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Category Carbonate mineral
(repeating unit)
(Na, Ce)(Y, REE)(HCO3)(OH)3·4(H2O)
Crystal system Monoclinic
Crystal class Sphenoidal (2)
(same H-M symbol)
Space group P2
Formula mass 375.77 g/mol
Color White, yellow
Crystal habit Blocky crystals, pseudo-tetragonal
Twinning Common on (101)
Cleavage [010] perfect, [101] parting
Fracture Uneven
Tenacity Very brittle
Mohs scale hardness 2–3
Luster Vitreous (Glassy)
Streak white
Diaphaneity Translucent to transparent
Specific gravity 2.30
Optical properties pseudouniaxial negative 2V (meas.) ≤ 5°
Refractive index nα = 1.40, nβ = 1.540, nγ = 1.540
Birefringence δ = 0.140
Other characteristics Radioactivity 770 Bq/g
References [1][2][3]

Thomasclarkite-(Y) is a rare mineral which was known as UK-93 until 1997, when it was renamed in honour of Thomas H. Clark (1893–1996), McGill University professor. The mineral is one of many rare-earth element minerals from Mont Saint-Hilaire. The only reported occurrence is in an alkalic pegmatite dike in an intrusive gabbro-nepheline syenite.

See also[edit]