Stability of Sapphirine: a Study of the Aluminous Part of the System MgO-Al₂O₃-SiO₂- H₂O

Abstract

The stability field of sapphirine has been located in the PT range 1 bar-7 kb, 600°-1500° C by experimental study of compatibility relations between the phases chlorite, cordierite, enstatite, spinel, sapphirine, corundum and mullite in the presence of excess water. In the pressure range from about 300 to at least 7,000 bars $$PH_{2}O$$ sapphirine forms from chlorite + spinel + corundum at temperatures between 600° and 760° C, depending on pressure. The assemblage chlorite + sapphirine is only stable over a narrow temperature range of at most 50° C. By the appearance of a stable tie line, enstatite + sapphirine at pressures above about 3.5 kb and temperatures in excess of 765° C sapphirine can also occur in relatively $$Al_{2}O_{3}$$-poor bulk compositions at these elevated PT conditions. The petrogenetic grid of the system $$MgO-Al_{2}O_{3}-SiO_{2}- H_{2}O$$ derived for the phases involved as well as data on the compatibility relations of sapphirine outside the PT range discussed explains most of the natural two-phase assemblages of sapphirine with phases of the system $$MgO-FeO-Al_{2}O_{3}-SiO_{2}-H_{2}O$$ pertaining to these rocks. The coexistence of magnesian cordierite with an iron-bearing spinel restricts the occurrence of sapphirine to highly magnesian bulk compositions. At higher FeO contents sapphirine disappears to form, for example, cordierite + spinel + corundum ± Al-silicate.