THE CRYSTAL CHEMISTRY OF THE KORNERUPINE-PRISMATINE SERIES. III. CHEMICAL RELATIONS

Abstract

The chemical formula of kornerupine sensu lato may be written as X Y 2 M 7 T 4 V O21 W , where X = □, Mg, Fe2+; Y = Mg, Fe2+; M = Al, Mg, Fe2+, Fe3+; T = Si, Al; V = Si, Al, B, Be; W = OH, F. The total number of cations is variable (14.2 to 14.4 apfu ), the sum of the cation charge is 43+, and the amount of vacancy ( pfu ) is given by □ = 15 − ( S 4+ + S 3+ + S 2+), where S Z+ is the sum of the cations of formal charge Z+. The T -site populations are coupled to both B and Fe2+ content; at moderate Si content (~3.71 Si apfu ), the structure accommodates the largest range in B (0–0.9 apfu ) and Fe2+ (0–1.3 apfu ). Flexibility in the kornerupine structure originates at the T (2)– T (3)– T (2) trimer, where coupled expansion–contraction of tetrahedra and rotation [ T (2)] proceed at the expense of sterically induced distortion of the neighboring M (1) and M (4) octahedra. Both Fe2+ and Mg are approximately disordered over M (1) and M (2), with the maximum [ M (1)Fe2+ + M (2)Fe2+] content coupled to the B content of the crystal. The X site shows the greatest affinity for Fe2+, with no apparent coupling of X Fe2+ to the B content. The Al contents of M (2) and M (3) + M (5) are strongly negatively and positively correlated, respectively, with B content. Condensing the general formula to X Y 2 M 7 T 5 O21 W , the principal (heterovalent) end-member compositions for Be-free crystals are as follows: | X | Y | M | T | | W | |:-- | --- | ------ | ----- | --- | - | | □ | Mg2 | Al5Mg2 | Si5 | O21 | F | | □ | Mg2 | Al7 | Si3B2 | O21 | F | | Mg | Mg2 | Al3Mg4 | Si5 | O21 | F | | Mg | Mg2 | Al7 | SiB4 | O21 | F | where Mg = [8,6](Mg,Fe2+), Al = [6](Al,Fe3+), Si = [4]Si, B = [4](Al,B) and F = (OH,F). These compositions can be represented in the orthogonal system X Mg– M Al– T Si, and define a plane on which the compositions of kornerupine can be graphically represented. Chemical exchange involves the two independent substitutions: X □ + M Al2 = X Mg + M Mg2 and M Mg + T Si = M Al + T B. The principal homovalent chemical variations in kornerupine are T Al ⇌ T B and X , M Fe2+ ⇌ X , M Mg. However, neither show a simple 1:1 substitution. For T Al ⇌ T B, the total amount of trivalent cations ( i.e. , T Al + B) varies with variation in the T Al:B ratio (and hence with a change in the Si content). For X , M Fe2+ ⇌ X , M Mg, the total amount of divalent cations ( i.e. , X + M Fe2+ + X + M Mg) varies with the variation in X + M Fe2+ : X + M Mg ratio (and hence with a change in the M Al or X □ content). However, the slopes of ensuing relations are significantly less than 1.0, as these substitutions are constrained by commensurability requirements arising from the layered nature of the structure and the distribution of sites over which these substitutions act.