Intracrystalline equilibria and im-miscibility along the join clinozoisite - epidote: An experimental and 57Fe Mössbauer study
Fehr, Karl Thomas; Heuss-Aßbichler, Soraya
published: Sep 10, 1997
ArtNo. ESP154017201004, Price: 29.00 €
Members of the clinozoisite-pistacite series (epidotes) are common minerals in caIcsilicate rocks, rodingites, skams and metabasites. The epidote structure contains 3 octahedral sites, two of them, M(I) and M(3), can be occupied by Al and Fe3+. The occupancies lead to an intracrystalline substitutional disorder state, which can be described by the reaction Fe3+, M (3) + AI, M(I) = Fe3+, M(I) + AI, M(3) (I) The intracrystalline equilibrium conditions of reaction (I) were determined for natural epidotes of various compositions (0.98 >= Fe pfu >= 0.39), heat treated at 500 up to 650 °C/0.3 GPa, HM buffer. The site occupancies of Fe3+ were determined by means of Mossbauer spectroscopy. Kinetic studies show that intracrystalline equilibrium is achieved within 5 days. The amount of Fe3+ on M(I) rises with increasing temperature up to 9.1 % and 4.2 % at 650 °C for epidotes with 0.98 and 0.79 Fe pfu, respectively. At temperatures: >= 500 °C all Fe3+ is exclusively accommodated on the distorted M (3) site. In epidotes with Fe pfu < 0.7 this is the case at any temperature. the values for equilibrium constants for the reaction (i) are significantly lower with respect to the data of bird & helgeson (1980), because their data were obtained on metastable disordered synthesis experiments. at intermediate compositions (0.5 < fe pfu < 0.7) two doublets of fe3+ on M (3) occur in the Mossbauer spectra due to immiscibility of two different short-range ordered epidote phases. The resulting solvus shows steep slopes and a noticeable asymmetric shape in the T-X-section, revealing a critical temperature of 740 °C/0.3 GPa, HM buffer. This solvus cannot be described by the mixing models of Bird & Helgeson (1980) and Perchuk & Aranovich (1979). At Fe-poor compositions 0.25 < Fe pfu < 0.5 a second miscibility gap occurs along the join clinozoisite-epidote. Its critical temperature exceeds 600 °C at 0.3 GPa, HM buffer.