ALKALIC PEGMATITES OF THE AFRIKANDA MASSIF
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Paragenetic sequences observed in ore-bearing and barren pegmatites associated with alkalic ultramafic rocks are described. The Afrikanda massif is located in the western part of the Kola peninsula and is represented by nepheline pyroxenite. In its central part are ore-bearing pegmatites containing concentrations of titanomagnetite, knopite, schorlomite garnet, nepheline, pyroxene, and phlogopite. Processes of replacement are described and a comparison is made of geochemical features of these pegmatites as compared to granitic pegmatites, nepheline syenite pegmatites, and gabbroic pegmatites. --M. Russell.Keywords:
Pegmatite
Massif
Nepheline
Nepheline syenite
Ultramafic rock
Pyroxene
Kola peninsula
Nepheline syenite
Massif
Nepheline
Kola peninsula
Ultramafic rock
Pegmatite
Layered intrusion
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Perovskite-group minerals from the Lovozero peralkaline complex, on the Kola Peninsula, Russia, are primarily loparite-rich members of the perovskite-lueshite-loparite-(Ce) solid-solution series. From early-forming poikilitic nepheline syenite to late eudialyte lujavrite, loparite compositions evolve by enrichment in Na, Sr, and Nb, and depletion in Ca, Ti and light rare-earth elements. The evolutionary trend is from calcian niobian loparite-(Ce) in the poikilitic nepheline syenite and rocks of the differentiated complex through niobian calcian loparite-(Ce) in the differentiated complex and eudialyte lujavrite to cerian lueshite in eudialyte lujavrite. This trend coincides with the proposed order of crystallization of the major intrusive series of the massif. Intra- and intergrain compositional variation and diverse patterns of core-to-rim zonation exhibited by loparite grains from the same sample are characteristic of most parageneses and may result from a combination of re-equilibration phenomena and late-stage metasomatic processes.
Nepheline syenite
Peralkaline rock
Nepheline
Kola peninsula
Massif
Pegmatite
Metasomatism
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Nenadkevichite NaCaTiNbSi4O14.4H20, gerassimovskite NbTi(OH) 9 and tundrite Na2Ce2TiSiO8.4H2O, which until now have been found only in the Lovozero alkaline massif in the Kola peninsula have also been found in the nepheline-sodalite syenites of the Ilímaussaq massif (South Greenland). Nenadkevichite and gerassimovskite occur in hydrothermaI veins in the poikilitic sodalite syenites (naujaites) as alteration products of epistolite. Tundrite was found in pegmatites in nepheline syenites (kakortokites). A new formula of tundrite is suggested.
Nepheline
Sodalite
Massif
Kola peninsula
Nepheline syenite
Pegmatite
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Citations (4)
The pressure-temperature conditions of formation of jadeite-bearing pyroxenes can be calculated from data on the reactions: NaAlSix3O8 + NaAlSiO4 = 2 NaAlSi2O6, NaAlSi3O8 = NaAlSi2O6 + SiO2, where feldspar, nepheline, and pyroxene occur as complex solid solutions. If it is assumed that (a) nepheline can be treated as a binary "sub-regular" solution, (b) pyroxene can be treated as a multicomponent "sub-regular" solution, and (c) the activity coefficient of albite in plagioclase coexisting with jadeitic pyroxene is 1.0, then experimental data yield a relationship between the compositions of coexisting minerals and their temperatures of equilibration. For nepheline-bearing assemblages this relation is: , where T is the temperature in °K, P the pressure in bars, , , and are, respectively, the mol fractions of jadeite, diopside, and hedenbergite in pyroxene, and the mol fractions of other (unspecified) components in pyroxene, the mol fraction of albite in plagioclase, and the mol fraction of KAlSiO4 in nepheline. The corresponding expression for quartz-bearing assemblages: , gives pressure-temperature conditions of equilibration for natural omphacitic pyroxenes in agreement with those estimated by other means. Jadeite-rich pyroxenes can form under amphibolite facies metamorphic conditions in nepheline-bearing assemblages, as demonstrated by metamorphosed alkaline rocks from central Labrador. The model predicts limits on the composition of omphacitic pyroxenes which agree with observation.
Nepheline
Pyroxene
Nepheline syenite
Diopside
Grossular
Alkali feldspar
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Citations (37)
Nepheline
Kola peninsula
Massif
Nepheline syenite
Peninsula
Excursion
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Citations (5)
Naujakasite, Na6(Fe,Mn)Al4Si8O26, long known from the Ilímaussaq alkaline complex, South Greenland, was not reported until 1999 from other occurrences of alkaline rocks in spite of the fact that the mineral is composed of common elements. In 1999, a variety of naujakasite rich in Mn was found in the Lovozero alkaline complex in the Kola Peninsula, Russia. This variety has been approved by the IMA as a new mineral, manganonaujakasite, Na6(Mn0.53Fe2+ 0.47)Al4Si8O26. At Ilímaussaq naujakasite is a rock-forming mineral in the highly evolved rock naujakasite lujavrite in which it may make up more than 75 vol.%; at Lovozero manganonaujakasite is a very rare constituent in mineralised lovozerite–lomonosovite lujavrite. Naujakasite appears to take the place of nepheline in hyper-agpaitic nepheline syenites characterised by exceptionally high Na/K ratios. The nepheline syenites at Ilímaussaq have an average Na/K (atomic) ratio of 3.08, and the naujakasite lujavrites have the extreme ratio 4.56. The nepheline syenites of the Khibina and Lovozero complexes are characterised by lower Na/K ratios, 1.27 for Khibina and 1.67 for Lovozero, and thus nepheline is stable in the hyper-agpaitic rocks and naujakasite occurs only in pegmatites.
Nepheline
Kola peninsula
Nepheline syenite
Pegmatite
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Citations (8)
Zirconolite
Nepheline
Kola peninsula
Nepheline syenite
Baddeleyite
Xenolith
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Citations (1)
Paragenetic sequences observed in ore-bearing and barren pegmatites associated with alkalic ultramafic rocks are described. The Afrikanda massif is located in the western part of the Kola peninsula and is represented by nepheline pyroxenite. In its central part are ore-bearing pegmatites containing concentrations of titanomagnetite, knopite, schorlomite garnet, nepheline, pyroxene, and phlogopite. Processes of replacement are described and a comparison is made of geochemical features of these pegmatites as compared to granitic pegmatites, nepheline syenite pegmatites, and gabbroic pegmatites. --M. Russell.
Pegmatite
Massif
Nepheline
Nepheline syenite
Ultramafic rock
Pyroxene
Kola peninsula
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