Formula: Na2B4O5(OH)4.8H2O
Hydrated borate containing hydroxyl
Crystal System: Monoclinic
Specific gravity: 1.715 measured, 1.7 calculated
Hardness: 2 to 2½
Streak: White
Colour: White, yellowish, seldom bluish, greenish
Solubility: Soluble in water
Melting point: 878 °C. The melt dissolves numerous metal oxides. Loses 5 mol of water when heated to 100°C, another 4 mol when heated to 150 °C, and the last mol at 400 °C. Rapidly dehydrates in air to tincalconite (Mindat).

Evaporite Deposits (typical)
Volcanic sublimates and hot spring deposits

Borax is the most widespread of the borate minerals. It is formed by evaporation of enclosed lakes and as an efflorescence on the surface in arid regions. It occurs in salt lake beds, hot spring deposits and volcanic vents, commonly well-crystallised; also as an efflorescence on soils in arid regions (HOM). Associated minerals include inyoite, thénardite, ulexite, colemanite, kernite, kurnakovite, trona, aphthitalite, glauberite, calcite, nitratine, gaylussite, hanksite, halite and gypsum (HOM, Mindat).


The type locality is Ladakh, India.

At Searle's Lake, California, USA, it is estimated that a borax crystal of 1 cm length can grow in just a matter of hours (ALM 11.4). Borax forms pseudomorphs after hanksite, and tincalconite forms pseudomorphs after borax here (KL p184).

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