Formula: NaCaB5O7(OH)4.3H2O
Hydrated borate containing hydroxyl
Specific gravity: 2.14
Hardness: 3½
Streak: White
Colour: Colourless, colourless in transmitted light
Solubility: Readily soluble in dilute acids. Partially decomposed in hot or cold water, with Na being leached relative to Ca (Mindat)

Fumeroles and Hot Springs

Probertite occurs in lake-bed deposits, the boron supplied by hydrothermal spring activity. Probertite may be an ore of boron. Associated minerals include kernite, colemanite, ulexite and borax (HOM).


At the type locality, the Baker Mine, U.S. Borax Mine, Kramer Borate deposit, Boron, Kern county, California, USA, Probertite and kernite are the main borates of the replacement deposit, and the crystallisation of the probertite has preceded or has been simultaneous with the crystallisation of the kernite. These two exceptional borates are evidently products of fumarolic or hotspring conditions in which the temperature was the important factor influencing the crystallisation of these two lower hydrates, as the main minerals of the deposit, instead of the usual and common borax and ulexite. The presence of realgar, orpiment and stibnite as associates of the kernite confirm this view of fumarolic action (AM 14.427-430).

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