Insights into the crystal chemistry of the serandite–schizolite–pectolite series

Abstract

ABSTRACT The compositional series {Na} [[M1+M2](Ca2XMn2–2X)Si3O8(OH)] includes the minerals serandite (X = 0), schizolite (X = 0.5), and pectolite (X = 1). Six crystals are structurally and chemically characterized in detail (four from Ilímaussaq, Greenland; two from Mont Saint-Hilaire, Québec, Canada): one serandite, one pectolite, and four schizolite crystals. Those originating from Greenland show up to 0.05 apfu LREE3+ (La + Ce + Pr + Nd + Eu + Gd). For each, all H atoms were located, and final R1 factors were below 4.4% (<R1> = 2.35%). The results are compared with previously published crystal structure data from an additional 16 samples, originating from worldwide (mostly) igneous environments. Across the series, for all investigated samples, Ca and Mn order preferentially at the octahedral M1 and M2 sites, respectively, following the exchange M2Ca + M1Mn ↔ M1Ca + M2Mn. Site-occupancies closely adhere to a two-site distribution coefficient, calculated here to be K = 20.0(5), for ideal mixing where activity coefficients approach unity. For the above order-disorder exchange, ΔHex is calculated to be –1.77 kcal. With knowledge of K, site assignment and species determination may be accurately made solely with compositional data, where 0 ≤ ΣCa < 0.55 apfu, serandite; 0.55 ≤ ΣCa < 1.45 apfu, schizolite; and 1.45 ≤ ΣCa ≤ 2 apfu, pectolite, with the dominant-constituent rule mandating M1Ca < M1Mn (M2Ca < M2Mn), serandite; M1Ca > M1Mn (M2Ca < M2Mn), schizolite; and M1Ca > M1Mn (M2Ca > M2Mn), pectolite. Polyhedral distortion and structural strain at the M1 and M2 sites, calculated using the equations of Robinson et al. (1971) and solutions to Kirchhoff network equations, respectively, show a predictable, cooperative variation across the entire compositional series; however, prominent discontinuities in distortion and strain behavior are observed for the schizolite composition.