Orthorhombic System — The Crystal Class of Topaz, Peridot, and Tanzanite
Orthorhombic System — The Crystal Class of Topaz, Peridot, and Tanzanite
One of the seven crystal systems, characterised by three perpendicular axes of unequal length
The orthorhombic system is one of the seven crystal systems of mineralogy, characterised by three mutually perpendicular crystallographic axes of three different lengths. Crystals in this system are biaxial, exhibit two optic axes, and show three principal refractive indices, with biaxial-positive or biaxial-negative optic sign depending on the relationship between the three values. Important gem species in the orthorhombic system include topaz, peridot (forsterite-fayalite olivine), chrysoberyl, tanzanite (zoisite), iolite (cordierite), andalusite, sinhalite, and danburite. The system is sometimes called the rhombic system in older literature.
Crystallographic features
The orthorhombic unit cell has three axes a, b, and c of unequal length meeting at right angles to one another. The system divides into three crystal classes — orthorhombic-pyramidal, orthorhombic-disphenoidal, and orthorhombic-dipyramidal — distinguished by their symmetry elements. Orthorhombic crystals show three perpendicular twofold axes of symmetry or three perpendicular mirror planes in the most symmetric class, and can show prismatic, dipyramidal, and tabular habits depending on the species and the conditions of crystallisation.
The biaxial optical character is the key gemmological consequence. Orthorhombic crystals have three principal refractive indices, traditionally denoted alpha (smallest), beta (intermediate), and gamma (largest). The two optic axes lie in the plane defined by alpha and gamma, with the angle between them — the optic axial angle 2V — varying with species. Biaxial sign is positive when beta is closer to gamma than to alpha, and negative when beta is closer to alpha than to gamma.
Gem species in the system
Topaz is the most widely traded orthorhombic gem species, with refractive indices approximately 1.609 to 1.643 (varying by composition between fluor- and hydroxyl-bearing endmembers), biaxial-positive optic sign, and characteristic perfect basal cleavage. Peridot, the gem variety of magnesium-rich olivine, has refractive indices approximately 1.654 to 1.690, biaxial-positive sign, and typical 2V around 88 degrees. Chrysoberyl has refractive indices 1.745 to 1.755, biaxial-positive, and is the host species for alexandrite and cat's-eye chrysoberyl varieties.
Tanzanite, the blue-violet gem variety of zoisite, has refractive indices 1.691 to 1.700 and biaxial-positive sign, and shows strong trichroism that is the basis of its colour-treatment chemistry. Iolite (cordierite) has refractive indices 1.534 to 1.560 and biaxial-negative sign, with strong trichroism that gives the species its alternative name dichroite. Andalusite (1.629 to 1.650, biaxial-positive) shows strong colour-change pleochroism between green and red-brown directions. Sinhalite, danburite, and the orthorhombic amphiboles round out the gem-relevant orthorhombic species list.
Identification by optic character
The orthorhombic system supplies a substantial fraction of the gemmologist's biaxial diagnostic work. On the polariscope, an orthorhombic stone shows the characteristic biaxial interference figure when oriented correctly, and the angle 2V can be measured or estimated. Refractive index measurements on the standard contact refractometer yield two of the three principal indices for any given stone orientation; rotation of the stone allows the gemmologist to step through the three principal values across orientations, producing the species identification together with optic sign and biaxial confirmation.
Pleochroism — the directional variation of colour visible through a dichroscope — is a major identification tool for orthorhombic species, since the three principal refractive directions can carry three different colours. Tanzanite is the textbook trichroic species, with violet, blue, and yellow-brown directions in unheated material. Iolite, andalusite, sinhalite, and several other orthorhombic species show strong trichroism that is essentially diagnostic.
In the trade
Understanding crystal system is essential for gem identification. The orthorhombic system in particular accounts for several of the most important coloured-stone categories — topaz, peridot, chrysoberyl, tanzanite — and the biaxial optical character of these species is the basis on which the gemmologist distinguishes them from the uniaxial corundum, beryl, and tourmaline. See also crystal system, biaxial, pleochroism, optic sign.