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 by
Milton et al. (1965), but its space group is
P31
m. The other phase, to be called ewaldite, is the major constituent of the polycrystals studied by
Desautels (1967) under the name mackelveyite. It is bluish green and crystallizes in
P6
3
mc with
a=5.284±7 (1
(2[`11] 0)(2\overline {11} 0)
and twin symmetry 6
Ba (Ca0.48 RE0.20 Na0.15 K0.11 Sr0.03 U0.02 \square 0.01 )(CO3 )2 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
. For
Z=2 we obtain
Dx=3.37 g/cm
3, as compared with
Dm=3.25±5 g/cm
3. 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 get
nE
(2[`11] 0)(2\overline {11} 0)
und Zwillingssymmetrie 6
Ba (Ca0.48 RE0.20 Na0.15 K0.11 Sr0.03 U0.02 \square 0.01 )(CO3 )2 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
. Für
. Die Elektronensonden-Untersuchung der Polykristalle zeigt eine gleichmäßige Verteilung der Hauptbestandteile; sie weist folglich darauf hin, daß wir es mit zwei kristallinen Formen mit im wesentlichen gleicher chemischer Formel zu tun haben. Die diffuse Röntgenstreuung zeigt, daß Mackelveyit die schlechter geordnete Struktur ist, so daß Mackelveyit durch eine Ordnungs-Umwandlung in Ewaldit übergeht.