Forming of natrolite after mesolite as a result of the ion exchange mechanism in metabasalts (spilites) of Karadag in the Mountain Crimea
Spiridonov E. M.
Zapiski RMO (Proceedings of the Russian Mineralogical Society). 2021. V. 150. N 3. P. 98-113
Like other pre-Late-Jurassic formations of the Mountain Crimean kimmerids, volcanites of the Karadag massif are subjected to regional low-grade immersion metamorphism of three stages. Calcedony, analcime and laumontite are relics of the first stage formations developed under conditions of the zeolite facies; pseudomorphs of prehnite over laumontite are relics of formations of the second stage appeared in conditions of the early prehnite-pumpellyite facies. The famous Karadag zeolite mineralization arose in the process of the third stage under conditions of the high-to-low temperature subfacies of zeolite facies. The zeoliteforming sequence is: (1) laumontite, (2) heulandite-(Ca), (3) scolecite, (4) mesolite, (5) natrolite, (6) analcime. Mesolite, natrolite and analcime associate with calcite. In metabasalts of the western Karadag, mesolite developes in amygdales and calcite-zeolite veins, forming spherulites and later open spherulites. Composition of mesolite in amygdales is as following: Na1.97–2.01Ca1.96–1.98[Al5.87–6.01Fe3+0-0.02Si9.04-9.11O30] ⋅ n(H2O). Mesolite forming spherulites has low-calc composition: Na1.96К0.01Сa1.93[Al5.93Si9.09O30] ⋅ n(H2O). Mesolite of open spherulites is poor in sodium: Na1.87–1.89Сa1.94–1.95[Al6.03–6.07Si9.00–9.04O30] ⋅ n(H2O). Later natrolite and analcime are concentrated in calcite-zeolite veins. Natrolite was formed by replacement of mesolite in open spherulites as a result of Na → Ca cationic exchange mechanism. There were formed variable types of natrolite pseudomorphs after mesolite, including spotted ones. The probable reaction of the mesolite replacement by natrolite is: Na2Сa2[Al6Si9O30] ⋅ n(H2O) + 2Na2[СО3]sol → Na6[Al6Si9O30] ⋅ n(H2O) + 2Са[СО3]. The composition of natrolite pseudomorphs is variable: Na5.84–6.01Сa0.02–0.06[Al5.92–5.98Si9.00–9.07O30] ⋅ n(H2O).
Keywords: zeolites, ion exchange, mesolite, natrolite, analcime, spilites, albite, orthoclase, titanite, clinopyroxene, the Karadag volcanic massif, the Mountain Crimea