New Mexico Mineral Symposium — Abstracts

Although a number of papers have been published on the rare and distinctive red beryls from the Wall Wah Mountains, Utah, only partial geological and mineralogical data have been presented. Our studies have attempted to extend and refine the knowledge of the geology and mineralogy of the red beryl-bearing rhyolite at the Violet Claims and surrounding area in the Wah Wah Mountains.

The host rock varies from rhyolite to alkali rhyolite and contains, by weight, 3.53 to 4.15% K₂0, 1.17 to 1.61% Na₂O, and approximately 0.3% F, but no detectable Cl. All the chemical data indicate that the rhyolite is very similar to other topaz-bearing rhyolites in the western USA. The red beryls occur principally along fractures that formed late in the cooling of the rhyolite. The fractures contain black manganese oxides and hydroxides and clay minerals, in addition to the beryls. Some crystals occur imbedded in the rhyolite, and these crystals frequently contain abundant inclusions of quartz and feldspar. The fractures typically are only a few millimeters wide at most and rapidly pinch and swell. The best crystals of beryl are enclosed within a white- to pink-colored mixture of smectite and kaolinite. Bixbyite often forms "seed" cores of the red beryls. The beryl crystals are distinctly color-zoned, and complete chemical data have been obtained for core and rim portions of several crystals. Crystals cut perpendicular to c show cores that are pale- to medium-orange and rims that are burgundy-violet red. "Hourglass" zoning is shown in crystals cut parallel to c. Many crystals show this type of color zonation, but others are all red. The red color of the rim portions of the beryls correlates directly with an increased content of Mn and Ti. Specific determinations for water yielded only approximately 0.2% total water (w/w); this demonstrates that the beryls are essentially anhydrous. Unit cell dimensions were measured for core and rim portions: core -- a 9.229(1)A, c 9.212(1)A; rim -- a 9.234(1)A; c 9.204(3)A. Refractive indices determined for core and rim portions were within the following ranges: = 1.567 to  1.568, w = 1.574. Few beryl crystals are completely free of inclusions of other minerals, but, when they are free of inclusions and also free of flaws, they are spectacular specimens and make excellent and striking gemstones. Most crystals of red beryl are several millimeters to 1 cm long and 1 to several millimeters wide. The largest fine crystal found to date is in excess.of 2 cm in length and more than 1 cm wide. Cut stones are generally a carat or less in weight, but larger stones occasionally have been faceted.

The characteristic features of the red beryl result from its formation within a rhyolite host-rock under low pressure and high temperature conditions in a volcanic environment. A similar trace-element chemistry, for example, Mn, Ti, Mg, Pb, Zr, and others, of both the rhyolite and the red beryl substantiates their genetic relationship. Red beryls occur in "topaz-bearing" rhyolites, a class of silica-rich volcanic rocks that are distinguished by the presence of topaz and the high contents of F and other elements, including Nb, Be, Zr, Mn, Pb, Cs, Rb, Li, and Be. Topaz rhyolites are derived from magmas that originate in the lower portions of the earth's crust in an anorogenic environment. The low pressure, high temperature (about 800°C and less), and low water content of the magma, combined with its relatively high F content, favored release of a gas phase from the rhyolite. The gem-quality beryl then crystallized in cavities, along fractures, or within the rhyolite itself.