THE MINERAL THENARDITE

Chemistry: Na₂SO₄, Sodium Sulfate.
Class: Sulfates
Uses: As a source of sodium to produce soda, various uses in the glass and paper industries and as mineral specimens.
Specimens

Thenardite is one of several non-marine evaporite Sulfate Class minerals. It is easily dissolvable in water and specimens should be stored with desiccant. Sulfates in general tend to be more soluble than most of the other mineral classes and simple sodium salts, such as thernardite, are always soluble. As one might imagine, thenardite forms in arid regions as a salty precipitate as well as in dry caves and mines as an efflorescence and as a crusty deposit around fumaroles. It is associated with other minerals that form in the deposits of playa lakes.

Thenardite, which is named for the French chemist Louis J. Thenard, has several distinctive properties in addition to its solubility. For one it has a salty taste similar to halite, but the different cleavages and crystal habits make the two minerals quite distinguishable from each other. Thenardite is also generally fluorescent showing a white color in shortwave UV and a yellow-green color in longwave UV. Thenardite also has a distinctive crystal habit in which crystals form a diamond-like shape (such as from a deck of cards). The crystals are generally flattened and are sometimes well formed showing good orthorhombic symmetry. Although not often seen in mineral shops, thenardite is now making a presence with well formed crystal clusters mostly coming from Searles Lake, San Bernardino County, California.

Thenardite specimens should be stored in closed containers as thenardite is only metastable. The specimens will gradually absorb water and convert to the mineral mirabilite. In volcanic caves on Mt. Etna, Italy; thenardite is an early secondary mineral forming the cave's speleothems. But when the temperature of the cave lowers and the humidity rises, the thenardite coverts to mirabilite.

PHYSICAL CHARACTERISTICS:

Color is typically gray, white, colorless or brownish white or yellowish white.
Luster is vitreous, pearly to resinous.
Transparency: Crystals are transparent to translucent.
Crystal System is orthorhombic: 2/m 2/m 2/m.
Crystal Habits include tabular, dipyramidal, flattened, diamond-shaped crystals; often intergrown. Twins form arrowhead like crystals and crossed interpenetration twins. Also as crusts, granular and as massive rock forming beds.
Cleavage is perfect in one direction (pinacoidal).
Fracture is uneven to hackly or splintery.
Hardness is 2.5 - 3.
Specific Gravity is approximately 2.7 (average for translucent minerals).
Streak is white.
Other Characteristics: Easily soluble in water, has a salty taste, is fluorescent white in shortwave UV and yellow-green in longwave UV and can color a flame yellow (for sodium).
Associated Minerals include halite, mirabilite, gypsum, glauberite, trona, blodite, alunite, picromerite, sborgite, borax and other borate and non-marine evaporite minerals.
Notable Occurrences include several California sites such as Searles Lake, San Bernardino County; Soda Lake, San Luis Obispo County; Bertram deposit, Imperial County and Furnace Creek district, Inyo County; as well as Camp Verde Yavapi County in Arizona and sites in Nevada, USA; Espartinas, Madrid Province, Spain; Siberia, Russia; Mt Etna, Sicily, Italy; Chile; Kazakhstan and Canada.
Best Field Indicators are crystal habit, environment of formation, associations, cleavage, taste and fluorescence.