite with peristerite structure. The pale blue to greenish material has refractive indices of approximately 1.531–1.539, birefringence 0.008, and specific gravity 2.623 ± 0.002. The study discusses its occurrence, composition, crystallographic structure and gemological properties, establishing the material as an unusual gem variety of sodic plagioclase. The paper includes eight color figures, four diagrams and two tables.