Monoclinic System — One Oblique Axis Among the Seven Crystal Systems
Monoclinic System — One Oblique Axis Among the Seven Crystal Systems
The crystallographic class hosting jadeite, kunzite, sphene, orthoclase, and a substantial portion of the gem mineralogy
The monoclinic system is one of the seven crystal systems used in mineralogy and gemmology to classify minerals according to the symmetry of their atomic arrangements. Monoclinic crystals are defined by three unequal axes (a, b, and c) of which two intersect at right angles while the third (typically taken as the b-axis) intersects the others at an oblique angle (the angle beta, conventionally not equal to 90 degrees). The monoclinic system is the second-lowest in symmetry after the triclinic system, and it hosts a substantial portion of the gem mineralogy: jadeite, the spodumene varieties (kunzite, hiddenite), sphene (titanite), orthoclase, diopside, and epidote are all monoclinic species.
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Crystallographic definition
The defining geometry of the monoclinic system is its single non-perpendicular axis. The cell parameters are conventionally written as a, b, c, alpha, beta, gamma, where alpha and gamma are 90 degrees while beta is some other angle (typically between 90 and 120 degrees in real minerals). The b-axis is the unique axis perpendicular to both a and c, and the c-axis sits in the plane defined by a and c at an oblique angle to a. The result is a unit cell that is a parallelepiped — a slanted brick-shaped form rather than the rectangular box of the orthorhombic system.
The monoclinic system contains three crystal classes (point groups) of differing symmetry: 2/m (the most common, including most gem-mineral species), m (lower symmetry, fewer minerals), and 2 (lowest symmetry, rare). Most gem-quality monoclinic minerals belong to the 2/m class, and the diagnostic optical properties of the system follow from this symmetry.
Optical properties
Monoclinic minerals are optically biaxial, meaning they have two distinct optical axes along which light passes without being split into two polarised rays. Light passing through the crystal in any other direction is split into two rays travelling at different speeds — birefringence — and showing different refractive indices and often different absorption behaviour (pleochroism). The biaxial character is one of the principal optical signatures of the monoclinic system and one of the gemmologist's first-line tests for identifying species.
The pleochroism of biaxial monoclinic minerals can be pronounced. Iolite (cordierite, technically orthorhombic but optically very similar in pleochroic effect) and kunzite (monoclinic, lithium-bearing spodumene) are well-known examples of strongly pleochroic biaxial gems whose face-up colour depends critically on the cutting orientation. Diopside, particularly the chromium-bearing chrome diopside, also shows strong pleochroism. Cutters orient the rough to maximise the most desirable pleochroic colour in the finished stone.
Important gem species in the monoclinic system
Jadeite, the higher-value of the two principal jade varieties, is monoclinic, with prismatic to tabular crystal habit and the characteristic interlocking microcrystalline aggregate texture that gives jadeite its toughness. Spodumene (LiAlSi2O6) is monoclinic, occurring as gem-quality kunzite (lilac to pink) and hiddenite (green) varieties from pegmatite deposits. Diopside (CaMgSi2O6) and the related pyroxene minerals are monoclinic, with chrome diopside being the principal gem-quality variety. Sphene or titanite (CaTiSiO5) is monoclinic, prized for its very high dispersion and consequent strong fire when cut as faceted gems.
Orthoclase feldspar (KAlSi3O8) is monoclinic and includes both transparent yellow gem-quality material from Madagascar and the variety adularia, the alpine low-temperature orthoclase associated with moonstone adularescence. Epidote is monoclinic. Various less common monoclinic gem species — including unakite (epidote-bearing rock), datolite, axinite, and others — round out the system's representation in the gem trade.
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Implications for cutting and care
Monoclinic minerals frequently show distinct cleavage in one or more directions, often along the planes of weakest atomic bonding within the crystal structure. Cleavage in monoclinic gem species ranges from the perfect basal cleavage of orthoclase and the perfect prismatic cleavage of spodumene (which makes kunzite a notoriously difficult stone to cut) to the more moderate cleavages of diopside and sphene. Cutters working monoclinic material must understand the cleavage directions and orient the rough accordingly to avoid losses during cutting and damage during setting and wear.
Care for monoclinic gem jewellery generally follows the same principles as for any gem with significant cleavage: avoid impact, avoid thermal shock, set in protected configurations where appropriate, and clean by mild soap and water rather than by ultrasonic or steam methods. The directional hardness common to lower-symmetry crystal systems (where hardness can vary measurably with the direction of measurement on the crystal face) is rarely a practical concern for most jewellery applications but is important for cutters and lapidaries working the rough. See also: crystal system; biaxial; pleochroism.
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