Contrasting trends of agpaitic crystallization in nepheline syenite in the Pilanesberg Alkaline Complex, South Africa

Abstract A peralkaline nephelinite lava ([Na+K]/Al 2.15) from the active carbonatite volcano Oldoinyo Lengai, contains combeite, Ba lamprophyllite, a phase with affinities to delhayelite, CeSrNb perovskite, a CaNa phosphate high in Sr, Ba and K, and peralkaline glass; in addition to Fe-rich nepheline, aegirine-rich clinopyroxene and FeK-rich sodalite. The high alkali concentrations relative to alumina in the bulk rock could not have been achieved by fractionational crystallisation of the known Al-rich phenocryst phases (nepheline and sodalite) and some other process must be invoked.

Peralkaline rock

Aegirine

Nepheline

Carbonatite

Sodalite

Nepheline syenite

Phenocryst

10.1180/002646198547567

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Citations (36)

Paragenesis and decomposition assemblage of a Mn-rich eudialyte from the Sushina peralkaline nepheline syenite gneiss, Paschim Banga, India

Lithos (2012)

Roger H. Mitchell Aniket Chakrabarty

Peralkaline rock

Aegirine

Nepheline

Nepheline syenite

Carbonatite

10.1016/j.lithos.2012.02.003

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Citations (15)

The miaskitic-to-agpaitic transition in peralkaline nepheline syenite (white foyaite) from the Pilanesberg Complex, South Africa

Chemical Geology (2016)

Tom Andersen Marlina Elburg Muriel Erambert

Peralkaline rock

Nepheline syenite

Nepheline

10.1016/j.chemgeo.2016.08.020

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Citations (32)

pH changes in peralkaline late-magmatic fluids

Contributions to Mineralogy and Petrology (2002)

Gregor Markl Lukas P. Baumgartner

Peralkaline rock

Nepheline

Aegirine

Nepheline syenite

Analcime

Sodalite

10.1007/s00410-002-0401-6

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Citations (62)

Geochemical behaviour of spilitization in alkaline magmatism, Trias-Lias, of the Iberian Chain and Mallorca (Spain)

Chemical Geology (1988)

M. Lago Dieter Zachmann R. Vaquer A. Pocoví

Aegirine

Peralkaline rock

Nepheline

Nepheline syenite

Amphibole

Titanite

Alkali feldspar

Ilmenite

Carbonatite

10.1016/0009-2541(88)90655-9

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Citations (3)

Fluid–rock interaction in autoliths of agpaitic nepheline syenites in the Ilímaussaq intrusion, South Greenland☆

Lithos (2006)

Jürg Schönenberger Michael A.W. Marks Thomas Wagner Gregor Markl

Aegirine

Nepheline

Peralkaline rock

Nepheline syenite

Enstatite

10.1016/j.lithos.2006.03.024

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Citations (44)

Compositions and petrogenetic significance of the eudialyte group minerals from Sushina, Purulia, West Bengal

Journal of the Geological Society of India (2012)

Aniket Chakrabarty Kamal Lochan Pruseth Amit Sen

Nepheline syenite

Peralkaline rock

Nepheline

Aegirine

10.1007/s12594-012-0069-0

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Citations (8)

Alteration of Feldspathoids Changes pH of Late-Magmatic Fluids: A Case Study from the Lovozero Peralkaline Massif, Russia

Minerals (2022)

Julia A. Mikhailova Yakov A. Pakhomovsky L. М. Lyalina Е. А. Селиванова

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

10.3390/min13010039

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Citations (2)

Contrasting trends of agpaitic crystallization in nepheline syenite in the Pilanesberg Alkaline Complex, South Africa

Lithos (2018)