It is famous for producing the world's finest specimens of blue boracite, as well as trembathite and hilgardite. Operated by Cleveland Potash Ltd., it has some of the deepest workings (1,200 meters) in Britain.
Hilgardite, [Ca.sub.2][B.sub.5][O.sub.9]Cl*[H.sub.2]O, is relatively common at the Boulby mine as well-developed, transparent, colorless to pinkish crystals in boracite nodules and more rarely as nodules in which it is the main insoluble component.
Relationship to other species: Related to hilgardite, [Ca.sub.2]([B.sub.5][O.sub.9])Cl*[H.sub.2]O.
Comments: The mineral was originally described by Yarzhemskiy in 1952 but was discredited by von Hodenberg and Kuhn in 1982 as "strontium hilgardite." Dr.
The Boulby potash mine has produced some of the world's finest specimens of blue boracite, ericaite and hilgardite. The locality continues to be actively mined and may yield further specimens in the future.
The mine has recently been reported in mineralogical journals as a new locality for well-crystallized boracite and hilgardite (Robinson and King, 1993; Cooper, 1994; Moore, 1994; Weiss, 1994).
Most of the unusual species described below, including anhydrite, boracite, ericaite, hilgardite, magnesite, rectorite and syngenite were identified by X-ray powder diffraction at Manchester University.
Hilgardite [Ca.sub.2][B.sub.5][O.sub.9]Cl[multiplied by][H.sub.2]O
Hilgardite was recorded in the Boulby Potash by Milne (1978) as a form of "parahilgardite" (using a provisional and now obsolete nomenclature).
Hilgardite is relatively common at the Boulby mine.
Evaporite rocks at the Boulby mine have provided a suite of minerals, including some of the finest known examples of the borate minerals ericaite, boracite and hilgardite. The latter two of these species occur as nodules in which they are intimately mixed with soluble chlorides.