Frondel and Bauer (1953) have shown that the friedelite family of minerals can be divided into three sections: pyrosmalite and mangan-pyrosmalite, with c = 7.15 Å; schallerite, with c≈ 14.3 Å (both Type I with As2O3 ≈ 12% and Type II with As2O3 ≈ 6 to 8%); and friedelite and ferroschallerite with c = 21.43 Å. Taking one analysis of manganpyro-smalite, they calculated the empirical unit-cell contents included (with a small correction) in Table 1 below (column 1); these clearly indicate a unit-cell formula [(Mn, Fe, Mg, Zn)16Si12O30(OH, Cl)20], neglecting the As, too small in amount to locate. Noting the simple multiple relation in their cell-dimensions, they concluded that the unit-cell contents of pyrosmalite, schallerite, and friedelite (including ferroschallerite, regarded as an arseniferous variety of friedelite) are respectively: 2[(Mn, Fe)8Si6O16(OH, Cl)10], 4[(Mn, Fe)8(Si, As)6O15(OH, Cl)10], and 6[(Mn, Fe)8(Si, As)6O15(OH, Cl)10].

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