Stillwellite-(Ce)

Stillwellite-(Ce) is a rare-earth boro-silicate mineral with chemical formula (Ce,La,Ca)BSiO₅.

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Stillwellite-(Ce)
1.3 cm stillwellite-(Ce) crystal in matrix
General
Category	Silicate mineral
Formula (repeating unit)	(Ce,La,Ca)BSiO5
IMA symbol	Swl[1]
Strunz classification	9.AJ.25
Dana classification	54.02.03.02
Crystal system	Trigonal
Crystal class	Pyramidal (3) H-M symbol: (3)
Space group	P31
Identification
Color	Red-brown to pale pink
Crystal habit	Flat rhombohedral crystals, massive
Twinning	Observed on {100}
Cleavage	Imperfect
Fracture	Conchoidal
Mohs scale hardness	6.5
Luster	Resinous
Streak	White
Diaphaneity	Transparent to translucent
Specific gravity	4.57–4.6
Optical properties	Uniaxial (+)
Refractive index	nω = 1.765, nε = 1.780
Birefringence	δ = 0.015
Other characteristics	Radioactive
References	[2][3][4]

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It occurs as a metasomatic replacement of metamorphosed limestones in the Mary Kathleen mine, Australia and in alkalic pegmatites associated with syenite in an alkaline massif in Tajikistan.^[2] It occurs in association with allanite, garnet, uraninite in the Australian deposit; with calcite, monazite, bastnasite, thorite, uranothorite and thorianite in the Desmont mine, Wilberforce, Ontario, Canada; and with pyrochlore, tienshanite, sogdianite, thorite, caesium kupletskite, reedmergnerite, steacyite, pectolite and quartz in the Tajikistan deposit.^[2] It has also been reported from Mont Saint-Hilaire, Quebec, Canada and from Mineville, Essex County, New York. Other occurrences include the Inagli massif, Yakutia, Russia, around Langesundsfjord, Norway, in the Ilimaussaq intrusive complex, southern Greenland and the Vico volcano, Lazio, Italy.^[2]