Abstract

This paper presents a microbeam (electron microprobe, Raman spectroscopic and X-ray microdiffraction) study of cancrinite-group minerals of relevance to alkaline igneous rocks. A solid solution is known to exist between cancrinite and vishnevite with the principal substitutions being forumla by forumla and Ca for Na. In the present study, several intermediate members of the cancrinite–vishnevite series from a syenitic intrusion at Cinder Lake (Manitoba, Canada), were used to examine how chemical variations in this series affect their spectroscopic and structural characteristics. The Cinder Lake samples deviate from the ideal cancrinite–vishnevite binary owing to the presence of cation vacancies. The only substituent elements detectable by electron microprobe are K, Sr and Fe (0.03–0.70, 0–0.85 and 0–0.45 wt.% respective oxides). The following Raman bands are present in the spectra of these minerals: ∼631 cm–1 and ∼984–986 cm–1 [forumla vibration modes]; ∼720–774 cm–1 and ∼1045–1060 cm–1 [forumla vibration modes]; and ∼3540 cm–1 and 3591 cm–1 [H2O vibration modes]. Our study shows a clear relationship between the chemical composition and Raman characteristics of intermediate members of the cancrinite–vishnevite series, especially with regard to stretching modes of the forumla and forumla anions. From cancrinite-poor (Ccn6.5) to cancrinite-dominant (Ccn91.3) compositions, the forumla vibration modes disappear from the Raman spectrum, giving way to forumla modes. X-ray microdiffraction results show a decrease in unit-cell parameters towards cancrinite-dominant compositions: a = 12.664 (1) Å, c = 5.173(1) Å for vishnevite (Ccn22); a = 12.613 (1) Å, c = 5.132(1) Å for cancrinite (Ccn71). Our results demonstrate that Raman spectroscopy and X-ray microdiffraction are effective for in situ identification of microscopic grains of cancrinite–vishnevite where other methods (e.g. infrared spectroscopy) are inapplicable. The petrogenetic implications of cancrinite–vishnevite relations for tracing early- to late-stage evolution of alkaline magmas are discussed.

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