Scapolite

Crystal system Tetragonal
Transparency Transparent to translucent
Luster Vitreous
Fracture Conchoidal
Cleavage Perfect prismatic
Specific Gravity 2.68
Hardness 6.5
Optical Character Uniaxial- Double Refractive
Refractive index 1.550 - 1.572
Birefringence 0.15 t 0.022
Dispersion 0.017
Fluorescence light yellow
Pleochroism Strong in blue stones
Chemical Formula
Comments
Streak

Scapolite (SCAP-oh-lite), which is used to some extent as a gem material for jewelry, is actually a group of minerals. Chemically, they are sodium-calcium-aluminum silicates, with some chlorine. They form an isomorphous mixture of a series of ratios between the sodium-end member and calcium end member. All of them are tetragonal in crystal structure, usually occurring in prismatic crystals; the name, in fact, comes from the Greek word meaning "rod", in allusion to the shape of the crystals.

Scapolite is most commonly used as a gemstone when it displays chatoyancy. In its various colors, stones with parallel, needlelike inclusions, which are essential to chatoyancy, are common. It varies from a colorless, transparent form through pink, green and violetish blue. Deep-yellow stones are comparatively common, also. The deeper the color and the stronger the eye, the more desirable the stone. One variety has been called "pink moonstone", because it shows a sheen similar to that of moonstone.

The major sources of scapolite are the Mogok, Burma, area; Madagascar; Espirito Santo, Brazil; Siberia; and the eastern portion of the United Stages. It occurs in metamorphic rocks, particularly metamorphosed limestones, and in near contact with igneous intrusions. Associated minerals are garnet, apatite, sphene and zircon.

The luster of scapolite is vitreous, its hardness is approximately 6 1/2, and its fracture is conchoidal. Because of perfect prismatic cleavage, its toughness is classed as only fair. Specific gravity varies from 2.62 to 2.74, and it has no inclusions that can be considered characteristic. The R.I. of yellow and pink stones is usually near 1.550 - 1.572, with a consequent birefringence of .022.

The violetish-blue stones usually read 1.545 - 1.561 or 1.562, and have a birefringence of .015 to .017. This color is also slightly lower in S.G.; approximately 2.63, as compared to the values mentioned above. The optic character is uniaxial negative, the dispersion is low (.017). Transparent violetish-blue stones show strong dichroism. Scapolite is attacked by acids and it fuses easily.

Fibrous material, which is cut in the cabochon style, must be oriented carefully, as with any chatoyant stone, so that the needle like inclusions are at right angles to the length of the cabochon. This material polishes satisfactorily on a felt wheel with either tin or cerium oxide. Linde A on a tin lap, using 42° crown angles and 43° pavilion angles, produces an acceptable faceted stone.