| Thomasclarkite-(Y) | |
|---|---|
 | |
| General | |
| Category | Carbonate mineral |
| Formula | (Na, Ce)(Y, REE)(HCO3)(OH)3·4(H2O) |
| IMA symbol | Tcl-Y [1] |
| Crystal system | Monoclinic |
| Crystal class | Sphenoidal (2) (same H-M symbol) |
| Space group | P2 |
| Identification | |
| 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 |  Radioactive 770 Bq/g |
| References | [2] [3] [4] |
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.