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

https://doi.org/10.31857/S0869605521030114

Full text is available on eLIBRARY.RU

Language: Russian 

Abstract

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: Na_1.97–2.01Ca_1.96–1.98[Al_5.87–6.01Fe³⁺_0-0.02Si_9.04-9.11O₃₀] ⋅ n(H₂O). Mesolite forming spherulites has low-calc composition: Na_1.96К_0.01Сa_1.93[Al_5.93Si_9.09O₃₀] ⋅ n(H₂O). Mesolite of open spherulites is poor in sodium: Na_1.87–1.89Сa_1.94–1.95[Al_6.03–6.07Si_9.00–9.04O₃₀] ⋅ 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: Na_5.84–6.01Сa_0.02–0.06[Al_5.92–5.98Si_9.00–9.07O₃₀] ⋅ n(H₂O).

Keywords: zeolites, ion exchange, mesolite, natrolite, analcime, spilites, albite, orthoclase, titanite, clinopyroxene, the Karadag volcanic massif, the Mountain Crimea