Ekanite

Ekanite is an uncommon silicate mineral with chemical formula: Ca₂ThSi₈O₂₀ or (Ca,Fe,Pb)₂(Th,U)Si₈O₂₀. It is a member of the steacyite group. It is among the few gemstones that are naturally radioactive. Most ekanite is mined in Sri Lanka, although deposits also occur in Russia and North America. Clear and well-colored stones are rare as the radioactivity tends to degrade the crystal matrix over time in a process known as metamictization.

Ekanite
A Cut crystal of Ekanite
General
Category	Silicate mineral
Formula (repeating unit)	Ca2ThSi8O20 or (Ca,Fe,Pb)2(Th,U)Si8O20
Strunz classification	9.EA.10
Crystal system	Tetragonal
Crystal class	Trapezohedral (422) H-M symbol: (4 2 2)
Space group	I422
Identification
Color	Green, yellow, dark red
Crystal habit	Pyramidal crystals, granular to massive
Cleavage	Distinct on {101}
Fracture	Brittle, uneven
Mohs scale hardness	4.5
Luster	Vitreous
Streak	White
Diaphaneity	Transparent to translucent
Specific gravity	2.95 - 3.28
Optical properties	Uniaxial (-)
Refractive index	nω = 1.580 nε = 1.568
Birefringence	δ = 0.012
2V angle	10 - 15°
Other characteristics	Radioactive, metamict
References	[1][2][3]

The type locality is Eheliyagoda, Ratnapura District, Sabaragamuwa Province, Sri Lanka,[1] where it was first described in 1955 by F. L. D. Ekanayake, a Sri Lankan scientist,[4][5] and it is named after him.[4]

In Sri Lanka the mineral specimens occur as detrital pebbles. In the Tombstone Mountains of Yukon, Canada, the mineral is found in a syenitic glacial erratic boulder.[2] In the Alban Hills of Italy it is found in volcanic ejecta.[1]