Beryl

Hand Specimen identification: Association, sometimes hexagonal crystal shape, very poor cleavage, color (if blue), and hardness identify beryl. See Figure 7.

Chemical Composition : Be3Al2Si6O18

Hardness : 71 2 to 8

Specific Gravity2.7 to 2.9

Cleavage/Fracture : poor (001)/even

Luster/Transparency : vitreous/transparent to translucent

Color : blue, colorless, variable

Streak : white

Optical Properties : In thin section, beryl may appear similar to quartz, apatite, or topaz, but quartz is 1+ 2, apatite has higher relief, and topaz is biaxial. Uniaxial 1- 2, v = 1.568, e = 1.562, d = 0.006.

Crystallography : Hexagonal, a = 9.23, c = 9.19, Z = 2; space group P6/m2/c2/c; point group 6/m2/m2/m.

Habit : Beryl typically forms hexagonal prisms. When pres-ent, terminating faces are pinacoids or, more rarely, pyramids. Single crystals (Figure 7) are common; columnar aggregates are less so.

Structure and composition : 6-membered tetrahedral rings form sheets that are linked by tetrahedral Be and octahedral Al. Some classification schemes group beryl with true ring sili-cates such as tourmaline, but in beryl the framework structure is not completely planar; there is much cross linking. Small amounts of Na, Rb, and Li may substitute for Be; minor H2O and CO2 may occupy spaces within the rings.

Occurrence and associations : Beryl is found in granitic rocks, notably in peg-matites. It may also be found in schists and in rare ore deposits.

Varieties : Beryl can be many different colors due to small amounts of trace elements: emerald is vivid green; aquamarine is pale greenish blue (Plates 3.3 and 8.5); morganite is rose colored; heliodor is gold.

Related Minerals : Cordierite, 1Mg,Fe22Al4Si5O181, is 2similar in structure to beryl. Euclase, BeAlSiO4 OH , is another of the rare beryllium silicates.