Roberta Oberti

Nb and Ta incorporation and fractionation in titanian pargasite and kaersutite: crystal–chemical constraints and implications for natural systems

New partition coefficients between liquid and pargasitic/kaersutitic amphiboles (Amph/LDNb,Ta) experimentally determined for Nb and Ta at upper-mantle conditions, combined with single-crystal structure refinement of the synthesised amphiboles, show that Amph/LDNb,Ta are strongly dependent on the structure and composition of both amphibole and melt. The correlation of the Amph/LDNb,Ta with the amphibole oxy-component is explained by the ordering of Nb and Ta at the M1 site, contributing with the fraction of Ti at M1 to locally balance the O3O2−↔O3(OH)− substitution. In our set of dehydrogenated amphiboles, variations in the SiO2 content of the melt from 41.5 to 54.6 correspond to a six-fold increase of the Amph/LDNb,Ta, in which Amph/LDNb varies from 0.14 to 0.71 and Amph/LDTa from 0.11 to 0.54. Partition coefficients for Nb and Ta abruptly increase in Ti-depleted compositions (Amph/LDNb up to 1.63 and Amph/LDTa to 1.00). The ratio of DNb to DTa (Amph/LDNb/Ta) varies from 0.71 to 1.63, and is a function of the M1 site dimension, which in turn depends on its Fe, Mg and Ti contents. The observed variations can be explained by assuming that the ionic radius of Nb is (∼0.01–0.02 A) larger than that of Ta, contrary to the common assumption that they are both equal to 0.64 A. We calibrated a simplified model for the prediction of Amph/LDNb/Ta values shown to be negatively related mainly to mg# [Mg/(Mg+Fe)] and to Ti content. High-mg# amphiboles have Amph/LDNb/Ta close to unity, so the low Nb/Ta found in convergent margin volcanics and in the continental crust cannot be explained by the involvement of amphibole in the mantle wedge. Amphibole in the subducting slab may have lower mg# and consequently high Nb/Ta values, and thus may give rise to subchondritic Nb/Ta values in coexisting melts. Nb/La is also negatively correlated with mg#, and strongly increases in Ti-depleted compositions.

Trace-element incorporation in titanite: constraints from experimentally determined solid/liquid partition coefficients

Titanite/liquid partition coefficients for most of the trace elements relevant in petrogenetic studies are provided for titanite-saturated liquids equilibrated at 1.5 GPa and 850 °C starting from lamproitic compositions. The high compatibility observed for REE, HFSE, Sr, V and Sc, and the strong incompatibility observed for actinides, large ion lithophile and light elements are discussed in terms of available crystal-chemical mechanisms for incorporation and crystal-structure control. The exchange vectors Na1Ca−1 and Al1Ti−1 allow local charge balance to be achieved after incorporation of REE3+ and R5+ at the Ca and Ti site, respectively. The significant amounts of H measured are also relevant in this regard [the exchange mechanism being (OH)1O−1 at the O1 site]. The incorporation of U4+ and Th4+ at the Ca site is scarce, and is likely balanced by that of Mg2+ at the Ti site; both these substitutions are responsible for strong structural strain. Titanite can thus be considered an important repository for REE and HFSE in metamorphic and igneous rocks, and its role must be accounted for when modelling trace-element residence during metamorphic reactions and late magmatic crystallisation history. Due to the measured differences in compatibility, titanite crystallisation increases the values of Nb/Ta and LREE/HREE ratios in residual liquids. The similar compatibility of U and Pb makes titanite suitable for U–Pb geochronology of igneous rocks only after common Pb correction. Finally, this study confirms that the titanite end member is not suitable for radioactive waste disposal due to the discussed crystal-chemical constraints, and that titanite-based waste forms should contain high amounts of Na+ and Mg2+.

Infiltration metasomatism at Lherz as monitored by systematic ion-microprobe investigations close to a hornblendite vein

Abstract Systematic investigations (electron- and ion-microprobe, and X-ray structure-refinement) of pyroxenes and amphiboles close to a cm-wide hornblendite vein have been carried out on a composite peridotite sample from the Lherz massif with the aim of constraining the processes of melt infiltration in the subcontinental mantle. Vein amphiboles have light rare earth element (LREE)-enriched patterns with the maximum at Nd and Sm, and positive Ba, Nb, Sr, and Ti anomalies. The liquid flowing in the vein had an alkaline geochemical signature (i.e., large-ion lithophile element (LILE)-enriched without significant high field strength element (HFSE) anomalies). Modal metasomatism, represented by crystallization of amphibole and consumption of clinopyroxene, occurred in a 2-cm thick layer of wall-rock. Newly formed amphiboles, i.e., those in the vein and in the modally metasomatised layer, have a lower [6] Al disorder than the pre-existing disseminated amphiboles, which formed during an older metasomatic event. Cryptic metasomatism is recorded beyond the modally altered layer in the cores of clinopyroxenes and in amphiboles. Moving away from the vein towards the peridotite, amphiboles and pyroxenes display systematic compositional variations, and approach equilibrium with the wall-rock within a few cm. Porphyroclastic clinopyroxenes record chromatographic separation of elements: less incompatible elements reach equilibrium with the wall-rock lherzolite at shorter distances than the more incompatible ones. The inverse correlation between the depth of the metasomatic effect and partition coefficients in porphyroclastic clinopyroxenes indicates that different concentration fronts developed for different elements during liquid/rock interaction, suggesting that liquid percolation along grain boundaries was responsible for clinopyroxene metasomatism. As in the clinopyroxene, abundances of incompatible elements in the amphibole decrease in the peridotite with distance from the vein. However, a less marked separation of elements is recorded by the amphiboles. Elements that are markedly more compatible in amphibole than in other minerals of a lherzolite, such as K, Ba, and Rb, have abrupt gradients which are confined to the modally-metasomatised region. The lack of metasomatic enrichment in these elements in the farther, disseminated amphiboles, indicates that amphibole crystallization was synchronous with melt infiltration and acted as a buffer for such elements. Micro-analytical investigations indicate that the geochemical effects associated with small-scale liquid infiltration can be successfully described by models which take into account both the chromatographic fractionation of elements and the influence of mineral-assemblage variations.

Fluoro-edenite from Biancavilla (Catania, Sicily, Italy): Crystal chemistry of a new amphibole end-member

Abstract Fluoro-edenite, ideally NaCa2Mg5(Si7Al)O22F2, was found both as prismatic or acicular crystals of millimetric size and as fibers in the rock cavities in gray-red altered benmoreitic lavas occurring at Biancavilla (Etnean Volcanic Complex, Catania, Italy). It is associated with feldspars, quartz, clino- and orthopyroxene, fluoro-apatite, ilmenite, and hematite, and probably crystallized from late-stage hydrothermal fluids. Fluoro-edenite is transparent, intense yellow, non-fluorescent, has vitreous to resinous luster, and gives a yellow streak parallel to the c axis; Mohs’ hardness 5-6, Dcalc = 3.09 g/cm3, perfect cleavage on {110}, and conchoidal fracture. In plane-polarized light, fluoro-edenite is birefringent (1st order), biaxial negative, α = 1.6058(5), β = 1.6170(5), γ = 1.6245(5), 2Vcalc = 78.09°, Y ≡ β ⊥ (010), and γ:Z = 26°. No pleochroism is observed. Fluoro-edenite is monoclinic, space group C2/m, a = 9.847(2) Å, b = 18.017(3) Å, c = 5.268(2) Å, β = 104.84(2)°, V = 903.45 Å3, Z = 2; the ten strongest X-ray diffraction lines in the powder pattern are [d(I, hkl)]: 3.125(10, 310), 8.403(6,110), 3.271(5,240), 2.807(4,330), 2.703(3,151), 1.894(2,5̄10), 2.938(2,221), 1.649(2, 461), 3.376(2,131), 1.438(2,6̄61). IR analysis showed absorption bands at 1066, 991, 791, 738, 667, 517, 475 cm-1, and no bands in the OH-stretching region. Structure refinement allowed determination of cation site-preference and ordering. Microprobe analysis of the refined crystal gave SiO2 52.92, TiO2 0.29, Al2O3 3.53, FeOt 2.50, MnO 0.46, MgO 22.65, CaO 10.83, Na2O 3.20, K2O 0.84, F 4.35, Cl 0.07 wt%, and the crystal-chemical formula obtained by combining all the available data is: A(Na0.56 K0.15) B(Na0.30 Ca1.62 Mg0.03 Mn0.05) C(Mg4.68 Fe2+0.19 Fe3+0.10 Ti4+0.03) T(Si7.42 Al0.58) O22O3(F1.98 Cl0.02)2.

On the symmetry and crystal chemistry of britholite: New structural and microanalytical data

Abstract We provide in this paper complete structural and micro-chemical characterization of two britholite samples with compositions (Mn0.04Ca4.75REE4.37Th0.72U0.12)Σ10.00(Si5.57P0.25B0.16)Σ5.98O24(OH0.23F1.77)Σ2.00 (from Latium, Italy) and (Na0.98Ca2.01REE6.97)Σ9.96(Si5.07P0.75)Σ5.82O24(OH0.53F1.47)Σ2.00 (from Los Islands, Guinea). The crystal-chemical formulae were calculated by combining electron-microprobe analyses for intermediate-Z elements (Na, Ca, Mn, Si, P), ion-microprobe analyses for low-Z (H, Li, Be, B, F) and high-Z elements (Ba, Y, REE and actinides), and high-quality (Robs 1.2-2.6) single-crystal structure refinements. Structure refinements indicate that the best approximation to the real symmetry is the P63 space group. In britholite, the lowering of symmetry with respect to the P63/m space group of apatite means that the O3 and O3a atoms are no longer equivalent and allows the tetrahedron to rotate up to ~4° around the Si-O1 bond. Consequently, the O3a atom moves closer to the REE1a site, whereas the O3 atom moves farther from the REE1 site and closer to the REE2 site, which thus assumes a [7+1] coordination. The infrared spectrum of britholite from Latium shows a unique and very broad band in the OH-stretching region at 3437 cm-1, which is consistent with the ordering of trivalent REE cations at the REE2 site. The calculated integral molar absorptivity εi is 23600 L · mol-1⋅cm-2. A remarkable constancy in the unit-cell volume along the whole apatite-britholite compositional range is observed for values of the aggregate ionic radius at the REE sites shorter than 1.12 and longer than 1.15 Å, whereas large variations are observed for intermediate values; this behavior suggests constraints due to the rigidity of the tetrahedral group, that are further enhanced at high symmetry.

Lithium in amphiboles: detection, quantification, and incorporation mechanisms in the compositional space bridging sodic and BLi-amphiboles

Lithium is an important constituent in amphiboles, where it can be incorporated up to a limit of three atoms per formula unit (apfu). Lithium can partition itself between the B-group sites (where it occurs at the [6+2]-coordinated M4’ position) and the C-group sites (where it occurs at the M3 site). Systematic analysis of the available chemical (EMP + SIMS) and structural data constrains lithium occurrence in amphiboles to the following compositions and exchange vectors: (1) B Li is incorporated according to M4’ Li M4 Na -1 , and no miscibility gap is apparent, despite the difference in the ionic radii; (2) C Li is incorporated according to M3 Li M2 Fe 3+ M3 Fe 2+ -1 M2 Fe 2+ -1 ; however, a partial bond-strength contribution is provided by Si at the T1 site and by Na or K at the A m site. Amphiboles with C Li > 0.5 apfu (root names: leakeite, kornite, whittakerite and pedrizite) have more than half-occupied A-group sites. Seven new amphibole end-members containing lithium have been discovered in epysienites (dequartzified and albitised granites) from the Pedriza Massif (Central Spain), where lithium incorporation and partitioning is controlled both by the composition of the fluid and the temperature conditions of crystallisation. This occurrence provides an unique opportunity to characterise the M4’ Li ⇔ M4 Na and M3 Li ⇔ M3 Fe 2+ solid solutions, as well as model crystal-chemical mechanisms and understand their dependence on intensive parameters. An accurate quantification and partitioning of lithium in amphiboles is not trivial, and requires a combination of ion-micro-probe analysis and structure refinement. Analysis of the available data provides criteria for calculating reliable H 2 O and Li 2 O values, as well as for obtaining reliable unit formulae from routine EMP results. These criteria can then be used to simplify petrological studies in Li-rich environments.

A new anhydrous amphibole from the Hoskins Mine, Grenfell, New South Wales, Australia; description and crystal structure of ungarettiite, NaNa 2 (Mn (super 2+) 2 Mn (super 3+) 3 )Si 8 O 22 O 2

Abstract Ungarettiite is a new amphibole species from the Hoskins mine, near Grenfell, New South Wales, Australia. It occurs with Mn-bearing oxides, silicates, and carbonates in a stratiform schist associated with metajasper, metabasalt, and metasiltstone. Ungarettiite is brittle, H = 6, Dmeas = 3.52 g/cm3, Dcalc = 3.45 g/cm3. In plane-polarized light, it is strongly pleochroic, X = orange red, Y ~ Z = very dark red; X Λ a = -2° (in β acute), Y = b, Z Λ C= 17° (in β obtuse), with absorption X < Y ≤ Z. Ungarettiite is biaxial positive, α = 1.717(2), β = 1.780(4), γ= 1.800(2); 2V= 51(2)°, dispersion r < ν. Ungarettiite is monoclinic, space group C2/m, a = 9.89(2), b = 18.04(3), c = 5.29(1) Å, β = 104.6(2)°, V = 912(1) Å3, Z = 2. The strongest ten X-ray diffraction lines in the powder pattern are [d(I, hkl )]: 2.176(10,171), 3.146(9,310), 2.544(9,2̅02), 1.447(9,3̅.11.1), 3.400(8,131), 1.656(8,461), 8.522(7,110), 2.299(7,1̅71), 2.575(6,241), and 2.047(6,202). Analysis by a combination of electron microprobe and crystal-structure refinement gives SiO2 50.66, Al2O3 0.04, TiO2 0.03, Fe2O3 0.50, FeO 0.00, Mn2O3 24.35, MnO 12.42, MgO 1.46, ZnO 0.10, CaO 0.18, Na2O 9.13, K2O 0.76, F not detected, sum 99.63 wt%. The formula unit calculated on the basis of 24 O atoms is (K0.15Na0.82)(Na1.97Ca0.03)(Mn2+1.66Mg0.34- Mn3+2.96Fe3+0.06Zn0.01)(Si7.99Al0.01)O2 and is close to the ideal end-member composition of NaNa2(Mn2+2Mn3+3)Si8O22O2. The crystal structure of ungarettiite was refined to an R index of ~ 1.5% using MoKα X-ray intensity data. Site-scattering refinement shows that the M1, M2, and M3 sites are occupied dominantly by Mn. The (M-O) distances are M1: 2.03; M2: 2.17; M3: 2.01 Å , compatible with the site occupancies M1~ M3 ~ Mn3+, M2 ~ Mn2+. All bonds to the O3 anion are very short, and a bond valence analysis indicates that the O3 site is occupied by a divalent anion: O2-, as suggested by the overall electroneutrality requirement of the structural formula.

Sodic-ferripedrizite, a new monoclinic amphibole bridging the magnesium-iron-manganese-lithium and the sodium-calcium groups

Abstract Sodic-ferripedrizite, ideally Na(LiNa)(Fe3+2 Mg2Li)Si8O22(OH)2, is the second new amphibole endmember found in episyenites formed after cordierite-bearing porphyritic granites in the East Pedriza Massif (Central System, Spain). It is green, vitreous, translucent, non-fluorescent and brittle, and has gray streak, H(Mohs) = 6, splintery fracture, perfect {110} cleavage, Dmeas = 3.15, Dcalc = 3.15 g/ cm3. In plane-polarized light, sodic-ferripedrizite is strongly pleochroic, X = green blue, Y = blue green, Z = yellow green (X > Y > Z); Y = b, X ^ c = 4°, Z ^ c = -8°. It is biaxial positive: a = 1.694 (1), b = 1.698(1), and g = 1.702(1); 2VZ = 83(2)° and 2VZ,calc = 85(3)°, dispersion r > v. Sodic-ferripedrizite is monoclinic, space group C2/m, a = 9.536(1), b = 17.789(2), c = 5.277(1) Å, β = 102.53°, V = 873.8(1) Å3. The five strongest lines in the X-ray powder-diffraction pattern [d in Å[I] (hkl) are 3.397[3](131), 3.056[10](310), 2.749[5](330), 2.699[6](151), and 1.639[4](461). Analysis by electron microprobe and flame photometry gave an average chemical formula of A(Na0.70K0.03) B(Li1.34Na0.58Ca0.08) (Mg1.75 Fe3+1.65Li0.88Fe2+0.32Al 0.21Ti0.11Mn2+0.07Zn0.01)Si 8.00O22(OH1.35F0.65). Structure refinement of one crystal of pedrizite and of another crystal with higher clinoholmquistite component, both of which were also analyzed by ion microprobe for light and volatile elements, allowed us to assess Li partitioning among the B- and C-group sites and the active crystal-chemical mechanisms; Li is ordered at the [6+2]-coordinated position in the M4 cavity and at the M3 site. Thus, sodic-ferripedrizite encompasses the different site-preferences and crystal-chemical mechanisms observed for Li in amphiboles of the Mg-Fe-Mn-Li group (clinoholmquistite series) and the Na-Ca group (leakeite series). These data and other recent results on synthetic amphiboles suggest that miscibility between the two groups of B-cations is far more extensive than previously expected.

An electron microprobe, LAM-ICP-MS and single-crystal X-ray structure refinement study of the effects of pressure, melt-H2O concentration and fO2 on experimentally produced basaltic amphiboles

Amphiboles were crystallized in sub-liquidus experiments at 0.5–2.0 GPa and 1000–1050 °C from hydrous nepheline basanite and olivine basalt starting compositions. The amphiboles and coexisting (quenched) melts were analysed for major, minor and trace elements by a combination of electron microprobe, laser ablation microprobe and inductively-coupled plasma mass-spectrometry (LAM ICP-MS). Individual amphiboles were also characterized by single-crystal X-ray structure refinement, and empirical estimates of dehydrogenation were obtained based on M1–M2 distances. The amphiboles display compositional variation that can be interpreted as crystal-chemical responses to: (1) increasing pressure, and (2) changes in oxygen fugacity ( f O 2 ) and the activity of H 2 O. As pressure increases, Al moves from the T1 tetrahedron (where it is replaced by Si) to the octahedral M2 site. This coupled substitution, which implies an increase in coordination number for Al, results in a decrease in the c and b unit-cell edges. The overall decrease in unit-cell volumes is kept small, however, by an increase in the B (Fe, Mg) content with increasing pressure, which in turn decreases the volume occupied by the B-cations but increases the sin β value. In this way, the entrance of minor K at the A site and Cl at the O3 site (K D s for both increase with pressure) is allowed, resulting in a slight lengthening of the a edge. The degree of dehydrogenation at O3 correlates inversely with the H 2 O concentration in coexisting melts. Generally, dehydrogenation is locally balanced by M1 Ti, with the Ti excess with respect to ½ O 2− ordered at the M2 site. In one sample, crystallized under more oxidizing conditions, O 2− is > 2Ti, and local charge balance requires the presence of Fe 3+ ordered at the M1 (and M3) sites. D amph/melt values measured for the high field strength elements Ti, Zr, Hf, Nb and Ta ( D HFSE ) correlate positively with O 2− and with [4] Al, suggesting that Ti, Zr, Hf, Nb and Ta (HFSE) are incorporated in both the M1 and the M2 sites. Partition coefficients for rare earth elements ( D REE ) correlate positively with [4] Al and negatively with [6] Al. Increased f O 2 results in increased Fe 3+ , [4] Al and D REE , but does not produce a noticeable increase in O 2 − or in D HFSE .

Ferripedrizite, a new monoclinic BLi amphibole end-member from the Eastern Pedriza Massif, Sierra de Guadarrama, Spain, and a restatement of the nomenclature of Mg-Fe-Mn-Li amphiboles

Abstract Ferripedrizite, ideally ANaBLi2C(Fe3+2Mg2Li)TSi8O22X(OH)2, is a new amphibole end-member found in episyenites formed after cordierite-bearing porphyritic granites in the Eastern Pedriza Massif (Central System, Spain). It contains the maximum amount of Li that can be incorporated in the amphibole structure. The name was approved by the IMA-CNMMN together with restriction of the use of the prefix sodic in the pedrizite series to compositions with BNa > 0.5 apfu; its use for compositions with Natot > 0.5 apfu has been maintained in the rest of the Mg-Fe-Mn-Li group. Complete solid solution between ferripedrizite and leakeite [ideally, ANaBNa2C(Fe3+2Mg2Li)TSi8O22X(OH)2] has been found in the Pedriza Massif. According to the present nomenclature rules, this join bridges three different amphibole groups. Samples with B(Mg+Fe+Mn+Li) ≥ 1.0 apfu and 0. 0 £ BNa £ 0.50 apfu belong to the Mg-Fe-Mn-Li group and are termed ferripedrizite; samples with B(Mg + Fe + Mn + Li) ≥1.0 apfu and 0.50 < BNa £ 0.99 apfu belong to the Mg-Fe-Mn-Li group and are called sodicferripedrizite; samples with B(Mg + Fe + Mn + Li) < 1.0 apfu and BNa ≥ 1.50 apfu belong to the sodic group and are named leakeite; samples with B(Mg+Fe+Mn+Li) < 1.0 apfu and 1.0 £ BNa < 1.50 apfu belong to the sodic-calcic group (albeit Ca is negligible) and deserve a new root name. The ferripedrizite sample from Pedriza is black, vitreous, translucent, non-fluorescent, and brittle, and has gray streak, H = 6, uneven fracture, perfect {110} cleavage, Dmeas = 3.15, Dcalc = 3.19 g/cm3. It is strongly pleochroic, X = yellow green, Y = green blue, Z = bluish green (Y = Z >> X), Z = b, Y ^ c = 15(6)°, X ^ a = 3°. It is biaxial positive: a = 1.695(1), b = 1.700(2), and g = 1.702(1); 2VZ = 125(17)°, dispersion r > v. It is monoclinic, space group C2/m, a = 9.501(1), b = 17.866(2), c = 5.292(1) Å, b = 102.17(2)°, V = 878.1(2) Å3. The ten strongest lines in the X-ray powder-diffraction pattern [d in Å,(I),(hkl)] are: 8.251(3)(110), 4.466(2)(040), 3.411(2)(131), 3.050(10)(310), 2.747(3)(330), 2.711(4)(151), 2.495(2)(2-02), 2.161(2)(261), 1.642(4)(461), 1.394(3)(6-61). Structure refinement and electron- and ion-microprobe analysis of a crystal with composition A(K0.04Na0.52)B(Na0.25Ca0.05Li1.70)C(Li0.64Fe3+1.64Mg1.49Fe2+0.85Al0.21Ti0.09Mn0.07Zn0.01)TSi8O22X(OH1.31F0.69) are provided, together with some discussion on cation ordering