Gerstley borate is composed of ulexite ([Na.sub.2]O 2CaO 5[B.sub.2][O.sub.3]3 16 [H.sub.2]O), with small amounts of colemanite (2CaO 3[B.sub.2][O.sub.3] 5[H.sub.2]O) and probertite
([Na.sub.2]O) 2CaO 5[B.sub.2] [O.sub.3] 10 [H.sub.2]O), resulting in low levels of pore water and much higher levels of chemically bound water.
Continuous mining machines were used as the main colemanite development mining system; drill-blast methods were used for probertite development mining, all access drift mining and all stope mining.
The orebody is mineralized with two main borate ore minerals, colemanite and probertite. A third borate ore, ulexite, occurs mainly in the upper part of the orebody.
The orebody continued to alter from probertite to colemanite, developing an envelope of colemanite around an inner core of probertite.
Probertite is believed to be either the dehydrated secondary mineral after the primary mineral of deposition, ulexite, or along with colemanite, a primary mineral of deposition.
The brecciated limestone suspended in probertite indicates an environment of much higher energy.
Nearly all of the fault tension of the orebody was released by deformation of the colemanite, leaving the probertite areas nearly free of faulting.
The slightly acid water easily dissolved probertite and colemanite and reacted with the limestone, gradually removing the limestone, liberating calcium ions and finally neutralizing the solution.
Most bedding structure survived this mineralogical change, indicating that the recrystallization of colemanite proceeded quickly after the dissolution of the probertite, thereby preventing total collapse of the mudstone beds.
Partially dissolved probertite fibers have occasionally been observed penetrating crystals of colemanite at this interface, indicating a wandering focus of dissolution and recrystallization and a solution varying in amount, acidity, and dissolved borate.