Thénardite
Thénardite is a rare sulfate mineral that is rarely found as crystals large enough for faceting. It is one of several arid regions, non-marine evaporate sulfate minerals and is associated with other minerals that form in the deposits of playa lakes. The Colour of this stone is mostly Colourless or very light grey or light brown. There are numerous minor occurrences of Thénardite but rarely in thick beds or as large crystals. Two notable localities for relatively large crystals are from Searles Lake, San Bernardino County, California, USA and Bärenstein Quarry, Niederschlag, Oberwiesenthal, Erzgebirge, Saxony, Germany.
| Formula: | Na2SO4 |
| Sodium sulfate | |
| Crystallography: | Orthorhombic – Dipyramidal |
| Crystal Habit: | Crystals dipyramidal {111}, or tabular {010} with large {101}, rarely prismatic along [100], to 20 cm; as pulverulent crusts and efflorescences |
| Twinning: | On {110}, commonly forming cruciform groups; as crude butterflylike twins on {011} |
| Cleavage: | [010] Perfect, [101] Fair, [100] Interrupted |
| Fracture: | Splintery, Uneven to Hackly |
| Tenacity: | Somewhat Brittle |
| Hardness (Mohs): | 2.5 – 3.0 |
| Density: | 2.664 – 2.700 (g/cm3) |
| Luminescence: | Fluorescent; SW UV = white, LW UV = yellow-green |
| Radioactivity: | Not Radioactive |
| Other: | Easily soluble in water |
| Color: | Colorless; Grayish White, pale Yellow, Yellow-Brown, Reddish due to inclusions |
| Transparency: | Transparent to Translucent |
| Luster: | Vitreous to Resinous |
| Refractive Index: | 1.471 – 1.484 Biaxial ( + ) |
| Birefringence: | 0.0013 |
| Dispersion: | Weak to None; r > v |
| Pleochroism: | None |
