CHRYSOBERYL
Chrysoberyl – BeAl2O4 – is a fairly rare oxide, characteristic of some granitic pegmatites. It can be a Be ore. As a collector’s mineral and as a gemstone, it can reach very high prices.
A chromiferous, gemological variety is alexandrite, which is trichroic. It has a dark yellow-green to bluish-green color in sunlight, but under incandescent light it shows a red to dark pink color, due to some substitution of Al by Cr. It generally reaches sizes of up to 3 carats. Synthetic alexandrite has been known for a long time, as have some minerals (corundum, spinel) similar to alexandrite because they also change color, but never in green tones.
There are four varieties of chrysoberyl. It may contain Fe, Cr, and Ti. It exhibits bluish opalescence or “chatoyancy,” especially in {010}. It may exhibit both asterism and the “cymophano” (“cat’s eye”) effect. Some crystals exhibit red or green fluorescence under shortwave ultraviolet light.
1. Characteristics
Crystal system: Orthorhombic bipyramidal.
Color: Emerald green, greenish-white, yellowish-green, greenish-brown, yellow, blue, black.
Habit: Tabular crystals, sometimes short prismatic ones, up to 22 cm.
Cleavage: {110} distinct, {010} imperfect, {001} bad. Striations in {001}, parallel to [100].
Tenacity: Brittle.
Twinning: Twins are very common along the {130} plane, both by contact and interpenetration. If the twin is formed by two crystals, it results in a heart-shaped twin. If the twin is formed by three crystals, it results in a pseudohexagonal twin (“trilling”: three tabular crystals forming an aggregate with six points) when viewed along [100], with or without penetrating angles. Twinned crystals are generally flattened perpendicular to the twin plane and have a feather-like striation on {001}.
Fracture: Irregular, conchoidal.
Mohs Hardness: 8.5
Parting: No.
Streak: White.
Lustre: Vitreous.
Diaphaneity: Transparent.
Density (g/cm³): 3.75
2. Geology and Deposits
Chrysoberyl is typical of some Be-rich granitic pegmatites associated with high-grade mica schists, ultramafic rocks, or dolomitic marbles. In these cases, it forms through the interaction of Be- and Al-rich liquids from the pegmatite with the surrounding rocks, by contact and regional metamorphism.
Alexandrite, the chromiferous variety, forms only when Be-rich liquids from pegmatites react with Cr-rich ultramafic surrounding rocks, a rather rare situation.
It can occur detrital in placers and sedimentary rocks.
3. Mineral Associations
It is associated with typical pegmatite minerals, such as quartz, feldspars (albite, microcline), micas (biotite, muscovite, phlogopite, clinochlore), tourmaline (schorlite), columbite, topaz, staurolite, kyanite, garnets (almandine, spessartine), and zircon.
It also occurs with other Be minerals, such as beryl and phenacite.
In pegmatite reaction zones, it also occurs with apatite and fluorite.
4. Transmitted Light Microscopy
Refraction indices: nα: 1.746 nβ: 1.748 nγ: 1.756
PLANE POLARIZED LIGHT – PPL
Color / Pleochroism: Colorless, it can be pleochroic in pale shades of: X = red, Y = yellow-orange, Z = emerald green.
Relief: High.
Cleavage: {110} distinct, {010} imperfect, {001} poor.
Habits: Tabular to short prismatic crystals, usually twinned.
CROSSED POLARIZED LIGHT – XPL
Birefringence and Interference Colors: Maximum birefringence of 0.011, corresponding to colors ranging from gray and white to first-order straw yellow, does not reach yellow or orange.
Extinction: Paralell.
Elongation sign: No information available.
Twins: Very common, through contact or interpenetration.
Zoning: No information available.
CONVERGENT LIGHT
Character: B(+)
2V angle: 70º
Alterations: No information available.
May be confused with: Other colorless minerals with high relief.
Corundum is U(-), at most anomalously biaxial with a 2V angle of 5 to 7º.
5. Reflected Light Microscopy
Reflected light microscopy is clearly not the recommended analytical method for identifying chrysoberyl. However, it is important to prepare a polished slide or section to identify the opaque minerals that occur associated with chrysoberyl.
Sample preparation: xx
PLANE POLARIZED LIGHT – PPL
Reflection color: xx
Pleochroism: xx
Reflectivity: xx
Bireflectance: xx
CROSSED POLARIZED LIGHT – XPL
Isotropy / Anisotropy: xx
Internal reflections: xx
May be confused with: xx
General Characteristics: xx