Adam Pieczka

Oxidized tourmalines — a combined chemical, XRD and Mössbauer study

Thermal oxidation of Fe 2+ in tourmalines of the schorl-dravite series induced by gradual annealing of powdered samples in air above 600°C was studied using X-ray diffraction and Mossbauer spectroscopy. The a unit-cell parameter distinctly decreases, while the c parameter even slightly increases during the oxidation as a result of thermal modification of the X, Y, Z, B and T polyhedra sizes, particularly those primarily occupied by Fe 2+ or traces of Mn 2+ . The Y octahedron shrinks proportionally to the initial content of Fe 2+ (Fe 2+ +Mn 2+ ), while the Z octahedron even shows a small expansion. The Δ max. and Δ max. values estimated for 3 Fe 2+ (Fe 2+ +Mn 2+ ) per formula unit (pfu) are -(0.072–0.070) A and +0.009 A, respectively. They indicate that in the tourmaline structure the primary Fe 2+ and traces of Mn 2+ occupied only the Y sites. In Mossbauer spectra of oxidized tourmalines the Fe 2+ doublets with QS > 2 mm/s decreased, while the doublets with QS 2.5+ mixed valence doublet and the Fe 3+ doublets progressively increased with advancing oxidation. These changes are explained in term of progressive generation of Fe 3+ in place of primary Fe 2+ within the triad of the Y sites. The process results in progressive formation of the Me 2+ Fe 2+ Fe 3+ Y-triads, followed by the Fe 2+ Fe 3+ Fe 3+ triads, sometimes with an electron delocalized among adjacent Fe 2+ and Fe 3+ . Finally, triads with the Y sites occupied only by Fe 3+ instead of the primary Fe 2+ are generated and tourmaline completely transformes into the oxidized form. Annealing of tourmalines after complete oxidation increases Fe 3+ —Al disordering among the Y and Z sites up to the structure9s decomposition at around 900°C. The Fe 2+ doublets with QS 2+ at the Z sites result rather from the presence of Fe 2+ at the Y sites and the temperature- or oxygen-fugacity-induced changes in the W site occupancy by hydroxyls and oxygens.

Limitations of Fe2+ and Mn2+ site occupancy in tourmaline: Evidence from Fe2+- and Mn2+-rich tourmaline

Abstract Fe2+- and Mn2+-rich tourmalines were used to test whether Fe2+ and Mn2+ substitute on the Z site of tourmaline to a detectable degree. Fe-rich tourmaline from a pegmatite from Lower Austria was characterized by crystal-structure refinement, chemical analyses, and Mössbauer and optical spectroscopy. The sample has large amounts of Fe2+ (~2.3 apfu), and substantial amounts of Fe3+ (~1.0 apfu). On basis of the collected data, the structural refinement and the spectroscopic data, an initial formula was determined by assigning the entire amount of Fe3+ (no delocalized electrons) and Ti4+ to the Z site and the amount of Fe2+ and Fe3+ from delocalized electrons to the Y-Z ED doublet (delocalized electrons between Y-Z and Y-Y): X (Na0.9Ca0.1) Y(Fe2+2.0Al0.4Mn2+0.3Fe3+0.2) Z(Al4.8Fe3+0.8Fe2+0.2Ti4+0.1) T(Si5.9Al0.1)O18 (BO3)3V(OH)3W[O0.5F0.3(OH)0.2] with a = 16.039(1) and c = 7.254(1) Å. This formula is consistent with lack of Fe2+ at the Z site, apart from that occupancy connected with delocalization of a hopping electron. The formula was further modified by considering two ED doublets to yield: X(Na0.9Ca0.1) Y(Fe2+1.8Al0.5Mn2+0.3Fe3+0.3) Z(Al4.8Fe3+0.7Fe2+0.4Ti4+0.1) T(Si5.9Al0.1)O18 (BO3)3V(OH)3W[O0.5F0.3(OH)0.2]. This formula requires some Fe2+ (~0.3 apfu) at the Z site, apart from that connected with delocalization of a hopping electron. Optical spectra were recorded from this sample as well as from two other Fe2+-rich tourmalines to determine if there is any evidence for Fe2+ at Y and Z sites. If Fe2+ were to occupy two different 6-coordinated sites in significant amounts and if these polyhedra have different geometries or metal-oxygen distances, bands from each site should be observed. However, even in high-quality spectra we see no evidence for such a doubling of the bands. We conclude that there is no ultimate proof for Fe2+ at the Z site, apart from that occupancy connected with delocalization of hopping electrons involving Fe cations at the Y and Z sites. A very Mn-rich tourmaline from a pegmatite on Elba Island, Italy, was characterized by crystal-structure determination, chemical analyses, and optical spectroscopy. The optimized structural formula is X(Na0.6□0.4) Y(Mn2+1.3Al1.2Li0.5) ZAl6TSi6O18 (BO3)3V(OH)3 W[F0.5O0.5], with a = 15.951(2) and c = 7.138(1) Å. Within a 3σ error there is no evidence for Mn occupancy at the Z site by refinement of Al ↔ Mn, and, thus, no final proof for Mn2+ at the Z site, either. Oxidation of these tourmalines at 700-750 °C and 1 bar for 10-72 h converted Fe2+ to Fe3+ and Mn2+ to Mn3+ with concomitant exchange with Al of the Z site. The refined ZFe content in the Fe-rich tourmaline increased by ~40% relative to its initial occupancy. The refined YFe content was smaller and the distance was significantly reduced relative to the unoxidized sample. A similar effect was observed for the oxidized Mn2+-rich tourmaline. Simultaneously, H and F were expelled from both samples as indicated by structural refinements, and H expulsion was indicated by infrared spectroscopy. The final species after oxidizing the Fe2+-rich tourmaline is buergerite. Its color had changed from blackish to brown-red. After oxidizing the Mn2+-rich tourmaline, the previously dark yellow sample was very dark brown-red, as expected for the oxidation of Mn2+ to Mn3+. The unit-cell parameter a decreased during oxidation whereas the c parameter showed a slight increase.

Modelling of some structural parameters of tourmalines on the basis of their chemical composition. I. Ordered structure model

This study models the mean sizes of X, Y, Z, B and T sites as well as the unit-cell parameters of tourmalines on the basis of their chemical compositions, using previously determined relations between the mean bond lengths in co-ordination polyhedra and the cation populations in those sites. For the octahedral sites of Al-rich tourmalines, these relations also take into account the possible modifying influences of the IInd co-ordination shells. For Al-poor tourmalines, the following relationships were determined: Fe 3+ , Mg 2+ and Ti 4+ ions supplementing the deficit of aluminium located at the Z octahedra have an effect on the mean size of the Z site, and these ions have a modifying influence on the adjacent Y sites. For a set of 43 tourmalines from various localities, representing various members of this mineral group, the observed unit-cell parameters were compared with the calculated values. The standard errors SE of the models: a calc . = K a a obs. and C calc . = k c c obs . are 0.0 13 A and 0.008 A, respectively, and the k a , k c coefficients are close to the theoretical value of 1 within the limits of a statistical error se = 0.0001-0.0002. These values suggest a highly satisfactory agreement of the measured and modelled values and indicate that the determined model equations can be used to predict the structural parameters of tourmalines. On this basis, the model predicts the structural characteristics of all end-member species known in the tourmaline group, assuming the ordering of octahedral ions between Y and Z sites. In some (Fe 3 +,Al)-rich tourmalines (buergerites and schorls) and Al-rich dravites small differences have sometimes been noted between the observed and predicted values of and mean sizes, as well as the a and c unit-cell parameters; the differences suggest a limited disordering of Fe 3+ and Al or Mg and Al between Y and Z sites.

Tourmalines from the Western Tatra Mountains (W-Carpathians, S-Poland) Their characteristics and petrogenetic importance

Tourmalines are the most important Fe-Mg minerals in the pegmatites and leucogranites intruding the crystalline core of the Western Tatra Mountains (South Poland). They also occur in the folded host rocks inside the contact zone. The chemical composition and the weak zonation of the investigated tourmalines are both consistent with the field observations which suggest their magmatic/late magmatic origin and a close association with the Li-poor leucogranites and Al-rich, Ca-poor metasediments. The irregular distribution of the tourmaline-bearing rocks can be interpreted as reflecting three factors, i.e. , limited boron and water availability in the metasedimentary rocks during anatexis, variable oxygen fugacity (controlled partly by the presence of graphite) and restricted mobility of the mafic cations, necessary for tourmaline formation. The exsolution of a boron-rich fluid phase, incompatible with silicate melt, and its escape along a shear zone is also considered. The differences in Fe 3+ /Fe 2+ characterising the Western Tatra tourmalines could result both from f O 2 variations in the source metasediments during anatexis and from the interaction of magmatic/postmagmatic fluids with the metamorphic host rocks.

Primary Nb-Ta minerals in the Szklary pegmatite, Poland: New insights into controls of crystal chemistry and crystallization sequences

Abstract An assemblage of primary, extremely As- and Sb-rich, Nb-Ta minerals from the Szklary pegmatite includes columbite-(Fe), columbite-(Mn), tantalite-(Mn), stibiocolumbite, stibiotantalite, an as yet unnamed (As,Sb,U)-rich (Ta,Ti)-oxide, Mn3UAs2Sb2Ta2Ti2O20, and holtite. Anomalous trends of Mn-Fe and Ta-Nb fractionation in the columbite group and crystallization sequences in the primary assemblage can be explained by the contamination of the pegmatite-forming melt by ultramafic and mafic wall-rocks, the competition among these minerals for Ta and Sb and with biotite and tourmaline for Mg, Fe, and Ti, and local variations in melt composition. A hot magmatic fluid, exsolved from the parental melt, reacted with the primary Nb-Ta oxides, inducing two different patterns of alteration. The columbite-group minerals were altered to fersmite, pyrochlore, and bismutopyrochlore locally grading to plumbopyrochlore, whereas stibiocolumbite, stibiotantalite, and the (As,Sb,U)-rich (Ta,Ti)-oxide altered to stibiomicrolite, uranmicrolite grading to betafite, and then to bismutomicrolite or Bi-dominant betafite. In all of the secondary pyrochlore-group minerals, Ta-Nb fractionation is comparable to, or only slightly greater, than that in the primary Nb-Ta oxides, indicating a modest differentiation of the residual melt coexisting with the fluid.

“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.

Holtite and dumortierite from the Szklary Pegmatite, Lower Silesia, Poland

Abstract The Szklary holtite is represented by three compositional varieties: (1) Ta-bearing (up to 14.66 wt.% Ta2O5), which forms homogeneous crystals and cores within zoned crystals; (2) Ti-bearing (up to 3.82 wt.% TiO2), found as small domains within the core; and (3) Nb-bearing (up to 5.30 wt.% Nb2O5,) forming the rims of zoned crystals. All three varieties show variable Sb + As content, reaching 19.18 wt.% Sb2O3 (0.87 Sb a.p.f.u.) and 3.30 wt.% As2O3 (0.22 As a.p.f.u.) in zoned Ta-bearing holtite, which constitutes the largest Sb+As content reported for the mineral. The zoning in holtite is a result of Ta-Nb fractionation in the parental pegmatite-forming melt together with contamination of the relatively thin Szklary dyke by Fe, Mg and Ti. Holtite and the As- and Sb-bearing dumortierite, which in places overgrows the youngest Nb-bearing zone, suggest the following crystallization sequence: Ta-bearing holtite → Ti-bearing holtite → Nb-bearing holtite → As- and Sb-bearing, (Ta,Nb,Ti)-poor dumortierite → As- and Sb-dominant, (Ta,Nb,Ti)-free dumortierite-like mineral (16.81 wt.% As2O3 and 10.23 wt.% Sb2O3) with (As+Sb) > Si. The last phase is potentially a new mineral species, Al6⃞B(Sb,As)3O15, or Al5⃞2B(Sb,As)3O12(OH)3, belonging to the dumortierite group. The Szklary holtite shows no evidence of clustering of compositions around ‘holtite I’ and ‘holtite II’. Instead, the substitutions of Si4+ by Sb3+ + As 3+ at the Si/Sb sites and of Ta5+ by Nb5+ or Ti4+ at the Al(1) site suggest possible solid solutions between: (1) (Sb,As)-poor and (Sb,As)-rich holtite; (2) dumortierite and the unnamed (As+Sb)-dominant dumortierite-like mineral; and (3) Ti-bearing dumortierite and holtite, i.e. our data provide further evidence for miscibility between holtite and dumortierite, but leave open the question of defining the distinction between them. The Szklary holtite crystallized from the melt along with other primary Ta-Nb-(Ti) minerals such as columbite-(Mn), tantalite-(Mn), stibiotantalite and stibiocolumbite as the availability of Ta decreased. The origin of the parental melt can be related to anatexis in the adjacent Sowie Mountains complex, leading to widespread migmatization and metamorphic segregation in pelitic-psammitic sediments metamorphosed at ∼390-380 Ma.

Occurrence of Sulphides in Sowia Dolina Near Karpacz (SW Poland) - An Example of ore Mineralization in the Contact Aureole of the Karkonosze Granite

Occurrence of Sulphides in Sowia Dolina Near Karpacz (SW Poland) - An Example of ore Mineralization in the Contact Aureole of the Karkonosze Granite The authors studied the poorly-known, uneconomic sulphide mineralization site in Sowia Dolina near Karpacz. Host rocks are hornfelses of the Velká Úpa schist series, which belongs to the Izera-Kowary Unit. Ore minerals assemblage includes: pyrrhotite, pyrite, chalcopyrite, arsenopyrite, sphalerite, galena and marcasite, accompanied by ilmenite and rutile. The oldest sulphide is high-temperature pyrrhotite crystallized at about 600°C, which is in good agreement with the temperature range of contact metamorphic conditions, revealed by muscovitesillimanite transformation. Low-temperature pyrrhotite and other sulphides formed at about 390°C (arsenopyrite geothermometer) down to 265°C (pyrrhotite geothermometer), whereas fluid inclusions studies of vein quartz demonstrated the temperature range 380-150°C. Mineralization in Sowia Dolina is similar to other ore hydrothermal deposits known from the proximal or distal contact zone of the Karkonosze granite. Wystąpienie siarczków w Sowiej Dolinie koło Karpacza - przykład mineralizacji kruszcowej w aureoli kontaktowej granitu Karkonoszy (Sudety, Polska) Hornfelsy Sowiej Doliny należą do jednostki izersko-kowarskiej i są częścią serii łupkowej grupy Velkej Úpy, przeobrażonej na kontakcie z waryscyjskim granitem Karkonoszy. W Sowiej Dolinie istnieją ślady dawnych robót górniczych, wyloty sztolni i hałdy, na których znaleźć można okazy z mineralizacją siarczkową. Okruszcowane hornfelsy odznaczają się dobrze zachowaną foliacją i lineacją. Przejawem metamorfizmu kontaktowego są poligonalne zarysy ziaren kwarcu oraz rozpad muskowitu na sillimanit, zgodnie z reakcją: muskowit + kwarc = Al2SiO5 + K-skaleń + H2O, która oznacza warunki metamorfizmu wysokiego stopnia i osiągnięcie temperatury powyżej 600°C, a także krystalizacja andaluzytu i kordierytu, całkowicie zamienionego w pinit. Efektem zmian kontaktowych jest również powstanie pseudomorfoz po granacie. Najbogatsze skupienia minerałów kruszcowych stwierdzone zostały w hornfelsach wzbogaconych w kwarc lub przecinanych żyłkami kwarcowo-skaleniowymi. Dominującym minerałem rudnym jest pirotyn, rzadziej pojawia się piryt. Minerały te tworząmasywne skupienia kilkucentymetrowej miąższości, niekiedy także żyłki lub struktury rozproszone. W mniejszych ilościach występują: chalkopiryt, galena, sfaleryt, arsenopiryt, bornit, markasyt oraz minerały Ti. Sukcesja minerałów kruszcowych została określona na podstawie przerostów mineralnych (tab. 2). Najstarsze minerały, ilmenit i rutyl, są związane przypuszczalnie z metamorfizmem regionalnym. Po minerałach Ti krystalizował pirotyn. Młodszy od niego jest chalkopiryt, którego starsza generacja tworzy zrosty z pirotynem, następna natomiast występuje jako odmieszania w sfalerycie. Po pirotynie i starszym chalkopirycie, w tym samym czasie powstawały sfaleryt, arsenopiryt i galena. Markasyt jest minerałem wtórnym, tworzącym się w początkowych stadiach procesu wietrzenia rud na hałdzie. Następstwo siarczków potwierdziła interpretacja geotermometryczna wyników analiz chemicznych w mikroobszarze. Wykazała ona, że temperatury powstawania pirotynu wahały się w zakresie temperatur 630-265°C, a arsenopiryt krystalizował w temperaturze około 390°C. Temperatury powstawania kwarcu żyłowego oznaczone za pomocą inkluzji ciekło-gazowych mieszczą się w zakresie temperatur 380-150°C. Porównanie obserwacji mikroskopowych rud z danymi chemicznymi i petrologicznymi pozwala na sugestię, że procesy metamorfizmu kontaktowego w temperaturach około 600°C odpowiadają krystalizacji wysokotemperaturowego pirotynu, natomiast pozostałe siarczki i kwarc żyłowy tworzyły się w procesach hydrotermalnych niższych temperatur, aż do około 150°C.

X and Q band EPR studies of paramagnetic centres in natural and heated tourmaline

Various members of the tourmaline group (schorl, dravite, and elbaite) as well as the products of their gradual oxidation were investigated by EPR spectroscopy in X and Q bands at 293 K and 77 K. In the EPR spectra of the schorl and dravite samples the signals at g ≈ 2 and 4.3 were attributed to clustered and isolated Fe3+ ions, respectively. The EPR spectra of Fe-poor elbaite are dominated by signals at g ≈ 2.5 and 3.5, assigned to Mn2+ ions. In the schorl and dravite samples, gradually annealed in air above 750 K, the total intensity of the EPR spectrum increased with increasing temperature, due to the oxidation of Fe2+ ( d 6) to Fe3+ ( d 5) ions. The Fe3+ ion being a product of thermal oxidation initially occupies sites with g ≈ 4.3 and after heating at temperatures above 1070 K forms clusters with g ≈ 2.0. In the Fe-poor elbaite the total intensity of the spectrum gradually decreased with the increasing oxidation temperature up to 1150 K, due to the transformation of paramagnetic Mn2+ ( d 5) into Mn3+ ( d 4) ions. Simultaneously, the signal of Fe3+ at g ≈ 4.3 became more pronounced. At still higher temperatures ( T > 1150 K) the intensity of the signal around g ≈ 2.0 increased indicating further oxidation of Mn3+ to Mn4+ ( d 3).

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.