Stillwellite-(Ce)

(Ce,La,Ca)BSiO₅IMA symbolSwl^[1]Strunz classification9.AJ.25Dana classification54.02.03.02Crystal systemTrigonalCrystal classPyramidal (3)
H-M symbol: (3)Space groupP3₁IdentificationColorRed-brown to pale pinkCrystal habitFlat rhombohedral crystals, massiveTwinningObserved on {100}CleavageImperfectFractureConchoidalMohs scale hardness6.5LusterResinousStreakWhiteDiaphaneityTransparent to translucentSpecific gravity4.57–4.6Optical propertiesUniaxial (+)Refractive indexn_ω = 1.765, n_ε = 1.780Birefringenceδ = 0.015Other characteristics RadioactiveReferences^[2][3][4]

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

Location

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]

Discovery

It was first described in 1955 for an occurrence at the type locality is the Mary Kathleen Mine, 55 km (34 mi) east of Mount Isa, Queensland, Australia. It was named for Austrian mineralogist Frank Leslie Stillwell (1888–1963).^[3][4]

Chemistry Breakdown

	(1)	(2)		(1)	(2)
SiO2	22.40	22.06	La2O3	27.95	19.12
UO2		0.22	Ce2O3	33.15	30.82
ThO2		5.41	Pr2O3		1.82
B2O3	12.23	[13.46]	Nd2O3		5.36
Al2O3	0.42		Sm2O3		0.34
Y2O3	0.74	0.28	Fe2O3	0.18

References