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Boracite


Boracite is a magnesium borate chloride. Nominally the pure compound is colorless, but often minor substitution of iron for magnesium causes blue coloration. Werner (1789) named the mineral for its boron content. Boracite crystals show cubic crystal forms, but are actually orthorhombic at room temperature. Mallard (1883) heated boracite to 265°C, and showed that the symmetry changes from orthorhombic to cubic. Such thermal inversions are the most precise geo-thermometers; these crystals presumably formed above 265°C.

Boracite crystals here reach 2 mm, building a mound of sea-foam color. This piece comes from a salt mine; boracite damages the crushing equipment due to its high hardness (Mohs 7-7½). So zones rich in boracite are thus avoided at this locality. Attractive!

Price: $100

Item code: MS1793

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