Compositions and petrogenetic significance of the eudialyte group minerals from Sushina, Purulia, West Bengal
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Keywords:
Nepheline syenite
Peralkaline rock
Nepheline
Aegirine
Baddeleyite
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Peralkaline rock
Pyroxene
Nepheline syenite
Aegirine
Mineral redox buffer
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Nepheline syenite
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Sodalite
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The 360-370-Ma-old Lovozero peralkaline massif (NW Russia) is a layered nepheline syenitic–foidolitic pluton. In the rocks of the massif, late-stage (auto)metasomatic alterations of rock-forming minerals are quite intense. We studied the products of the alteration of nepheline and sodalite via microtextural, microprobe, and spectroscopic methods. We found that these minerals are extensively replaced by the association between natrolite + nordstrandite ± böhmite ± paranatrolite in accordance with the following reactions: 3Nph + 4H2O → Ntr + Nsd + NaOH; 6Nph + 9H2O → Ntr + Pntr + 2Nsd + 2NaOH; Sdl + 4H2O → Ntr + Nsd + NaOH + NaCl, where Nph is nepheline, Ntr is natrolite, Nsd is nordstrandite, Pntr is paranatrolite, and Sdl is sodalite. As a result, about one-third of the sodium from nepheline (and sodalite) is set free and passes into the fluid. This leads to an increase in the Na/Cl ratio and, hence, the pH of the fluid. An increase in pH stabilizes hyperagpaitic minerals (e.g., ussingite, villiaumite, thermonatrite, and trona), which can crystallize in close proximity to pseudomorphized nepheline and sodalite. Thus, the alteration of feldspathoids increases the pH of late-magmatic fluids, which in turn can lead to the crystallization of hyperagpaitic minerals.
Nepheline
Peralkaline rock
Sodalite
Massif
Nepheline syenite
Aegirine
Metasomatism
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Nepheline
Nepheline syenite
Aegirine
Peralkaline rock
Ilmenite
Carbonatite
Alkali feldspar
Amphibole
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