The importance of P-V-T data for internally consistent data bases - Some examples

It is shown that the position of the mineral equilibria: 2 diaspore + 4 alpha-quartz/coesite = pyrophyllite (1) 6 zoisite = 4 grossular + 5 kyanite + coesite + 3H(2)O (2) 12 zoisite = 7 g rossular + 8 kyanite + coesite + 3 lawsonite (3) 2 zoisite + kyanite + coesite + 7H(2)O = 4 lawsonite (4) in P-T space is very sensitive to changes in P-V-T data. Calculations of reaction (1) using different internally consistent data bas es (Berman, 1988; Holland and Powell, 1990; Chatterjee et al., 1994; Gottsc halk, 1997) are highly inconsistent with respect to the dP/dT slope and abs olute position. These differences are mainly caused by using different valu es for V-298(0) (pyrophyllite) from the literature (cf. Theye et al., 1997) . Based on experimental reversals for curve (1) (Theye et al., 1997), it is recommended to use V-298(0) (pyrophyllite) = 127.82 cm(3) mol(-1) followin g Krupka et al. (1979). As a second example, several equilibria involving z oisite and lawsonite are discussed (reactions 2-4). Calculations of Holland ed al. (1996) based on their new P-V-T measurements of these minerals are contradictory to the experiments of Schmidt and poll (1994), Skrok ed al. ( 1994), and poll and Schmidt (1998). Using the Berman (1988) data base and i ncluding P-V-T data by Comodi and Zanazzi (1996, 1997), Grevel and Nowlan ( 1996), and Grevel et al. (1996), which are almost similar, calculation of r eactions (2)-(4) is in good agreement with the experimental results.