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Thenardite is a delicate sulfate mineral that comes from arid dry or saline lake deposits. It is similar to Mirabilite, a chemically similar mineral to Thenardite that, unlike Thenardite, contains water in its structure. Mirabilite is an unstable mineral, and will dehydrate and lose its water when exposed to air, thereby changing to Thenardite. These altered specimens are in fact Thenardite pseudomorphs after Mirabilite. Some of these crystals, especially those of outstanding crystal form, are artificially grown as by-products of borax mining operations. Thenardite is named after the French chemist Louis Jacques Thenard (1777-1826), a professor at the University of Paris. |
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Chemical Formula |
Na2SO4 |
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Composition |
Sodium sulfate |
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Color |
White, yellowish, light brown, gray |
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Streak |
White |
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Hardness |
2.5 - 3 |
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Crystal System |
Orthorhombic |
Crystal Forms
and Aggregates |
As intergrown clusters of distorted crystals, oftenbipyramidal in shape. Individual crystals, which are rare, are tabular and short prismatic. Sometimes in well-formed twinned crystals that bisect each other. Also massive, grainy, encrusting, and in coral-like masses. |
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Transparency |
Transparent to translucent |
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Specific Gravity |
2.7 |
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Luster |
Vitreous |
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Cleavage |
1,1 - basal |
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Fracture |
Uneven to hackly |
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Tenacity |
Brittle |
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Other ID Marks |
1) Has a weak salty taste.
2) Slowly soluble in water.
3) Fluorescent white in shortwave ultraviolet light and yellow-green in longwave ultraviolet light; may also be phosphorescent. |
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In Group |
Sulfates; Anhydrous Sulfates |
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Striking Features |
Weak taste and fluorescence |
| Environment |
Dry lakes and saline lakes in evaporite deposits, mainly in arid regions. |
| Rock Type |
Sedimentary |
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Popularity (1-4)
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3 |
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Prevalence (1-3)
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2 |
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Demand (1-3) |
2 |
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Thenardite has come from Espartinas, Spain; Mt. Etna, Vesuvius, Italy; the Bilma Oasis, Niger; and Wadi el Natrun, Egypt. In the U.S., it occurs as sharp, twinned crystals in Soda Lake, San Luis Obispo Co., California. Thenardite also occurs in other dry lake deposits in California, including Searles Lake, San Bernardino Co.; Boron, Kern Co.; and Borax Lake, Lake Co. It also has come from the Verde Valley, Yavapai Co., Arizona.
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Glauberite - Less soluble in water, forms different crystals.
Halite - Much saltier taste, occurs in different crystals, has perfect cubic cleavage.
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