Magdalena Dumańska-Słowik

“Silicified” pyrochlore from nepheline syenite (mariupolite) of the Mariupol Massif, SE Ukraine: A new insight into the role of silicon in the pyrochlore structure

Abstract Pyrochlore-supergroup minerals containing relatively high Si concentration are quite common in various geochemical parageneses, e.g., carbonatites, alkaline syenites, pegmatites. However, the role of Si and the mechanism of its incorporation into the structure of these minerals, although widely discussed, have not been explained definitively. Our paper reports the results of comprehensive SEM, EPMA, XRD, TEM, and MAS-NMR studies performed for the first time on a natural pyrochlore, which is the late-magmatic to early hydrothermal accessory component of the nepheline syenite in the alkaline Mariupol massif in Ukraine. It represents partly metamict, patchy-zoned, A-cation depleted, REE-, U-, and Th-bearing fluornatropyrochlore, locally exceptionally rich in SiO2 (up to 13.01 wt%) that underwent both primary and secondary alterations, leading to kenopyrochlore or hydropyrochlore species. The primary alteration was induced by high-temperature, Ca2+- and Si4+-rich, and F- moderate fluids, which affected only some domains of the pyrochlore crystals and resulted in filling the A site vacancies mainly by Ca2+, but also Mn2+, Sr2+, and K+. The secondary alteration, induced by the exposure of the host rock to ground water driving fluid-mediated coupled dissolution-reprecipitation process, affected the whole pyrochlore crystals (both Si-enriched and Si-free domains) and caused, among others, the leaching of some A- and Y-site components. TEM investigations indicate that the selected-area electron diffraction patterns taken from Si-poor areas show strong and sharp diffraction spots related to well-crystalline pyrochlore, whereas the Si-rich areas show weaker spots with a diffuse diffraction halo that are typical of metamict material. Due to the fact that no intergrowth with other Si-bearing phases was observed in the TEM images even at very high magnification, it might be concluded that Si4+ can occupy severely a-decay damaged and chemically altered portions of this structure. The absence of Si in the sixfold-coordinated B site has been corroborated both by compositional relationships, and by the lack of any [6]Si4+ signal around -200 ppm in the MAS-NMR spectrum. A broad signal in the spectrum appearing at around -84 ppm, points to an amorphous species with tetrahedrally coordinated Si, close to Q(2) species defined as Si atom with two bridging O atoms, i.e., [Si(OSi)2(-)2], in the form of finite-length chain-like structures, located in the damaged A and B sites of the primary structure.

Cancrinite from nepheline syenite (mariupolite) of the Oktiabrski massif, SE Ukraine, and its growth history.

Secondary cancrinite, (Na5.88K<0.01)∑5.88(Ca0.62 Fe0.01Mn0.01Zn<0.01 Mg<0.01)∑0.64[Si6.44Al 5.56O24](CO3)0.67(OH)0.26(F<0.01,Cl<0.01)·2.04H2O), was found as accessory component of mariupolite (albite-aegirine nepheline syenite) from the Oktiabrski massif in the Donbass (SE Ukraine). It probably crystallized from a subsolidus reaction involving nepheline (and sodalite?) and calcite dissolved in the aqueous-carbonic fluid at the maximum temperature of 930 °C, decreasing to hydrothermal conditions. It is depleted in sodium, calcium and carbon, what results in the occurrence of vacant positions at both cationic and anionic sites. Ca-deficient cancrinite crystallized from the same hydrothermal Si-undersaturated fluids enriched in the ions such as Na(+), Ca(2+), Cl(-), F(-), HCO3(-), which formed calcite, sodalite, natrolite and fluorite. It has dark-red CL colours with patchy zoning, what indicates the variable/diverse fluid composition during its formation. In the CL spectrum of cancrinite only one broad emission band at 410 nm is observed, which can be attributed to O* center (the recombination of a free electron with an O(-) hole center). The formation of secondary CO3-rich species, i.e. cancrinite and calcite in mariupolite suggests that redox conditions in the Oktiabrski massif were oxidizing at the postmagmatic stage.

Micas from mariupolite of the Oktiabrski massif (SE Ukraine): an insight into the host rock evolution--geochemical data supported by Raman microspectroscopy.

Muscovite and two dark mica varieties (the coarse-crystalline, pegmatitic, and fine-crystalline with signs of early weathering) representing members of the biotite series, originating from mariupolite of the Oktiabrski massif, (Ukraine), were investigated along with their solid inclusions using electron microprobe and Raman micro-spectroscopy to discuss their genesis and relationship to the parental magma. The coarse-crystalline, pegmatitic biotite, (K1.90Rb0.02Na0.01)(Fe3.56(2+)Mg1.34Ti0.36Fe0.34(3+)Mn0.03)[(Si5.73Al2.10Fe0.17(3+))O20](OH3.24 F0.76) represents the primary, magmatic annite that crystallized from an alkaline, Fe-rich and Mg-depleted host magma, whereas the fine-crystalline biotite, partly altered to vermiculite, (K1.75Rb0.03Na0.03)(Fe3.23(3+)Fe1.16(2+)Mg0.26Mn0.04Ti0.10)[(Si5.16 Al2.84)O20](OH)4.00, devoid of F, represents a re-equilibrated or secondary, post-magmatic Fe(3+)-bearing mica crystallized from alkaline to the subalkaline host magma. Muscovite, (K1.96Na0.06)(Al3.97Fe0.06(2+))[(Si5.99Al2.01)O20](OH)4, with low Na/(Na+K) ratio, low Fe and devoid of Ti and also F, forms only tiny, subhedral flakes formed in the post-magmatic, hydrothermal stage. The primary, unaltered biotite contains numerous solid inclusions of primary origin (albite, aegirine, zircon, K-feldspar, nepheline, pyrochlore, magnetite) and secondary origin (natrolite, hematite, Ti-Mn oxides/hydroxides); most of them are accompanied by a carbonaceous substance, all confirmed by scanning electron microscopy and Raman microspectroscopy.

Inclusions in topaz from miarolitic pegmatites of the Volodarsk-Volynski Massif (Ukraine) – A Raman spectroscopic study

The differently coloured (colourless, brown-pinkish and blue-pinkish) crystals of topaz from granitic pegmatites of Volodarsk-Volynsky Massif (VVM) have been investigated by scanning electron microscopy (SEM) and Raman microspectroscopy (RS) methods. Topaz (287, 522, 855, 929 cm(-1)), goethite (390 cm(-1)), pyrite (377-379 cm(-1)), marcasite (397 and 331 cm(-1)) and monazite (460 and 970-1070 cm(-1)) were identified as mineral inclusions in analysed crystals. On the basis of RS spectra some of this inclusions contain also organic matter, represented by carbonaceous matter (D-band at ca. 1320-1340 cm(-1) and G-band at ca. 1590-1600 cm(-1)) and liquid simple hydrocarbons consisting of aliphatic and aromatic groups (1240, 1325 and 1420 cm(-1)). Other solid phases found the host topaz, i.e. quartz, orthoclase, very rare minerals (micas) as lepidolite, zinnwaldite and also beryl and rutile, were identified with SEM-EDS analyses. All these mineral inclusions have been formed by post-magmatic, fluid-induced processes, extended from pegmatite to hydrothermal stages of magma crystallization.

The transformation of nepheline and albite into sodalite in pegmatitic mariupolite of the Oktiabrski Massif (SE Ukraine)

Sodalite, Na8Al6Si6Cl2, from a pegmatitic variety of mariupolite in the Oktiabrski Massif (SE Ukraine) was studied using electron microprobe, electron microscopy, spectroscopic cathodoluminescence and Raman techniques to determine its growth history during the evolution of the host rock. Three generations of the mineral were distinguished: (1) the oldest forms patches with a pink-violet cathodoluminescence colour, (2) a younger one, with a dark blue colour, forms the matrix of the crystals, and (3) the youngest generation forms veins with light blue cathodoluminescence in the older sodalite generations; all are undoubtedly secondary phases formed during the post-magmatic evolution of the host rock. The close spatial association of the sodalite with coexisting albite, nepheline, natrolite and K-feldspar, forming inclusions in each other, and the embayed contacts of sodalite with nepheline and albite, and the patchy appearance of sodalite under CL, together suggest that the two older sodalite varieties formed from the conversion of nepheline and albite under the action of Na-, Cl- and Al-bearing, but Si undersaturated basic fluids released during cooling of the host. The excess of SiO2 (aq.) released as a result of albite metasomatism could be accommodated by natrolite occurring as tiny inclusions within the sodalite crystals. The youngest, veinlet, generation was probably formed via a fluid-mediated dissolution-recrystallization process, perhaps simultaneously with the coexisting veins of natrolite.

“Watermelon” tourmaline from the Paprok mine (Nuristan, Afghanistan)

The “watermelon” tourmaline from the Paprok mine (Nuristan, Afghanistan) shows different tourmaline components. Four zones differ widely what is refl ected by different chemistry and various colours (pink core and green rim). All zones show a relatively low Mn content (~0.05 apfu) but different Fe contents. While Fe is in the (pink) core at the detection limit, it increases signifi cantly in the outer (green coloured) zones. In the core (zone I) “fl uor-elbaite” could be identifi ed for the fi rst time for the Nuristan region. Tourmaline compositions of this zone show an elbaitic component of ~55 mol%, ~30 mol% rossmaniteand ~15 mol% liddicoatite-component. Zone II shows an increasing schorland rossmanite-component (up to 40 mol%), while the liddicoatiteand elbaite-components are decreasing. The intermediate zone III shows the highest schorl-component of all zones (up to ~11 mol%), while the elbaite-component is decreasing. In the rim (zone IV) the schorl-component decreases to ~6 mol%, while simultaneously the elbaite-component increases. Our investigation shows that during tourmaline crystallization only the amounts of Fe and Li, which were available for the tourmaline crystallisation, have changed to a signifi cant degree. We believe that at beginning of the crystallisation of the Fe-bearing zones the formation of Fe-rich tourmalines (schorl, foitite) in this pegmatite was in a fi nal stage and therefore in this pegmatitic system Fe increasingly was available. The Mn/(Mn+Fe) ration ranges from ~0.10 to ~1.00. The presence of different mineral inclusions such as stannite and Ca carbonates point to a hydrothermal origin of this tourmaline.

Characteristics and origin of agates from Płóczki Górne (Lower Silesia, Poland): A combined microscopic, micro-Raman, and cathodoluminescence study

Agates from Płóczki Górne hosted by Permian basaltic rocks are predominantly made of length-fast chalcedony, and subordinately megaquartz and quartzine. Moganite occurs in traces mainly in transparent, outer, darker regions of white-grey coloured agates. Silica matrix of agates comprises a wide variety of solid inclusions represented by celadonite, plagioclases, hematite, goethite, barite, calcite, heulandite-clinoptyloite, nontronite-saponite, and Mn-dioxides (ramsdellite). Mineral phases are locally accompanied by black aggregations of carbonaceous matter, which gives a Raman signature of disordered carbon. These organic components were probably deposited from a hydrothermal fluids at low-temperature conditions and originated from sedimentary rocks found in the surrounding area of Płóczki Górne. The abundance of various SiO2 phases, mineral inclusions as well as various micro-textures (colloform, comb, feathery, and jigsaw-puzzle) in agates resulted from physicochemical fluctuations of SiO2-bearing mineralizing solutions at various stages of these gems formation. Agates from Płóczki Górne formed during post-magmatic stage of basaltic host rocks evolution. Not only were the hydrothermal fluids enriched in silica, but also they contained other elements such as Na, Ca, Al, Mg, Mn, Fe, Ba, SO4, and CO2, which were mobilized from host rocks or surrounding area.