Formula: (Na,Ce)(Y,REE)(HCO3)(OH)3·4H2O
Origin of name: Named after Thomas Henry Clark (1893-1996), McGill University, Montreal, Quebec, Canada, noted for his studies of the geology of the St. Lawrence Lowlands.
Crystal system: Monoclinic, pseudotetragonal
Crystal class: 2
Twinning: On {101}, ubiquitous
Unit cell: a = 4.556, b = 13.018, c = 4.566, β = 90.15°
Colour: White to yellow
Diaphaneity: Semitransparent
Luster: Vitreous
Habit: As blocky crystals to 2 mm, showing {010}, {010}, {101}, {101}, {111}, {111}, {111}, {111}, singly and in groups.
Hardness: 2-3
Specific gravity: 2.30
Cleavage: 2, {010} perfect, parting on {101}
Tenacity: Brittle, uneven fracture
Streak: White
Synonyms/varieties:
Comments: Occurs as a late hydrothermal phase in alkaline pegmatite dikes in nepheline syenite.
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