Be
Hexagonal -- Dihexagonal bipyramidal
Environment
Almost exclusively a pegmatite mineral, rarely in high-temperature veins and in rhyolite seams and pockets where it has formed from beryllium-bearing gases or very hot solutions.
Crystal description
One of the most beautifully crystallized minerals, usually in prismatic hexagons, sometimes several feet long and weighing many pounds (kilos). Rarely in tabular crystals; the pink cesium-bearing beryls are more likely to develop this flat habit. Also massive and embedded as grains or columnar masses.
Physical properties
White, blue, yellow, green, pink, red.
Luster
glassy;
hardness
8;
specific gravity
2.6-2.8;
fracture
conchoidal;
cleavage
poor basal;
clarity
gemmy and transparent, to translucent; sometimes weakly fluorescent yellow (emerald may fluoresce pink to deep red also, especially as a synthetic).
Composition
Beryllium aluminum silicate (14.0% BeO, 19.0% Al
2
O
3
, 67.0% SiO
2
, sodium, lithium, and cesium may replace part of the beryllium, thus reducing the BeO content and lowering its value as an ore of beryllium). Impurities create many of the characteristic colors; iron is responsible for aquamarine, chromium and/or vanadium for emerald, and manganese for red beryl.
Tests
Glows white, but does not decrepitate violently like quartz, instead remaining whitened but intact in the flame. Edges fuse with great difficulty to a white glass. Insoluble in the common acids.
Distinguishing characteristics
The pegmatitic occurrence and six-sided crystal outline are very characteristic. Only likely to be confused with apatite (which is much softer, often fluorescent, and soluble in acid), with white, massive topaz (wholly infusible and with a good cleavage), and with quartz (which, before attaining near-red heat, decrepitates violently).
Occurrence
Ordinary beryl is the chief ore of beryllium. Transparent varieties have gem value and are called aquamarine (blue and blue-green), emerald (green), golden beryl (yellow-brown), morganite (pink), and ruby (red). Since as a rule it is a mineral of once deeply buried rocks, it will be found primarily on gneiss and schist roofs above batholiths, where pegmatites have been exposed on the surface as weathering has removed overlying formations. New England has many pegmatite localities; their beryls, sometimes enormous, usually appear as well-formed crystals in quartz and feldspar pegmatites. Crystals ordinarily break free of their matrix without much difficulty. Farther south, North Carolina pegmatites are also sources of common beryl, with emeralds from the chromium-colored variety occurring at several localities in the state. Tabular beryl crystals (generally thought to indicate a high cesium content) have been found in some abundance in New Mexico at Dixon. Large crystals are found in the Black Hills of South Dakota, though gem material is unlikely. Short-prismatic pink (morganite) beryls occur with the aquamarine, kunzite, and colored tourmaline in San Diego Co., California. An unusual occurrence for beryl is that of ruby red crystals in the Wha Wha Mountains, Utah. where they are more or less embedded in a white rhyolite, 1-1Ɖ in. (2-3 cm) long. Smaller, flatter, and paler raspberry-pink crystals and rose-colored clusters have been found in the gas cavity in the Thomas Range (Utah) rhyolite, best known for its topaz and bixbyite.
In a freak paragenesis, emeralds are found in calcite veins in a black limestone at Muso, and in almost-pegmatite veins at Chivor, and Gachala, Colombia. Biotite schists bordering a pegmatite are considered the source of the chromium coloring emeralds in the Ekaterinberg, (Sverdlovsk) Russia district, accompanied there by alexandrite, chrysoberyl, and phenakite. Large crystals of aquamarine and fine morganites come from Madagascar. Brazil has long been the chief source of aquamarine, and crystals weighing several hundred pounds (100-150 kg) have been recovered from streambeds and dikes, principally in Minas Gerais, Brazil. In a tin mine of South Africa, clusters of slender needles are unusual gangue minerals in an unusual vein deposit. Numerous pegmatites have lately been worked in the mountainous Afghanistan-Pakistan border area; large numbers of splendid aquamarine crystals have been found, though most are too pale to have other than specimen value.
