Microtextural and Powder-Diffraction Study of Analcime Phenocrysts in Volcanic Rocks of the Crowsnest Formation, Southern Alberta, Canada

Abstract

We present results of powder X-ray-diffraction and microstructural studies of analcime from the volcanic rocks of the Crowsnest Formation, Alberta, Canada. The powder X-ray diffractogram of only one of the analcime samples contains a peak inconsistent with the cubic symmetry that has been attributed to conversion from leucite. We re-evaluate this criterion, and suggest that primary igneous analcime may be cubic as well; caution thus is required in deducing paragenesis from crystal symmetry. A more compelling argument for the primary nature of the analcime is that fresh fracture-surfaces are clean and planar, the characteristics suggested by previous workers to be indicative of crystallization from a melt or fluid. The fracture surfaces lack the porous microstructure previously attributed to the conversion of leucite to analcime. SOMMAIRE Nous présentons les résultats d'études en diffraction X (méthode des poudres) et microstructurales de l'analcime des roches volcaniques de la formation de Crowsnest, en Alberta, Canada. Un des trois échantillons contient une raie en violation de la symétrie cubique, antérieurement attribuée à une transformation de l'analcime à partir de la leucite. Nous ré-évaluons la validité de ce critère, et nous préférons l'hypothèse que la symétrie cubique pourrait aussi caractériser l'analcime ignée primaire. Il est donc nécessaire d'utiliser ce critère paragénétique avec circonspection. Il semble plus sûr de se fier sur la microstructure des phénocristaux. Les surfaces de fractures sont propres et planaires dans le cas de l'analcime fraiche et primaire, indication d'une croissance à partir d'un bain fondu ou bien d'une phase fluide. Les surfaces de fractures sont dépourvues des microstructures poreuses attribuées dans la littérature à la transformation de la leucite en analcime.(Traduit par la Rédaction)

Phase equilibria in NaAlSiO4–KAlSiO4–SiO2–H2O at 100 MPa pressure: equilibrium leucite composition and the enigma of primary analcime in blairmorites revisited

“…The formation of primary analcime described above took place in a deep crustal chamber (P H 2 O 800À1000 MPa; depth 28À35 km) at 600À700ºC and we agree with Luth and Bowerman (2004) that the absence of amphibole and biotite indicates crystallization under oxidizing conditions. The absence of amphibole suggests that the oxygen fugacity was more oxidizing than the QFM buffer, perhaps with logf O2 in the range À21 to À19 (Thomas, 1982;Gilbert and Popp, 1982).…”

“…Figure 5f shows different levels through the lamellar structure of the rim with the smooth surface at the bottom right corner representing unaltered core material. The texture of the altered rim is identical to that described as a weathered analcime surface by Luth and Bowerman (2004). The glassy surfaces of the fresh analcime are identical to those assumed to characterize primary analcime (Luth and Bowerman, 2004) and we concur with their conclusion that these analcimes are indeed primary phenocrysts.…”

“…Pearce (1970Pearce ( , 1993 suggested that clear/glassy yellow analcime phenocrysts from Crowsnest are unaltered and represent primary material and Peterson et al (1997) concluded that O-isotope data for such samples are consistent with primary magmatic formation at~700 MPa, 700ºC. Luth and Bowerman (2004) showed that the freshlyfractured grains of unaltered Crowsnest analcime phenocryts (honey-brown in colour) have clean, planar fractures rather than the porous, finegrained textures typical of X-type analcime. Luth and Bowerman (2004) also suggested that the absence of hydrous minerals at Crowsnest reflects crystallization under oxidizing conditions (cf.…”

“…Luth and Bowerman (2004) showed that the freshlyfractured grains of unaltered Crowsnest analcime phenocryts (honey-brown in colour) have clean, planar fractures rather than the porous, finegrained textures typical of X-type analcime. Luth and Bowerman (2004) also suggested that the absence of hydrous minerals at Crowsnest reflects crystallization under oxidizing conditions (cf. Bailey, 1969;Gilbert and Popp, 1982).…”

“…Several authors have advocated that the large analcime macrocrysts found in analcime phyric phonolites (blairmorites) from Crowsnest Pass, Alberta, Canada (Pearce, 1970(Pearce, , 1993Roux and Hamilton, 1976;Ferguson and Edgar, 1978;Peterson et al, 1997;Luth and Bowerman, 2004) and Lupata Gorge, Mozambique (Woolley and Symes, 1976) are examples of primary igneous minerals crystallized intratellurically at depth before eruption. However, other authors have suggested that such analcimes are pseudomorphs after primary trapezohedral leucite phenocryts (X-type analcimes) and are formed by low-temperature Na-ion exchange (e.g.…”