16.60: Scapolite

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Scapolite
Chemical composition	Complex sodium aluminium silicate
Crystal system	Tetragonal
Habit	Prismatic and massive
Cleavage	Distinct to perfect (depending on direction)
Fracture	Conchoidal
Hardness	6
Optic nature	Uniaxial -
Refractive index	1.545-1.580 (depending on color)
Birefringence	0.016-0.02
Dispersion	Low, 0.017
Specific gravity	2.634-2.74 (depending on color)
Lustre	Vitreous
Pleochroism	Medium to strong (depending on color)

Diagnostics

Polariscope

Scapolite can be easily confused with quartz - in particular the violet version with amethyst - due to overlapping refractive indices. The optic character for scapolite is uniaxial with a negative sign while quartz is uniaxial with a positive sign. In addition; quartz will usually show a bull's-eye. Separating the two is, in general, an easy task (30 seconds) when one is familiar with the conoscope addition to the polariscope.

Tenebrescence

File:Tenebrescent scapolite.jpeg

Figure \(\PageIndex{1}\): Tenebrescent scapolite before and after exposure to UV light.
Photo courtesy of Scott Davies, americanthai.com

A fairly recent find (2005) in Badakhshan, Afghanistan is tenebrescent scapolite. This colorless to silvery material is unearthed near the hackmanite deposits and shows an aquamarine color after exposure to SWUV light. The intensity of this color (blue) depends on the time it has been exposed to the UV lighting. Exposure to a UVP UVG4 SWUV lamp for 15 minutes triggered an almost Santa Maria aquamarine blue color that faded gradually during the following 2-3 minutes in natural daylight.