Crystal-chemical Mechanisms Research Articles

New Data on the Isomorphism in Eudialyte-Group Minerals. 2. Crystal-Chemical Mechanisms of Blocky Isomorphism at the Key Sites

The review considers various complex mechanisms of isomorphism in the eudialyte-group minerals, involving both key positions of the heteropolyhedral framework and extra-framework components. In most cases, so-called blocky isomorphism is realized when one group of atoms and ions is replaced by another one, which is accompanied by a change in the valence state and/or coordination numbers of cations. The uniqueness of these minerals lies in the fact that they exhibit ability to blocky isomorphism at several sites of high-force-strength cations belonging to the framework and at numerous sites of extra-framework cations and anions.

Incorporation of Cr, Mn and Ni into goethite (α-FeOOH): mechanism from extended X-ray absorption fine structure spectroscopy

The crystal-chemical mechanisms by which transition metals are associated with goethite are fundamental to our understanding of the solubility and bioavailability of micronutrients and heavy metals in soils, and in the formation of laterite ore deposits. Transition metals such as Cr, Mn and Ni may sorb onto goethite by forming surface precipitates, surface complexes or by replacing Fe3+ in the goethite structure. In the work reported here, we investigated the local coordination environment of Cr, Mn and Ni in synthetic goethite using EXAFS spectroscopy. We demonstrate the isomorphous substitution for Fe3+ by Cr3+ (up to 8 mol.%), Mn3+ (up to 15 mol.%) and Ni2+ (up to 5 mol.%). We find, however, that the next-nearest-neighbour coordination environment changes with composition. The perturbations are likely to be responsible for limiting the accommodation of Cr3+, Mn3+ or Ni2+ in the FeOOH structure.

Synthetic hypersilicic Cl-bearing mica in the phlogopite-celadonite join: A multimethodical characterization of the missing link between di- and tri-octahedral micas at high pressures

A hypersilicic Cl-bearing mica was synthesized at 4 GPa and 1200-1250 °C, close to the solidus of the join diopside-jadeite-KCl, in association with diopside-jadeite pyroxene, K-rich alumino silicate glass and/or sanidine and (K,Na)Cl. The mica shows a negative correlation between tetrahedral Si and octahedral (AI + Mg), suggesting an Al-celadonitic substitution (Si + VIAl + VIO = IVAI+ VIMg) and a chemical formula: Kl01(Mg245AlolDo35h~lSi352Alo48h~401O[(OH,0)166Clo34)h~2' The presence of hydroxyl was confirmed by OH stretching modes at 3734 and 3606 cm' in the Raman spectra. Singlecrystal X-ray diffraction data provide the unit-cell parameters (space group C2/m, 1M polytype): a = 5.299(4), b = 9.167(3), C = 10.226(3) A, ~=100.06(4)°, V= 489.1(4) N. The structure refinement shows the presence of vacancies on the octahedral sites (15% for Ml and 6.5% for M2). Chlorine occupies a position about 0.5 A from 04 with partial occupancy (0.39 apfu). Crystal-chemical mechanisms seem to govern chlorine incorporation in mica, since a large A site is necessary to locate the anion in the structure. A large A site results when the six -tetrahedra ring is hexagonal and the tetrahedral rotation angle exis 0°. Such a geometry is achieved either by increasing the annite component in biotite or by increasing the hypersilicic character of phlogopite through the Al-celadonite substitution. The present Si-rich mica shows a partial dioctahedral character due to the Al-celadonite substitution, which lowers the exangle and expands its stability field at high pressure. High aK20 conditions, like in potassium-rich brine or potassic carbonatitic melts, increase the Alceladonite component in the phlogopite solid solution, explaining the association of Si-rich micas with inclusions of potassic liquids in kimberlitic diamonds.

Trace-element partitioning in olivine: modelling of a complete data set from a synthetic hydrous basanite melt

Olivine/liquid partition coefficients for most trace elements relevant in petrogenetic studies are provided for a hydrous basanite melt equilibrated at 1.4 GPa and 1055 °C. The partitioning results determined by a combination of EMP and SIMS analyses are discussed in terms of available crystal-chemical mechanisms and crystal-structure control. The critical evaluation of this data set and of those reported in the literature, in the frame of the present lattice-strain model, provides useful suggestions for future studies designed to clarify incorporation mechanisms and site preference in olivine. A further useful suggestion is that the measured Ol/L D values for Li, Mg and Sc allow a straightforward but reliable estimate of the D 0 for 1+, 2+ and 3+ cations, respectively. The assessment of D 0 for VIHFSE 4+ is probably biased by their partitioning between the T and M sites. Anyway, it can be reasonably assumed to be close to Ol/L D Zr.

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.

Incorporation of Cr, Mn and Ni into goethite (α-FeOOH): mechanism from extended X-ray absorption fine structure spectroscopy

AbstractThe crystal-chemical mechanisms by which transition metals are associated with goethite are fundamental to our understanding of the solubility and bioavailability of micronutrients and heavy metals in soils, and in the formation of laterite ore deposits. Transition metals such as Cr, Mn and Ni may sorb onto goethite by forming surface precipitates, surface complexes or by replacing Fe3+ in the goethite structure. In the work reported here, we investigated the local coordination environment of Cr, Mn and Ni in synthetic goethite using EXAFS spectroscopy. We demonstrate the isomorphous substitution for Fe3+ by Cr3+ (up to 8 mol.%), Mn3+ (up to 15 mol.%) and Ni2+ (up to 5 mol.%). We find, however, that the next-nearest-neighbour coordination environment changes with composition. The perturbations are likely to be responsible for limiting the accommodation of Cr3+, Mn3+ or Ni2+ in the FeOOH structure.

Incorporation of Cr, Mn and Ni into goethite (α-FeOOH): mechanism from extended X-ray absorption fine structure spectroscopy

The crystal-chemical mechanisms by which transition metals are associated with goethite are fundamental to our understanding of the solubility and bioavailability of micronutrients and heavy metals in soils, and in the formation of laterite ore deposits. Transition metals such as Cr, Mn and Ni may sorb onto goethite by forming surface precipitates, surface complexes or by replacing Fe3+ in the goethite structure. In the work reported here, we investigated the local coordination environment of Cr, Mn and Ni in synthetic goethite using EXAFS spectroscopy. We demonstrate the isomorphous substitution for Fe3+ by Cr3+ (up to 8 mol.%), Mn3+ (up to 15 mol.%) and Ni2+ (up to 5 mol.%). We find, however, that the next-nearest-neighbour coordination environment changes with composition. The perturbations are likely to be responsible for limiting the accommodation of Cr3+, Mn3+ or Ni2+ in the FeOOH structure.

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 Na 1Ca −1 and Al 1Ti −1 allow local charge balance to be achieved after incorporation of REE 3+ and R 5+ 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 U 4+ and Th 4+ at the Ca site is scarce, and is likely balanced by that of Mg 2+ 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 Mg 2+.

CHARACTERIZATION OF GADOLINITE-GROUP MINERALS USING CRYSTALLOGRAPHIC DATA ONLY: THE CASE OF HINGGANITE-(Y) FROM CUASSO AL MONTE, ITALY

Important data concerning chemical composition of a mineral of the gadolinite group can be obtained from a refinement of its crystal structure. In spite of the complexities, an entirely crystallographic procedure performed on a single grain can afford a practical way to identify and characterize these species; such a procedure is often successful, provided the quality of the cry stal allows it. We consider the crystal-chemical mechanisms corresponding to the most important substitutions, and apply such a method to identify the Fe-poor and Ca-rich gadolinite-group minerals from Cuasso al Monte, Varese, Italy. These crystals can be ascribed to hingganite-(Y), with minor amounts of gadolinite (0.26–0.31 molar fraction) and datolite components (0.15–0.25 molar fraction) in solid solution. The component minasgeraisite-(Y) is not present, even as a minor one, in the samples studied .

Fractionation of Nb and Ta from Zr and Hf at Mantle Depths: the Role of Titanian Pargasite and Kaersutite

Selective enrichment or depletion in either Zr and Hf (HFSE) INTRODUCTION or Nb and Ta (HFSE) is a feature commonly observed in many Because of their similar crystal-chemical properties (i.e. mantle-derived melts and amphiboles occurring as either disseminated high charge and ionic radius), high field strength elements minerals in mantle xenoliths and peridotite massifs or in vein (HFSE: Ti, Zr, Hf, Nb and Ta) are considered to behave assemblages cutting these rocks. The fractionation of Nb from Zr as a coherent group of substituents in rock-forming silicate seen in natural mantle amphiboles suggests that their incorporation minerals, and thus are not expected to fractionate greatly is governed by different crystal-chemical mechanisms. An extensive during partial melting and crystallization processes (Sun set of new partitioning experiments between pargasite–kaersutite and & McDonough, 1989). Nevertheless, fractionation of melt under upper-mantle conditions shows that HFSE incorporation HFSE from HFSE is a common feature in many and fractionation depends on amphibole major-element composition primitive mantle melts such as island-arc volcanics and and the presence or absence of dehydrogenation. Multiple regression intra-plate alkaline melts, suggesting either the presence analysis shows that DNb/Zr is strongly dependent on the mgof a solid phase with DNb,Ta significantly different from number of the amphibole as a result of a combination of amphibole DZr,Hf, or of two distinct phases controlling these element and melt structure effects, so that the following generalizations pairs. Experimental investigations of HFSE decoupling apply: (1) high-mg-number amphiboles crystallized from unmodified have been so far focused on Ti minerals (rutile and mantle melts more easily incorporate Zr relative to Nb leading to ilmenite), because of their high compatibility for HFSE an increase of the Nb/Zr ratio in the residual melt; (2) low-mgand their consequent capability of altering HFSE signumber amphiboles, such as those found in veins cutting peridotites, natures of equilibrium melts. Rutile has been shown to may strongly deplete the residual melt in Nb and cause very low have partition coefficients for Ta and Nb exceeding that Nb/Zr in residual melts. Implications and applications to mantle for Zr by one to two orders of magnitude, with Zr a environments are discussed. further one to two orders higher than other trace elements ( Jenner et al., 1993; Foley et al., 2000). Rutile could thus cause significant decoupling between Nb and Zr, and has the potential to produce the pattern typical of islandarc basalts (Foley & Wheller, 1990; Foley et al., 2000), in

A crystal chemical re-evaluation of amphibole/melt and amphibole/clinopyroxene DTivalues in petrogenetic studies

Constraints on the calculation and use of mineral/melt and two-mineral partition coefficients for Ti (D Ti ) have been derived from current knowledge of the distinct crystal-chemical mechanisms for the incorporation of Ti 4+ in the amphibole structure as follows: (1) mineral/melt partition coefficients for Ti, and other tetravalent high field-strength elements (HFSE), can be compared only when considering the fraction of Ti 4+ that enters the same structural site; (2) accurate two-mineral partition coefficients can be obtained only when considering the fraction of Ti 4+ that is involved in the same crystal-chemical mechanism in the two relevant phases (i.e., M2 Ti 4+ and M1 Ti 4+ for amphibole and clinopyroxene, respectively). The complete crystal-chemical characterization of synthetic titanian pargasite and kaersutite and of synthetic richterite (all crystallized under P , T , X , f O 2 conditions of interest for upper-mantle studies) shows that the site preference of Zr and Hf differs between the two amphibole compositions; these elements are essentially ordered at M2 in pargasite and kaersutite, but preferentially enter M1 in richterite. In the latter case, Ti segregates into the split M1′ site with distorted coordination and shorter Ti-O3 distances, whereas Zr and Hf most likely prefer the larger and more regular M1 site. The observed site preference is strongly controlled by the relative dimensions of the available sites. The crystal-chemical mechanisms that govern the incorporation of octahedral high-charge cations are the local charge balance of [IV] Al (by R 3,4+ at M2) and of dehydrogenation (by R 3,4+ at M1); thus the incorporation of Zr and Hf depends on distinct intensive parameters in the two amphibole compositions. Calculation of partition coefficients and of elastic-site parameters under the assumption that all Ti and other HFSE 4+ order at the M2 site in amphibole, as is currently done in geochemical studies, is strongly biased. In the presence of significant dehydrogenation, amphibole/melt D 0 values obtained from modeling based on the elastic-strain theory starting from the more-accurate site populations for Ti may be only 1/4 of those obtained by using the total Ti content, and the derived site parameters E and r 0 are more consistent with octahedral coordination. This result has important consequences for the prediction of D values under P - T conditions different from those of the experimental work. Applying the above concepts to data from natural assemblages, we obtained a significantly narrower (0.3–2.4 vs. 1.5–8.9) and more reasonable range of variation for amphibole/clinopyroxene D Ti . A relationship between these values for D Ti and pressure is also now apparent.

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

Sodic-ferripedrizite, ideally \(Na(LiNa)(Fe_{2}^{3+}Mg_{2}Li)Si_{8}O_{22}(OH)_{2}\) , is the second new amphibole end-member 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, D meas = 3.15, D calc = 3.15 g/cm 3 . 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: α = 1.694 (1), β = 1.698(1), and γ = 1.702(1); 2 V Z = 83(2)° and 2 V Z,calc = 85(3)°, dispersion r > v . Sodic-ferripedrizite is monoclinic, space group C 2/ m , a = 9.536(1), b = 17.789(2), c = 5.277(1) A, β = 102.53°, V = 873.8(1) A 3 . The five strongest lines in the X-ray powder-diffraction pattern [ d in A[ 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 \(^{A}(Na_{0.70}K_{0.03})^{B}(Li_{1.34}Na_{0.58}Ca_{0.08}) (Mg_{1.75}Fe_{1.65}^{3+}Li_{0.88}Fe_{0.32}^{2+}A1_{0.21}Ti_{0.11}Mn_{0.07}^{2+}Zn_{0.01})Si_{8.00}O_{22}(OH_{1.35}F_{0.65})\) . 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.

Distinct site preferences for heavy and light REE in amphibole and the prediction of Amph/L D REE

New experimental amphibole/melt partition coefficients from a variety of geologically relevant amphibole (pargasite, kaersutite, and K-richterite) and melt compositions obtained under conditions of interest to upper-mantle studies are combined with the results of X-ray single-crystal structure refinement. The ideal cation radii (r0), calculated using the lattice-site elastic-strain model of Blundy and Wood (1994) under the hypothesis of complete REE (rare earth elements) ordering at [8]M4, mostly differ significantly from those obtained from both the structure refinement and the ionic radius of [8]Ca2+. Heavier REE may also strongly deviate from the parabolic trends defined by the other REE. On the basis of the crystal-chemical knowledge of major-element site-preference in amphibole and the occurrence of two sites with different co-ordination within the M4 cavity (M4 for Ca and Na, M4′ for Fe2+ and Mg), we propose a new model for REE incorporation. LREE order at the [8]M4 site, whereas HREE prefer the M4′ site with lower co-ordination in amphiboles with a significant cummingtonite component, and may also enter the M2 octahedron, at least in richterite. This more complex model is consistent with the observed Amph/LD, and drops the usual assumption that REE behave as a homogeneous group and order at the M4 site. The availability of multiple crystal-chemical mechanisms for REE3+ incorporation explains why measured and estimated Amph/LDHREE may differ by up to one order of magnitude. When REE enter two different sites within the same cavity, a fit performed on the basis of a single curve may appear correct, but the values obtained for r0 are biased towards those of the dominant site, and the Young's modulus is underestimated. When REE are incorporated in multiple sites in different cavities, the observed pattern cannot be reduced to a single curve, and the partition coefficients of heavy REE would be strongly underestimated by a single-site fit. The simplistic assumption that REE occupy a single site within the amphibole structure can thus substantially bias predictive models based on the elastic-strain theory. Our combined approach allows linkage between fine-scale site preference and the macroscopic properties of minerals and provides more reliable predictive models for mineral/melt partitioning. After the possible site-assignments have been identified, the shape of the Onuma curves constructed from accurately determined Amph/LDREE now allows the active mechanisms for REE incorporation in amphiboles to be recognised even where site populations are not available. The REE preference for polyhedra with smaller size and lower co-ordination than those occupied by Ca invalidates the general idea that Ca acts as a “carrier” for REE.

Structure and Bonding Anisotropy in Intergrowth Oxides: A Clue to the Manifestation of Bidimensionality in T-, T′-, and T*-Type Structures

The manifestation of bidimensionality in T-, T′-, and T*-type structures of A2BO4 oxides occurs in terms of cooperative structure–bonding properties involving both the metal–oxygen A(B)–O bonds and the A–A cationic repulsions. The analysis of the A-metal network gives evidence of ninefold cationic coordination very similar to the ninefold oxygen coordination. The short A–A apical distance results in A–A repulsions which are able to balance the A–O apical overbonding. In La2NiO4 as compared to La2CuO4, the contribution of these repulsions is more important (dA–A=3.25 Å in La2NiO4; 3.66 Å in La2CuO4). The perovskite (P)/rock salt (RS) intergrowth, T-type structure, due to strong geometrical constraint, has excess anisotropy which needs to be relieved. In contrast, the perovskite (P)/calcium fluoride (CF), T′- type structure, is nearly free from any excess anisotropy to be relieved. In the (P)/(RS)/(P)/(CF) intergrowth, T*-type structure, owing to a more regular distribution of the A–O bonds, the excess anisotropy is further restricted. Crystal chemical mechanisms such as A substitutions and the oxidation of Cu, which lower the excess charge in the anionic and cationic (P) and (RS) slabs, respectively, do not operate. The useful role of controlled excess structure–bonding anisotropy is emphasized.

Detection, crystal-chemical mechanisms and petrological implications of [6]Ti4+ partitioning in pargasite and kaersutite

Detection, crystal-chemical mechanisms and petrological implications of [6]Ti4+ partitioning in pargasite and kaersutite

On the Correct Determination of DTi in Amph/Liq and Amph/Cpx Partitioning: The Key Role of the Crystal-Chemical Mechanisms

Recent work on spinel-facies mantle assemblages has shown that Amph/CpXDTi (a) are always higher than 1; (b) may vary in the range 2.7-8.9, and (c) are significantly higher than those of REEs and Zr (Vannucci et al. , 1995). Therefore, in amphibolebearing systems Ti can strongly fractionate relative to REEs and Zr, with important implications for the modelling of geochemical processes. The behaviour of Ti 4§ in amphibole is rather complex, and has recently been the subject of a number of systematic studies based on complete (EMP+SIMS) chemical characterization and X-ray single-crystal structure-refinement (SREF) of calcic, sodic-calcic and sodic amphiboles (Oberti e t al., 1992, Hawthorne et al. , 1993,1998). These studies have shown that Ti 4§ can enter the amphibole structure according to three different crystalchemical mechanisms: (a) T2Ti4+ T2Si4+ 1; (b) M1Ti4+ O302MI(Mg,Fe)2_~ ~ (c) M2'M3Ti4+