Synthesis of aenigmatite

• Main Authors: R. N. Thompson, J. E. Chisholm
• Other Authors: The Pennsylvania State University CiteSeerX Archives
• Format: Text
• Language: English
• Published: 1969
• Subjects: Greenland; Kelsey
• Online Access: http://citeseerx.ist.psu.edu/viewdoc/summary?doi=10.1.1.622.6686; http://www.minersoc.org/pages/Archive-MM/Volume_37/37-286-253.pdf

at 7oo ~ and iooo bars Pn~o, from an oxide starting material. The oxygen fugacity was controlled by means of an iron-wiistite buffer. An indexed X-ray powder pattern of synthetic aenigmatite is given, together with comparative data on two natural samples. One of the aenigmatites, from Kola, U.S.S.R., is known to be triclinic (Kelsey and McKie, 1964). The powder patterns of this and the synthetic aenigmatite are so similar that it is concluded that the latter is also triclinic, at least at room temperature. AENIGMAT1TE (cossyrite) is a fairly common constituent of sodium-rich peralkaline igneous rocks. The unit cell of aenigmatite was recently determined by Kelsey and McKie (1964), who also surveyed the available chemical analyses. They found that modern analyses did not vary greatly, and suggested an idealized formula of Na4[Fe~Ti2] Si18040. Ernst (I 962) synthesized a titanium-free ' aenigmatite ' during the course of a study of alkali amphibole stability relations, using the bulk composi-tion Na20.5FeO~. 8SiO2+excess H20. An X-ray powder diffractometer trace of this mineral gave similar d-spacings for principal peaks to those of a naturally occurring aenigmatite (U.S.N.M. 95499) from Julianchaat, Greenland. The titanium-free ' aenig-