Pergamon Compurers & Geosciences Vol. 22, No. 2, pp. 165-179, 1996 009?3-3004(95)oooS9-5 Copyright 0 1996 ElsevierScience Ltd Printed in Great Britain. All rights reserved 0098-30&t/96 S15.00 + 0.00 MINENT: A FORTRAN PROGRAM FOR PREDICTION OF ENTHALPY OF FORMATION FROM ELEMENTS OF MINERALS WITH KNOWN CRYSTAL REFINEMENTS PHILIPPE VIEILLARD Laboratoire Argiles, sols et Alterations, URA 721 CNRS, 40, Avenue du Recteur Pineau, 86022 Poitiers Cedex, France (Received and accepted 28 February 1995) Abstract-The computer code MINENT uses the crystallographic properties (bond length, cation oxygen and its estimated standard deviation, nature of cation occupying a site, molecular volume, space group) and the optical properties of any mineral to calculate its enthalpy of formation from elements and its accuracy. This method of computation is applied to all minerals belonging to the following sys- tem: Li,C&Na,O-K,O-BeO-MgO-CaO-SrO-BaO-MnC&Fe0-Co0-Ni0-Znl~ 0, Cr, Or-Fe, O,- Mn,O,-Si02-ZrO,-H,O (20 different cations). This method of computation takes into consideration the following properties: - hypothesis of a unique effective ionic radii for oxygen; - polarizability additivity assumption; - electronegativity based on the parameter A,O*- cation; - difference in electronegativity between the oxide state and the compound is a function of the surrounding of cation (bond length, shortest bond length, polarization of the cation and of oxygens); - presence or absence of bridging oxygen atoms between any two consecutive sites; - presence or absence of hydrogen bondings; - provides a prediction error related to the estimated standard deviation of mean bond length. Two minerals, ferrobustamite, and natrolite, have been selected to show the different steps in the computations. Key Words: Enthalpy of formation, Polarizability, Ferrobustamite, Natrolite, Bond length, Crystal structure, Software, Hydrogen bondings. INTRODUCTION Methods for thermodynamic data prediction are numerous, easy to use, and need only a minimal knowledge of input data, yet produce, in some situ- ations, great discrepancies. Two recent methods, Chermak and Rimstidt (1989) and Zuyev (1987) are the most elaborate methods of prediction and are based on the polyhe- dral model and on electronegativity differences, re- spectively. These two recent methods of prediction provide a reasonable error of estimation: 1% for Zuyev’s method and 0.5% for Chermak and Rimstidt (1989). Limitations of these methods are encoun- tered, however, when some minerals contain ions such as Mn2+, Ni*+, Co2+, Cr3+, Sr2+, Ba2+, and &In’+. By testing with a reduced number of minerals belonging to the system MgO-FeO-ZrO,SiO,, Aja, Wood, and Williams-Jones (1992) have shown that the polyhedral model is too simple because consider- ation of nearest neighbor ionic interaction in minerals structures is absent. Vieillard (1982) and Vieillard and Tardy (1988) have proposed a method for estimating the enthalpy of formation from oxides of any mineral based on the knowledge of crystal structures. This method, which is the result of a series of papers (Tardy and Garrels, 1974, 1976, 1977; Tardy and Gartner, 1977; Tardy and Vieillard, 1977; Vieillard, 1978; Tardy, 1979) is based on the empirical parameter A02- cation in aqueous state by the following expressions: AHO’-MZ+ = AH,” M,,,O(,, - 2/zAH; M;& (1) A.c02-M’+ = AG,“M,,,O,,, - 2/tAG,“M&$. (2) This method of prediction was tested first on compounds with two different oxides (Vieillard and Tardy, 1988) then gradually extended to simple minerals composed of three or more sites. An ex- pression of enthalpy of formation from constituent oxides was proposed for minerals and synthetic com- pounds having several sites or different oxides. A more elaborate method of prediction of enthalpy of formation from elements based on the knowledge of crystal refinements of minerals and synthetic com- pounds exhibiting several sites, or oxides, has been developed (Vieillard, 1994a), and has been applied to a wide number (180) of different minerals and C.4OEO 22,2--E 165