|| Sibirskite (CaHBO ) and parasibirskite ₃ are in the relation of polymorphism, these are regarded as the alteration
products of takedaite (Ca₃B₂O₆), occurs at Fuka mine, Okayama Prefecture. The synthesis of parasibirskite was
reported by the Schäfer (1968a, b) etc. and the synthesis of sibirskite was reported by the Sun et al. (2011).
To determine the formation condition of sibirskite and parasibirskite in the natural, they were synthesized from
the natural takedaite (Ca₃B₂O₆). As a result, both of sibirskite and parasibirskite were formed by the duration
time of 7 days with 1000 kg/cm2 and 300 oC, on the other hand, only sibirskite was formed by the duration time of
14 days with the same condition.
In addition, the mixtures of sibirskite and parasibirskite were examined under the various temperatures and
pressures of hydrothermal conditions. As a result, the upper limit of the stable temperature and pressure
conditions of sibirskite was estimated between 350 oC and 400 oC at 1500 kg/cm2, and between 400 oC and 450 oC
at 500 kg/cm2 . In the higher temperature than the condition, sibirskite were changed into shimazakiite (Ca₂B₂O₅).