Crystal-structure refinements were performed on clintonite-1M crystals [ideal composition (super ) Ca (super ) (Mg 2 Al) (super ) (SiAl 3 )O 10 (OH) 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 (super ) Al (sub -1) (super ) Mg (sub -2) (super ) Si (super ) (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 obs from 0.027 to 0.037 for six samples. The (super ) Al (super 3+) for (super ) Si (super 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 (super 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 Aa) between tetrahedral and octahedral sheets is mostly compensated by the distortion of the tetrahedral ring (tetrahedral rotation angle alpha : 23.1< or =alpha < or =24.9 degrees ). Relative to phlogopite, the interlayer separation in clintonite is reduced by about 0.6 Aa.