The chemical composition of carpholite can be represented as M2Al4Si4O12(0H)8 (M = Mn2+, Fe2+, Mg2+). The nonstoichiometric Li+-, K+-, F- -bearing carpholite that we investigated occurs as white to straw-yellow needles in miarolitic cavities in the Sawtooth batholith, Idaho, and has the composition Kx(Mn2-xLix)Al4Si4(OH)4F4 with x ≈ 0.65. It is orthorhombic, space group Ccca, with a = 13.715(5), b = 20.302(7), c = 5.138(3) Å, and Z = 4. The atomic positions including H have been refined to an R factor of 0.022 on the basis of 852 X-ray reflections. The crystal structure consists of pyroxene-like slabs [T-O-T layers with single silicate chains and Al(1) octahedral bands] parallel to the c axis, crosslinked by (Li+, Mn2+) and Al(2) octahedral bands. Half the (OH)- ions in normal carpholite are replaced by F-ions in an ordered fashion. Li+, replacing Mn2+ in a distorted octahedral coordination, occupies a site distinct from the Mn2+ position. The K+ ions partially occupy a large cavity with sixteenfold coordination [O8F8], which can be described as a truncated octagonal bipyramid. These cages share rectangular faces with adjacent ones to form channels parallel to c, which are peculiar to the carpholite structure. There is an additional channel site comparable to the A site in the amphiboles. The carpholite structure type is amenable to a large variety of cationic and anionic substitutions.

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