Chemical composition, statistical analysis of the unit cell, and electrostatic modeling of the structure of Al-saturated chlorite from metamorphosed rocks

Abstract

Natural Al-saturated chlorite having a wide range of Mg:Fe ratios [Mg/(Mg + Fe) = 0.357 to 0.943] was studied to determine the effect of Mg-Fe substitution on the unit-cell parameters. With a nearly constant Al content [Al_T/(Al_T + Mg + Fe) = 0.362 to 0.416, where Al_T represents total Al contents in both tetrahedral and octahedral sites], the only major variable affecting the unit-cell dimensions of natural chlorite is the Mg:Fe ratio. The value of about 0.4 for the Al content represents apparent Al saturation for chlorites in metamorphosed rocks. Unit-cell parameters were determined by least-squares refinement from 39 chlorite samples obtained over a large range in metamorphic grades, from X-ray data derived from Gandolfi pattern simulations (via single crystals and a single-crystal diffractometer) from this study and from traditional powder diffractometer data from the literature. Nine of the samples were analyzed for their Fe³⁺ contents (= 0.147 to 0.304 atoms) by Mössbauer spectroscopy; chemical compositions were generally obtained by electron-microprobe analysis.

Statistical tests show that the change of the crystallographic a and b axes are linear over the range of Mg:Fe ratios studied, whereas the crystallographic c axis and the β angle have no significant relationship to these Mg:Fe ratios. Equations relating the effect of Mg:Fe ratio on a and b are: a (Å) = −0.092 x + 5.408; x = Mg/(Mg + Fe) and b (Å) = − 0.162 x + 9.370, respectively.

Electrostatic models involving Pauling.s electrostatic valency principle for Al-saturated chlorite were determined assuming: (1) Al-Al avoidance for Al-substituted tetrahedra, (2) no vacancies, and (3) no Al³⁺ substitution in the M1 sites. Electrostatic restrictions occur for R³⁺ → R²⁺ substitutions in the M2 sites of the 2:1 layer. These restrictions produce an Al saturation in chlorite at Al_T/(Al_T + Mg + Fe) ratios of near 0.4. This saturation limit requires that 37.5% of the tetrahedral sites (1.5 out of 4 sites) are occupied by Al, and the electrostatic charge (electrostatic valency units, e.v.u.) close to +1 e.v.u. is favored for the interlayer to offset the net negative charge on the 2:1 layer.