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HILGARDITE with Boracite

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This large cabinet specimen comes from deep under the North Sea from the remarkable Boulby Potash mine. At a depth of 1,400 m (4,600 ft) Boulby is Europe's second deepest mine and has over 600 miles of tunnels which extend out beneath the seafloor. Mineralogically, Boulby mine is best known for producing some the world's finest Boracite specimens, but it has also provided some of the best Hilgardite too. In this specimen the two borate minerals occur side by side, with salmon pink/orange Hilgardite crystals infilling large spaces between pale grey nodular and crystallized Boracite. The Boracite is texturally quite interesting, grading from massive to well-crystallized. The contact between Hilgardite and Boracite is sharp, indicating a sudden change in geochemistry from magnesium-rich (Boracite) to calcium-rich (Hilgardite). Due to the extreme difficulty of access to the underground workings very few specimens of these unusual borate species are in circulation. This is compounded by the extreme hardness of Boracite (harder than Quartz) rapidly wearing down cutting tools, meaning that the tunnels generally avoid Boracite-rich zones.