Clintonite-1M: crystal chemistry and its relationships to closely associated Al-rich phlogopite

Crystal-structure refinements were performed on clintonite-1M crystals [ideal composition [6]Ca[6][([Mg.sub.2]Al).sub.[4](Si[Al.sub.3])[O.sub.10][(OH).sub.2]] from skarns of the Predazzo-Monzoni area and Adamello Massif (northern Italy) with the aim of characterizing some aspects of their crystal chemistry and their relationships with closely associated phlogopite-1M. In the clintonite samples examined, the tetrahedral composition ranges from [Si.sub.1.19][Al.sub.2.78][Fe.sub.0.03] to [Si.sub.1.28][Al.sub.2.70][Fe.sub.0.02] indicating that the extent of the exchange vector [4][Al.sub.-1][6][Mg.sub.-2][4]Si[6](Al, ??), which links trioctahedral with dioctahedral Ca-bearing brittle micas, was very limited. Single-crystal X-ray diffraction data were collected and structure refinements completed in space group C2/m converging to [R.sub.obs] from 0.027 to 0.037 for six samples. The [4][Al.sup.3+] for [4][Si.sup.4+] substitution, which is close to 70%, produces more regular and flatter tetrahedra than in the case of phlogopite, together with an increase in the thickness and in the lateral dimensions of the sheet; the presence of [Al.sup.3+] in octahedral coordination, on the other hand, reduces the dimensions of both M1 and M2 sites with a consequent decrease in the thickness of the sheet. The volume, the flattening angle [Psi], and the central cation off-center shift (BLD) of the trans M1 octahedral site are greater than those of the cis M2 site, thus indicating a normal octahedral ordering. The high misfit value (from 1.187 to 1.326 [Angstrom]) between tetrahedral and octahedral sheets is mostly compensated by the distortion of the tetrahedral ring (tetrahedral rotation angle [Alpha]: 23.1 [less than or equal to] [Alpha] [less than or equal to] 24.9[degrees]). Relative to phlogopite, the interlayer separation in clintonite is reduced by about 0.6 [Angstrom].