a Determination of the Angle Between the Cleavage Faces of Calcite by Use of X-Rays

The dihedral angle was determined by measuring the angle through which the crystal must be turned to reflect the MoKa line, first from one cleavage face and then from the adjacent face. Of the six crystals studied, one gave imperfectly cleaved surfaces, one showed slightly rippled faces, the others gave for the mean dihedral angle 74^∘{} 55{'}.0±{}.2{'} in excellent agreement with optical determinations. This corresponds to an angle between the edges of the crystal of β=101^∘55.0±.2^'. From the coefficients of thermal expansion the change of β with temperature is dβdt=-.051^' per degree. The volume of a rhombohedron the distance between whose opposite faces is unity comes out 1.09630±{}.00007 at 20^∘{}C.