Prolonged anhydrous and hydrothermal heating experiments made with natural and synthetic materials have confirmed the following polymorphic relations: Monoclinic celsian is stable from room temperature up to 1590°C, where it inverts sluggishly and re-versibly to hexagonal hexacelsian. The latter, stable to the melting point at about 1760°C, readily persists metastably on cooling below 1590°, and inverts rapidly and reversibly to an orthorhombic form on cooling through 300°C. Paracelsian appears to be a metastable phase, and changes monotropically through hexacelsian to celsian at temperatures at least as low as 500° and possibly at lower temperatures also.

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