硅线石角岩形成时变质反应的热力学计算

THERMODYNAMIC CALCULATIONS FOR METAMORPHIC REACTIONS INVOLVED IN THE FORMATION OF SILLIMANITE-HORNFELS

北京房山花岗闪长岩体周围岩石的接触热变质作用发育良好，从内向外，依次出现辉石角岩相、角闪石角岩相、绿帘石角岩相，环绕侵入体形成良好的接触变质圈。1967年，由于教学工作的需要，对岩体西南侧，东山口-关坻一带的泥质岩石的接触变质进行了矿物共生分析。其中辉石角岩相及角闪石角岩相的矿物共生组合如图1、2所示。

Four categories of metamorphic reaction may be established in the light of mineralassemblages of pyroxene-hornfels and hornblende-hornfels in this region. Calculationsfor the reaction (2) KAl_3Si_3O_(10)(OH)_2 + SiO_2--→Al_2SiO_5 + KAlSi_3O_8 + H_2Oare conducted using thermodynamic data from Nawmnov, Robie & Waldbaum and Zen(for △G_(f, 298, muscovite)~0 = -1,339.9 kcal/gf). The results show that the sillimanite-hornfelsin this region are formed at approximately 660℃ and 2,000 bars. Calculated equi-librium temperatures and pressures of this reaction are in good agreement with theexperimental data. But using the △G_(f, 298)~0 value of muscovite (-1,330 kcal/gfw) aslisted by Robie and Waldbaum, the calculated results are obviously discrepant withfield observations. This value thus is thought to be uncorrect and that calculated fromhydrothermal phase equilibrium data (-1,339.9 kcal/gf) should therefore be recom-mended.