Abstract
Stability of "ferriannite," KFe 3 (super +2) Fe (super +3) Si 3 O 10 (OH) 2 , has been determined as a function of total pressure ( P H 2 O + P H 2 ), temperature, and O fugacity. Ferriannite will react with such a gas to form the following assemblages: iron-feldspar + hematite, iron-feldspar + magnetite, melt + magnetite, melt + magnetite + wustite + fayalite, melt + wustite + iron + fayalite. X-ray powder data show that ferriannite is monoclinic with the following cell dimensions: a = 5.430 + or - 0.002 Aa, b = 9.404 + or - 0.003, c = 10.341 + or - 0.006, beta = 100 degrees 4' + + or - 10'. The indices of refraction are: alpha = 1.698 + or - 0.003 to 1.715 + or - 0.003; beta nearly equal gamma = 1.739 + or - 0.002 to 1.755 + or - 0.005. Variations in indices of refraction correspond to variations in the H fugacity of experiments in which ferriannite is the stable phase.
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