Afghanite, Sar-e-Sang, Badakhshan Province, Afghanistan

摘要

Feldspathoids are peculiar minerals. Compositionally, most appear to be complex chemical repositories whose major purpose seems to be the accommodation of some ions that might best go elsewhere (such as into a feldspar in the case of aluminum) if there were only enough silica in the system to get things reorganized. Familiar examples include nepheline, leucite, and members of the sodalite and cancrinite groups. Most collectors are familiar with the vibrant blue of sodalite and the yellow of cancrinite; alas, both are typically in massive, generally interesting, though not particularly desirable, specimens. Recently, however, the legendary lapis lazuli (lazurite) occurrence at Sar-e-Sang, Afghanistan, has produced peculiar specimens containing large, relatively sharp lazurite and sodalite crystals in white marble. Associated with these, though much rarer, are exceptional crystals of afghanite, a mineral that was only formally described as a new species in the late 1960s (Bariand 1968). An unusually fine afghanite specimen from this find has been nominated for this issue's Connoisseur's Choice (fig. 1). Afghanite is typically blue, though it tends toward colorless in smaller grains and is colorless in thin section. It exhibits {101-(bar)0} cleavage and a conchoidal fracture. Its hardness is 5.5-6, and its density is about 2.6. It has a vitreous luster and is optically positive. It crystallizes in the hexagonal system; its point group is thought to be 61m 2/m 21m, although there is debate on its structure and symmetry. Most crystals are tabular and range from stout to thin (slender laths are described in thin sections). Some grains are rounded, exhibiting only poorly developed faces.