Abstract

A bluish beryl from a pegmatite dike in Mohave County, Arizona, differs distinctly from other known beryls in physical properties and chemical composition. The highest measured value of ω for this beryl is 1.610, and the analyzed sample has ω = 1.608, ε = 1.599. Only four published analyses of beryls with an ω index higher than 1.592 are known, and the highest index heretofore recorded for any beryl, recognized as such, is 1.602. G = 2.921.

In composition, this mineral differs markedly from other beryls. It has the lowest percentage of SiO2 and of Al2O3 of any known beryl. It contains 4.69 per cent of oxides of bivalent elements other than beryllium, chiefly ferrous iron and magnesium. Its content of Cs2O, 6.68 per cent, is much greater than that of nearly all other known beryls. On the other hand, its content of Li2O, 0.23 per cent, is only about one-tenth of what would be expected for an alkali-rich beryl.

The composition and physical properties are incompatible with distinctive compositional trends determined for beryls by Schaller and Stevens. In general the ω index of refraction ranges from 1.570 for low-alkali beryls to 1.592 for high-alkali beryls. Low-alkali beryls are close to the composition indicated by the standard beryl formula Be3 · Al2 · Si6O18. The high-index beryls can be interpreted in terms of the standard beryl formula combined with the sodium-lithium beryl formula Na·Be2Al·AlLi·Si6O18 and its cesium-lithium analogue. In various solid solutions of these end-members the atomic ratios of Si and A1 do not vary, but increases in Li are accompanied by equal increases in Na or Cs and by equal decreases in Be.

In contrast to the compositional trends for most beryls, the Arizona beryl is deficient in Al; it is high in Na and Cs but low in Li, and its Be content is abnormally high for a high-index, high-alkali beryl. These relationships are attributed to the presence of another end-member represented by the generalized formula (Na, Cs)·Be3·Al(Fe2+, Mg)·Si6O18. The Arizona beryl contains more than 50 per cent of this femag end-member.

Most of the Arizona beryl with the highest indices of refraction occurs in the finegrained border zone of an irregular pegmatite dike, where it typically forms small podlike masses. Each mass is a single skeletal crystal of beryl that contains numerous inclusions of quartz, microcline, albite, fluorite and sphene. More euhedral and prismatic crystals of beryl, considerably less crowded with inclusions, in the inner parts of the dike also have high indices of refraction and unusual chemical composition.

The composition seems to be a direct reflection of a relatively high iron and magnesium content of the pegmatite fluid during its crystallization. These elements probably were derived in large part from digestion of mafic country rock. The incorporation of bivalent iron and magnesium into the beryl structure, in place of some trivalent aluminum, probably resulted in the strong attraction of large monovalent cations, chiefly Cs+ and Na+, into the tubular channels of the structure during formation of the crystals.

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