J. D. H. Donnay

Ewaldite, a new barium calcium carbonate

Mackelveyite type specimens turn out to be polycrystals consisting of two phases in syntactic intergrowth. One phase, for which the name mackelveyite is retained, is lime yellow, but usually shiny black owing to amorphous carbonaceous inclusions. Optically biaxial, nearly uniaxial, it is hexagonal by x-ray diffraction and has the cell dimensions reported byMilton et al. (1965), but its space group isP31m. The other phase, to be called ewaldite, is the major constituent of the polycrystals studied byDesautels (1967) under the name mackelveyite. It is bluish green and crystallizes inP63mc witha=5.284±7 (1 σ) andc=12.78±1 (1 σ) A. The intergrown crystals are turned 30° with respect to each other about a commonc direction; their cell dimensions obey the relations 3ce∼2cm and 2ae cos 30°∼am, whence 9Ve∼2Vm. Mackelveyite shows twinning by merohedry, twin plane\((2\overline {11} 0)\) and twin symmetry 6′m′ m. Ewaldite has not been observed to twin. The ewaldite formula is

Crystal data : determinative tables

Ba (Ca_{0.48} RE_{0.20} Na_{0.15} K_{0.11} Sr_{0.03} U_{0.02} \square _{0.01} )(CO_3 )_2

Omission solid solution in magnetite: Kenotetrahedral magnetite

. ForZ=2 we obtainDx=3.37 g/cm3, as compared withDm=3.25±5 g/cm3. Electron-probe analysis of polycrystals shows uniform concentration of major elements, thus indicating that we are dealing with two crystalline forms of essentially the same chemical formula. For nearly pure ewaldite we getnE≤1.572,nO=1.646±3. Polycrystals in which ewaldite is the dominant constituent show a morphology that is characteristically that of mackelveyite. Ewaldite is thus pseudomorphous after mackelveyite. X-ray diffuse scattering shows mackelveyite to be the more disordered structure, so that mackelveyite goes to ewaldite by an ordering transformation.

Contribution to the crystallography of uranium minerals

Two charts are presented. The first one gives, from a rotation photograph, the direct lattice translation of the rotation axis. From a zero‐layer undistored Weissenberg photograph, the second chart gives the reciprocal lattice translations perpendicular to the rotation axis. The nomographs are constructed for the standard camera radius (R = 180/2π mm), but they can be used with any wavelength.