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1.3 cm stillwellite-(Ce) crystal in matrix
Category Silicate mineral
(repeating unit)
Strunz classification 9.AJ.25
Dana classification
Crystal system Trigonal
Crystal class Pyramidal (3)
H-M symbol: (3)
Space group P31
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 [1][2][3]

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

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.[1] 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.[1] 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.[1]

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 Australian mineralogist Frank Leslie Stillwell (1888-1963).[2][3]