Pyriboles that consist of mixed double chains (anthophyllite), alternating double and triple chains (chesterite), and triple chains (jimthompsonite) coexist with phlogopite and Zn-rich spinel at Orijärvi, southwestern Finland. Each of these three pyribole-structure types has been observed as lamellae as wide as a few thousand ångströms, and electron-diffraction photographs give strong and sharp reflections for all three of these phases. However, the pyriboles are predominantly composed of large disordered areas, in which the proportions of triple-chain units to double-chain units are highly variable. Electron-microprobe analyses indicate that SiO2, A12O3, and Na2O increase and FeO and CaO decrease in the jimthompsonite relative to the anthophyllite. These chemical variations can be explained by different proportions of A sites, octahedral sites, and tetrahedral sites in the double- and triple-chain structures.

Assuming that Fe2+ and Mg are partitioned similarly between the distorted octahedral (dM) and regular octahedral (rM) sites of the anthophyllite and the jimthompsonite, the differing proportions of dM and rM in the two structures allow the estimation of site occupancies and the calculation of an intersite distribution coefficient
(Kd)FeMgdMrM
= 13.868. On the basis of the work of Seifert (1977), this value suggests a metamorphic temperature of about 600°C, which is consistent with the 550 to 630°C estimated for this area by Schreurs (1985).
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