“Henry's law” behaviour of Sm in a natural plagioclase/melt system: Importance of experimental procedure

We report an experimental study of the partitioning of Sm in a natural plagioclase/melt system as a function of concentration of Sm at constant pressure, temperature and bulk composition. Both radioactive ¹⁵¹Sm and non-radioactive Sm₂O₃ were used. In experiments in which the sample was initially held at a temperature above the liquidus for only one hour, we find that the plagioclase/ melt partition coefficient for Sm increases with decreasing concentration of Sm. There is also evidence for isotopic heterogeneity in the initially molten charge. In experiments in which the sample was initially held at the same temperature above the liquidus for 24 hours, the partition coefficient was constant as a function of concentration and no evidence for isotopic heterogeneity was observed. These experiments indicate that partition coefficients obtained from experiments involving tracer isotopes are very sensitive to experimental procedure. Our experiments also indicate that the partition coefficient for Sm between plagioclase and melt is constant over the range 3 to 50,000 ppm Sm in the melt. That range encompasses most concentrations in natural systems.     

An experimental study of the partitioning of a rare earth element (Gd) in the system diopside—aqueous vapour

The partitioning of Gd in the experimental system diopside-aqueous vapor as a function of temperature, pressure, composition of the phases, time, grain size, solid-liquid ratio and Gd concentration has been investigated. A radioactive tracer measurement was used to determine Gd concentration in the separated phases. Diposides were reacted with aqueous vapor containing tracer Gd and reversibility was tested by reacting Gd-doped diopsides with pure aqueous vapor. Equilibration of Gd between the bulk of the diopside and the liquid was found to be limited by the slow rate of Gd diffusion in diopside, maximum value of D = 2 × 10^?15cm²sec^?1 at 800°C and 1 kb. Depending on whether the diopside was previously synthesized or synthesized from an oxide mix during the experiment, Gd concentrations were zoned in the crystal such that higher concentrations existed at the edges or center, respectively. Equilibrium is difficult to achieve in these experiments, but at the optimum experimental conditions for equilibration, the Gd diopside-aqueous vapor distribution coefficient is 20 ± 6 (800°C, 1 kb) in approximate agreement with previous results of 55 ± 23. Changing the composition of the aqueous vapor indicated that possible mechanisms for Gd substitution included coupling of Gd³⁺ with H⁺ or Na⁺ replacing 2Ca²⁺, or substitution of 2Gd³⁺ for 3Ca²⁺ with formation of a cation vacancy.     

Defect structure of the low temperature α-cristobalite phase and the cristobalite ⇆ tridymite transformation in (Si-Ge)O2

Using minimum exposure techniques, it is feasible to perform high resolution electron microscopy on the α-cristobalite phase of (Si_0.9 Ge_0.1)O₂, which is extremely radiation sensitive. Such images reveal atomic scale information of twins and tridymite-like stacking faults on (1 1 1)_β planes, as well as of domain boundaries resulting from the β→α transition. Polytype structures are formed in certain cases. Morphological features suggest that the phase transformation cristobalite → tridymite proceeds by means of a zonal dislocation mediated synchro-shear process on (1 1 1)_β planes; the geometry of this process is analyzed. Received: 13 June 1999 / Accepted: 30 October 1999     

Assessment of spatial distribution and possible sources of heavy metals in the soils of Sariseki-Dörtyol District in Hatay Province (Turkey)

In this study, total heavy metal content of soil and their spatial distribution in Sar?seki-Dörtyol district were analyzed and mapped. Variable distance grids (0.5, 1.0 and 2.0 km) were established, with a total of 102 soil samples collected from two different soil depths (0–5 and 5–20 cm) at intersections of the grids (51 sampling point). Soil samples were analyzed for heavy metals (Cd, Co, Cr, Cu, Pb, Zn, Mn, Fe, and Ni). The most proper variogram models for the contents of all heavy metal were spherical and exponential ones. The ranges of semivariograms were between 1.9 and 31.1 km. According to the calculated geoaccumulation (I _geo) values, samples from both soil depths were classified as partly to highly polluted with Cd and Ni and partly polluted with Cr and as partly polluted-to-not polluted with Pb and not polluted with Cu, Fe and Mn. Similar results were also obtained when evaluated by the enrichment factor. The contamination levels of the heavy metals were Ni > Cd > Cr > Pb > Zn > Cu > Co > Fe > Mn in decreasing order. The soils in the study area are contaminated predominantly by Cd and Ni, which may give rise to various health hazards or diseases. Cadmium pollution results primarily from industrial activities and, to a lesser extent, from vehicular traffic, whereas Ni contents in the study area result from parent material, phosphorus fertilizer, industries, and vehicles.     

REE and HFS element behaviour in the alteration facies of the Erenler Dağı Volcanics (Konya,Turkey) and kaolinite occurrence

Mainly high-K, calc-alkaline, Late Miocene to Pliocene volcanic rocks cropped out of the Konya area in Central Anatolia, Turkey. The volcanic rocks are predominantly andesitic to dacitic in composition and rarely basalt, basaltic andesite, basaltic trachyandesite and pyroclastics. Kaolinite, illite, Ca-montmorillonite, alunite, jarosite, minamiite and silica polymorphs were formed by widespread and intense hydrothermal alteration in or around the volcanic products. To investigate the effects of hydrothermal alteration on the chemistry of volcanic rocks, the whole rock chemical composition (major and trace elements, including rare-earth elements (REE) was analysed. The results of the study demonstrate notable differences in the REE behaviour in the different sample groups. REE trends of fresh parent rocks to weakly-, moderately-altered, kaolinitic and alunitic rocks are characterised by strong LREE enrichment ((La/Lu)_cn = 14.57, 11,8 to 15.20, 4.54 to 13.30, 12.5 to 24.2 and 34.6 to 47.26, respectively). Most of the samples have pronounced negative and/or weakly-negative Eu anomalies ranging from 0.75 to 0.98 while three samples have weakly-positive Eu anomalies. LRE element contents are higher than those of HREE in the samples. The LRE elements were strongly enriched in the kaolinitic and alunitic alteration; in weakly- and moderately-altered rocks. LREE are nearly immobile whereas HRE elements show different behaviour in different rock groups. The HFS and TRT elements are slightly mobilised in weakly-altered rocks, but enriched in other alteration types. Elements commonly assumed to be immobile (e.g. Y, Zr, Nb, Hf, TiO₂, Al₂O₃, REE) show variation in mass calculation. LIL elements showed enrichment over LREE and MREE, and similar behaviour, in contrast with HFSE. A clear increment of trans-transition elements (TRTE) was found mainly in alunitic and partly in kaolinitic samples.     

Ra’s Abdah of the north Eastern Desert of Egypt: the role of granitic dykes in the formation of radioactive mineralization,evidenced by zircon morphology and chemistry

Syenogranitic dykes in the north of Egypt’s Eastern Desert are of geological and economic interest because of the presence of magmatic and supergene enrichment of radioactive mineralization. Zircon crystal morphology within the syenogranitic dykes allows precise definition of sub-alkaline series granites and crystallized at mean temperature of about 637 °C. The growth pattern of the zircons suggest magmatic and hydrothermal origins of radioactive mineralization. Hydrothermal processes are responsible for the formation of significant zircon overgrowth; high U-zircon margins might have occurred contemporaneously with the emplacement of syenogranitic dykes which show anomalous uranium (eU) and thorium (eTh) contents of up to 1386 and 7330 ppm, respectively. Zircon chemistry revealed a relative increase of Hf consistent with decreasing Zr content, suggesting the replacement of Zr by Hf during hydrothermal activity. Visible uranium mineralization is present and recognized by the presence of uranophane and autunite.     

Uranium-Lead and Lu–Hf Isotope Systematics of Zircon and Sm–Nd Isotopes in Rocks of the Dyumtaley Ore-Bearing Intrusion (Taimyr,Russia): New Evidence for the Role of the Depleted Mantle in Its Genesis

Doklady Earth Sciences - This study utilized SHRIMP, LA MC–ICP–MS and ID-TIMS analytical techniques and presents for the first time U–Pb age, Lu-Hf-isotope features of zircon and...     

Spatial distribution of heavy metals and arsenic in soils of Aragón (northeast Spain): controlling factors and environmental implications

Legal regulation of heavy metal contents is an important issue in many European countries, where laws still do not exist establishing the heavy metal levels permitted in soils. As a first step to determine the reference levels of heavy metals, it is necessary to know their contents in soils under natural conditions. To achieve this goal in the Autonomous Community of Aragón, a total of 133 sites have been sampled. A balanced allotment of the sampling sites, according to soil spatial distribution in the region, has been carried out by selecting 9 soil types that represent 97.5% of its surface area. Fifteen elements (Cr, Cu, Ba, As, Sb, Hg, Sn, Mn, Fe, Al, Zn, Ni, Co, Cd and Pb) have been analysed by (ICP–AES) after a partial acid extraction. The content of analysed elements has been correlated to some soil parameters such as organic matter, pH, and granulometric fractions. Results of the statistical analyses have shown a large variety and complexity in some of these relationships. The main factors for variation in the heavy metal contents are both the soil type and the lithology. Gypsisols and Calcisols developed on sedimentary rocks have the lowest contents while Leptosols overlying metamorphic and igneous rocks have the highest contents. The spatial distribution of heavy metal contents shows a large variability with the highest contents in the mountain ranges (Iberian and Pyrenees) and the lowest in the plains of the central Ebro valley.     

Assessment of pollution levels and human health risk of heavy metals in dust deposited on Yerevan’s tree leaves (Armenia)

The total concentrations of Cd, As, Pb, Cr, Ni, Co, Zn, Cu, Ag, Hg, and Mo were determined in the atmospheric dust of the city of Yerevan by atomic absorption spectrometry (AAnalyst PE 800). Heavy metal pollution levels were evaluated by calculating geo-accumulation (I _geo ) and summary pollution (Z_c) indices. Potential human health risk was assessed using the United States Environmental Protection agency’s human health risk assessment model. The results show that mean contents of all elements tested except Ni and Cr were substantially higher than local geochemical background values. According to the I _geo , Yerevan territory is strongly-to-extremely polluted by As, Ag, Hg, Mo, and Cd. The Z_c assessment indicated that very high pollution was detected in 36 % of samples, high in 32 %, average in 12 %, and low in 20 %. The health risk assessment revealed a non-carcinogenic risk (HI >1) for children at 13 samplings sites and for adults at one sampling site. For children the risk was due to elevated levels of Mo, Cd, Co, and As, while for adults, only Mo. Carcinogenic risk (>1:1,000,000) of As and Cr via ingestion pathway was observed in 25 and 14 samples, respectively. This study, therefore, is the base for further detailed investigations to organize problematic site remediation and risk reduction measures.     

A hitherto unrecognised band in the Raman spectra of silica rocks: influence of hydroxylated Si–O bonds (silanole) on the Raman moganite band in chalcedony and flint (SiO<Subscript>2</Subscript>)

Chalcedony is a spatial arrangement of hydroxylated nanometre-sized α-quartz (SiO₂) crystallites that are often found in association with the silica mineral moganite (SiO₂). A supplementary Raman band at 501 cm⁻¹ in the chalcedony spectrum, attributed to moganite, has been used for the evaluation of the quartz/moganite ratio in silica rocks. Its frequency lies at 503 cm⁻¹ in sedimentary chalcedony, representing a 2 cm⁻¹ difference with its position in pure moganite. We present a study of the 503 cm⁻¹ band’s behaviour upon heat treatment, showing its gradual disappearance upon heating to temperatures above 300 °C. Infrared spectroscopic measurements of the silanole (SiOH) content in the samples as a function of annealing temperature show a good correlation between the disappearance of the 503 cm⁻¹ Raman band and the decrease of structural hydroxyl. Thermogravimetric analyses reveal a significant weight loss that can be correlated with the decreasing of this Raman band. X-ray powder diffraction data suggest the moganite content in the samples to remain stable. We propose therefore the existence of a hitherto unknown Raman band at 503 cm⁻¹ in chalcedony, assigned to ‘free’ Si–O vibrations of non-bridging Si–OH that oscillate with a higher natural frequency than bridging Si–O–Si (at 464 cm⁻¹). A similar phenomenon was recently observed in the infrared spectra of chalcedony. The position of this Si–OH-related band is nearly the same as the Raman moganite band and the two bands may interfere. The actually observed Raman band in silica rocks might therefore be a convolution of a silanole and a moganite vibration. These findings have broad implications for future Raman spectroscopic studies of moganite, for the assessment of the quartz/moganite ratio, using this band, must take into account the contribution from silanole that are present in chalcedony and moganite.     

Crystal chemistry of perovskite-type compounds in the tausonite-loparite series, (Sr1−2 x Na x La x )TiO3

Perovskite-type compounds in the series tausonite-loparite, (Sr_{1?2 x} Na _x La _x )TiO₃, were synthesized by solid-state reaction (final heating at 1200–1300?°C), and studied using “conventional” and synchrotron X-ray powder diffractometry. The structures of intermediate compositions were determined using the Rietveld profile refinement method. In the compositional range 0?≤x?≤ 0.1, the series comprises perovskites characterized by an undistorted cubic structure (space group Pm3¯m, a?≈ 3.905–3.902?Å, Z?=?1). Intermediate compounds in the range 0.15?≤?x?≤?0.35 crystallize with tetragonal symmetry (I4/mcm, a?≈? , c?≈? , Z?=?4) derived from the cubic aristotype by antiphase rotation of the TiO₆ octahedra about a fourfold axis. The angle of rotation estimated from the positional parameters of oxygen atoms ranges from 2.5(7)° to 5.5(4)°. The cubic-to-tetragonal transition arises from substitution of Sr²⁺ by the comparatively smaller Na¹⁺ and La³⁺ cations. A further transition from the tetragonal to rhombohedral symmetry (R3¯c, a?≈? , c?≈?2 , Z?=?6) occurs between x?=?0.35 and 0.40, and apparently does not involve formation of perovskite with an intermediate two-tilt structure (Imma). The rhombohedral structure is characterized by a multicomponent octahedral tilt about a threefold axis ranging in magnitude from 6.5(2)° to 7.7(2)°. In the series (Sr_{1?2 x} Na _x La _x )TiO₃, the unit-cell dimensions decrease, and the degree of structural distortion increases with x.     

The role of trace element partitioning between garnet,zircon and orthopyroxene on the interpretation of zircon U–Pb ages: an example from high-grade basement in Calabria (Southern Italy)

The recognition of the coeval growth of zircon, orthopyroxene and garnet domains formed during the same metamorphic cycle has been attempted with detailed microanalyses coupled with textural analyses. A coronitic garnet-bearing granulite from the lower crust of Calabria has been considered. U–Pb zircon data and zircon, garnet and orthopyroxene chemistries, at different textural sites, on a thin section of the considered granulite have been used to test possible equilibrium and better constrain the geological significance of the U–Pb ages related to zircon separates from other rocks of the same structural level. The garnet is very rich in REE and is characterised by a decrease in HREE from core to outer core and an increase in the margin. Zircons show core–overgrowth structures showing different chemistries, likely reflecting episodic metamorphic new growth. Zircon grains in matrix, corona around garnet and within the inner rim of garnet, are decidedly poorer in HREE up to Ho than garnet interior. Orthopyroxene in matrix and corona is homogeneously poor in REE. Thus, the outer core of garnet and the analysed zircon grains grew or equilibrated in a REE depleted system due to the former growth of garnet core. Zircon ages ranging from 357 to 333 Ma have been determined in the matrix, whereas ages 327–320 Ma and around 300 Ma have been determined, respectively, on cores and overgrowths of zircons from matrix, corona and inner rim of garnet. The calculated DREE_zrn/grt and DREE_opx/grt are largely different from the equilibrium values of literature due to strong depletion up to Ho in zircon and orthopyroxene with respect to garnet. On the other hand, the literature data show large variability. In the case study, (1) the D _zrn/grt values define positive and linear trends from Gd to Lu as many examples from literature do and the values from Er to Lu approach the experimental results at about 900 °C in the combination zircon dated from 339 to 305 Ma with garnet outer core, and (2) D _opx/grt values define positive trends reaching values considered as suggestive of equilibrium from Er to Lu only with respect to the outer core of garnet. The presence of a zircon core dated 320 Ma in the inner rim of garnet suggests that it, as well as those dated at 325–320 Ma in the other textural sites and, probably, those dated at 339–336 Ma showing depletion of HREE, grew after the garnet core, which sequestered a lot of HREE and earlier than the HREE rich margin of garnet. The quite uniform REE contents in orthopyroxene from matrix and corona and the low and uniform contents of HREE in the zircon overgrowths dated at about 300 Ma allow to think that homogenisation occurred during or after the corona formation around this age. The domains dated around 325–320 Ma would approximate the stages of decompression, whereas the metamorphic peak probably occurred earlier than 339 Ma.     

The crystal chemistry of lithium in the alluaudite structure: a study of the (Na1− x Li x )1.5Mn1.5Fe1.5(PO4)3 solid solution (x = 0 to 1)

Summary Phosphates of compositions (Na_1–xLi_x)_1.5Mn_1.5Fe_1.5(PO₄)₃ were synthesized by solid state reactions in air, and pure alluaudite-type compounds were obtained for x=0.00, 0.25, and 0.50. Rietveld refinements of X-ray powder diffraction data indicate the occurrence of Mn²⁺ in the M(1) site, and of Fe³⁺ and Mn²⁺ in the M(2) site. For x=0.25 and 0.50, A(1) is occupied by Li⁺ and Na⁺, whereas A(2) is occupied by Na⁺ and vacancies. A careful examination of the number of electrons occurring in the A sites of the alluaudite-type compounds (Na_1–xLi_x)MnFe₂(PO₄)₃ and (Na_1–xLi_x) CdIn₂(PO₄)₃ confirms that lithium occupies only the A(1) crystallographic site of the alluaudite structure.     

Fault zones and stress fields of the Earth’s crust in the vicinity of Ulaanbaatar (Mongolia) at the modern stage of tectogenesis

The seismic and emanation activity of fault zones at a geodynamic test site arranged in the vicinity of Ulaanbaatar to investigate dangerous geological processes in a heavily populated area of Mongolia is analyzed. It has been found that, at the modern stage of tectogenesis, destruction of the Earth’s crust occurs in the area of the strike-slip fault transformed from the Indo-Asian collision zone. Thus, a network of fault zones with seismic and emanation activity is developed in this area. In the vicinity of Ulaanbaatar, there are four systems of fault zones. The spatial pattern of the systems shows a nodal junction of comparatively large rightlateral and left-lateral strike-slip faults of submeridional and sublatitudinal orientation. These strike-slips are barely connected with a network of ancient faults. The northwestern and northeastern zones developed by compression and tension, on the contrary, inherit disjunctive structures of the most recent age located in a nodal junction at the tops of dihedral angles. Determination of the pattern of recent faulting is essential to estimate the seismic hazard for the capital of Mongolia, where more than one-third of the nation lives.     

Sulfur–selenium isomorphous substitution and morphotropic transition in the Ag3Au(Se,S)2 series

Gold–silver sulfoselenides of the series Ag3AuSexS2–x (x = 0.25; 0.5; 0.75; 1; 1.5) were synthesized from melts on heating stoichiometric mixtures of elementary substances in evacuated quartz ampoules. According to X-ray single-crystal analysis, compound Ag₃Au₁Se_0.5S_1.5 has the structure of gold–silver sulfide Ag3AuS2 (uytenbogaardtite) with space group R3c. The volume of this compound is 1.5% larger than that of the sulfide analog. According to powder X-ray diffraction, compounds Ag₃AuSe_0.25S_1.75 and Ag₃AuSe_0.75S_1.25 also show trigonal symmetry. Compounds Ag₃AuSeS and Ag₃AuSe_1.5S_0.5 are structurally similar to the low-temperature modification of gold–silver selenide Ag₃AuSe₂ (fischesserite) with space group I4132. These data suggest the existence of two solid solutions: petzite-type cubic Ag₃AuSe₂–Ag₃AuSeS (space group I4132) and trigonal Ag₃AuSe_0.75S_1.25–Ag₃AuS₂ (space group R3c).It was found that fischesserite from the Rodnikovoe deposit (southern Kamchatka) contains 3.5–4 wt.% S. At the Kupol deposit (Chukchi Peninsula), fischesserite contains up to 2.5 wt.% S and uytenbogaardtite contains up to 5.3 wt.% Se. At the Ol’cha and Svetloe (Okhotskoe) deposits (Magadan Region), uytenbogaardtite contains up to 0.5 and 1.8 wt.% Se, respectively. Literature data on the compositions of silver–gold selenides and sulfides from different deposits were summarized and analyzed. Analysis of available data on the S and Se contents of natural fischesserite and uytenbogaardtite confirms the miscibility gap near composition Ag3AuSeS.     

Ab initio crystal structure and elasticity of tuite, γ-Ca<Subscript>3</Subscript>(PO<Subscript>4</Subscript>)<Subscript>2</Subscript>, with implications for trace element partitioning in the lower mantle

Tuite forms by the breakdown of apatite at high pressure and is thus expected to play a role in extending the phosphorus cycle beyond the stability field of apatite and into the lower mantle. With its large, high-coordination cation sites, tuite is thought to be able to dissolve large quantities of incompatible elements such as rare earth elements, Sr, Th, and U, and is potentially an important mantle reservoir for these elements. In this paper, ab initio calculations of the structure and elasticity of tuite to lower mantle pressure are presented and used to probe trace element incorporation. The calculated zero-pressure volumes of the M1 and M2 cation sites were 50.23 and 36.61 Å³, while the corresponding bulk moduli K ₀ are 116.1 and 94.2 GPa, significantly lower than the 234.1 GPa calculated for the M site of CaSiO₃ perovskite (cpv), another likely host for incompatible elements in the mantle. The partitioning of impurities between tuite and cpv is investigated using a lattice strain model, parameterized by the ab initio calculations, to calculate isovalent substitution energies across a range of pressures and impurity sizes. Additionally, energies of strontium and barium defects in tuite are compared with those of equivalent defects in cpv, and it is found that both elements will partition strongly from cpv into tuite.     

Thermodynamics of arsenates,selenites and sulfates in the oxidation zone of sulfide ores: XII. Mineral equilibria in the Cd–Se–H<Subscript>2</Subscript>O system at 25°C

Understanding the mechanisms of cadmium and selenium behavior under near-surface conditions is very important for solving certain environmental problems. The principal aim of this study is physicochemical analysis of the formation conditions of synthetic cadmium selenite CdSeO₃ · H₂O and experimental investigation of its thermal stability and dehydration and dissociation mechanisms. The synthesis was performed by boiling-dry aqueous solutions of cadmium nitrate and sodium selenite. The obtained samples were identified with electron microprobe and powder X-ray diffraction. Complex thermal analysis (thermogravimetry and differential scanning calorimetry) have shown that CdSeO₃ · H₂O is dehydrated at 177–241°C in two stages, apparently corresponding to the formation of CdSeO₃ · 2/3H₂O. The Eh–pH diagrams were calculated using the Geochemist’s Workbench (GWB 9.0) software package. The Eh–pH diagrams have been calculated for the Cd–Se–Н₂О and Cd–Se–CO₂–H₂O systems for the average content of these elements in underground waters. The formation of cadmium selenite, CdSeO₃ · H₂O in the oxidation medium is quite possible. The existence of CdSeO₃ is possible at high temperature.     

Dualistic distribution coefficients of trace elements in the system mineral–hydrothermal solution. III. precious metals (Au and Pd) in magnetite and manganmagnetite

Distribution coefficients D of Au and Pd between magnetite (manganmagnetite) and ammonium chloride hydrothermal solution and the structural D^str and surface-related D^sur terms of these coefficients were determined at 450 and 500°С and a pressure of 1 kbar using internal sampling techniques. Quantitative data on the speciation of precious metals are obtained using the technique of statistical selections of analytical data on single crystals SSADSC and compared with LA-ICP-MS data. Both Pd and Au are elements compatible with magnetite and its manganoan variety: D^str is ≈3 for Pd and ≈1 for Au, although Au seems to weakly enrich fluid at 500°C: D^str ≈ 0.5–0.8. The trends of postmagmatic Pd and Au fractionation can thus strongly depend on the presence of spinel-group minerals, first of all, magnetite and its solid solutions. The dualistic nature of the distribution coefficients provides sound grounds to believe that both elements are highly compatible, with regard not only for the structural but also for the surface-related modes of their occurrence (D^sur ≈ 17 and ≈50–70 for Au and Pd, respectively). The maximum concentrations of structural modes of the elements are 5.3 ppm for Au and 5.1 ppm for Pd and were found in the solid solution whose jacobsite mole fractions were 0.82 and 0.49, respectively. The principal distribution patterns of the elements in crystals are confirmed by LA-ICP-MS data. Data on this system testify that the distribution coefficients of minor and trace elements are geochemically dualistic because of the abnormal absorption properties of nanometer-sized nonautonomous phases on the surface of ore minerals, and this dualism plays an important geochemical role.