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Formula: (NH4)3B15O20(OH)8.4H2O
Hydrated pentaborate, paramorph
of larderellite
Crystal system: Monoclinic
Specific gravity: 1.765 measured, 1.76 calculated
Streak: White
Colour: White; colourless in transmitted light
Luminescence:
Solubility: Slowly and completely soluble in cold water
Environments
Localities
The type locality is Larderello, Pomarance, Pisa Province, Tuscany, Italy. The region of Larderello has experienced
occasional phreatic eruptions, caused by explosive outbursts of steam trapped below the surface. The water is
contained in metamorphic rocks where it is turned to steam, which is then trapped beneath a dome of impermeable
shales and clay. The steam
escapes through faults in the dome and forces its way out in the hot springs. It possesses a dozen explosion craters,
30 to 250 m in diameter.
Larderello now produces 10% of the world's entire supply of geothermal electricity, amounting to 4,800 GWh per year
and powering about a million Italian households. Its geology makes it uniquely conducive to geothermal power
production, with hot granite rocks lying unusually close to the surface, producing steam as hot as 202°C.
Contrary to popular belief, Larderello is not a volcano, as no eruptions of magma have occurred there at any point
in history.
Ammonioborite is granular, white and lustrous, forming a compact mass with
sassolite and associated with
larderellite
(Mindat).
Ammonioborite from Larderello - Image
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