Compositions, Proportions, and Equilibrium Temperature of Coexisting Two-feldspar in Crystalline Rocks

• Published in: Acta geologica Sinica (Beijing) Vol. 91; no. 3; pp. 875 - 881
• Main Authors: MA, Hongwen, YANG, Jing, SU, Shuangqing, YAO, Wengui
• Format: Journal Article
• Language: English
• Published: Richmond Wiley Subscription Services, Inc 01.06.2017; School of Materials Science and Technology,China University of Geosciences,Beijing 100083,China
• Edition: English ed.
• Subjects: activity equation; Alkalies; Approximation; Approximation method; Calcium; Chemistry; Classification; Composition; Crystalline rocks; Equilibrium; equilibrium temperature; feldspar proportion; Feldspars; Isotopes; Lime; mass balance principle; Minerals; Petrogenesis; Petrography; Petrology; Phase rule; Phases; Potassium; Potassium oxides; Rock; Rocks; Sodium; Temperature; Temperature effects; Thermodynamic equilibrium; mass balance principle; activity equation; equilibrium temperature; feldspar proportion; crystalline rocks
• ISSN: 1000-9515; 1755-6724
• DOI: 10.1111/1755-6724.13315

Compositions, proportions, and equilibrium temperature of coexisting two-feldspar in crystalline rocks are of great importance to classification in petrography and interpretation of petrogenesis. Crystalline rocks are usually composed of 4-6 minerals （phases）, depending on their independent chemical components and the equilibrium temperature of crystallizations. In general, number of mineral phases can be determined by the ＂Phase Rule＂. According to the mass balance principle, bulk composition of coexisting two-feldspar could be evaluated from the bulk chemistry of a rock, provided that the compositions of the coexisting mafic mineral phases containing calcium, sodium, and potassium oxides are determined, e.g., by microprobe analysis. The compositions, proportions, and temperature of two-feldspar in equilibrium can thus be simultaneously resolved numerically from bulk composition of the rock, by incorporating the activity/composition relations of the ternary feldspars with the mass balance constraints. Upon the numerical approximation method presented in this paper, better-quality, internally consistent data on feldspar group could usually be obtained, which would be expected more realistic and accurate in consideration of thermodynamic equilibria in the system of crystalline rocks, as well as bulk chemistry of a rock and the composing minerals.