Thermal behavior of ferric selenite hydrates (Fe2(SeO3)3·3H2O,Fe2(SeO3)3·5H2O) and the water content in the natural ferric selenite mandarinoite

Any progress in our understanding of low-temperature mineral assemblages and of quantitative physico-chemical modeling of stability conditions of mineral phases, especially those containing toxic elements like selenium, strongly depends on the knowledge of structural and thermodynamic properties of coexisting mineral phases. Interrelation of crystal chemistry/structure and thermodynamic properties of selenium-containing minerals is not systematically studied so far and thus any essential generalization might be difficult, inaccurate or even impossible and erroneous. Disagreement even exists regarding the crystal chemistry of some natural and synthetic selenium-containing phases. Hence, a systematic study was performed by synthesizing ferric selenite hydrates and subsequent thermal analysis to examine the thermal stability of synthetic analogues of the natural hydrous ferric selenite mandarinoite and its dehydration and dissociation to unravel controversial issues regarding the crystal chemistry. Dehydration of synthesized analogues of mandarinoite starts at 56–87?°C and ends at 226–237?°C. The dehydration happens in two stages and two possible schemes of dehydration exist: (a) mandarinoite loses three molecules of water in the first stage of the dehydration (up to 180?°C) and the remaining two molecules of water will be lost in the second stage (>180?°C) or (b) four molecules of water will be lost in the first stage up to 180?°C and the last molecule of water will be lost at a temperature above 180?°C. Based on XRD measurements and thermal analyses we were able to deduce Fe₂(SeO₃)₃·(6-x)H₂O (x?=?0.0–1.0) as formula of the hydrous ferric selenite mandarinoite. The total amount of water apparently affects the crystallinity, and possibly the stability of crystals: the less the x value, the higher crystallinity could be expected.     

Experimental investigations of the reaction path in the MgO–CO2–H2O system in solutions with various ionic strengths,and their applications to nuclear waste isolation

The reaction path in the MgO–CO₂–H₂O system at ambient temperatures and atmospheric CO₂ partial pressure(s), especially in high-ionic-strength brines, is of both geological interest and practical significance. Its practical importance lies mainly in the field of nuclear waste isolation. In the USA, industrial-grade MgO, consisting mainly of the mineral periclase, is the only engineered barrier certified by the Environmental Protection Agency (EPA) for emplacement in the Waste Isolation Pilot Plant (WIPP) for defense-related transuranic waste. The German Asse repository will employ a Mg(OH)₂-based engineered barrier consisting mainly of the mineral brucite. Therefore, the reaction of periclase or brucite with carbonated brines with high-ionic-strength is an important process likely to occur in nuclear waste repositories in salt formations where bulk MgO or Mg(OH)₂ will be employed as an engineered barrier. The reaction path in the system MgO–CO₂–H₂O in solutions with a wide range of ionic strengths was investigated experimentally in this study. The experimental results at ambient laboratory temperature and ambient laboratory atmospheric CO₂ partial pressure demonstrate that hydromagnesite (5424) (Mg₅(CO₃)₄(OH)₂ · 4H₂O) forms during the carbonation of brucite in a series of solutions with different ionic strengths. In Na–Mg–Cl-dominated brines such as Generic Weep Brine (GWB), a synthetic WIPP Salado Formation brine, Mg chloride hydroxide hydrate (Mg₃(OH)₅Cl · 4H₂O) also forms in addition to hydromagnesite (5424).     

Biachellaite, (Na,Ca,K)8(Si6Al6O24)(SO4)2(OH)0.5 · H2O, a new mineral species of the cancrinite group

Biachellaite, a new mineral species of the cancrinite group, has been found in a volcanic ejecta in the Biachella Valley, Sacrofano Caldera, Latium region, Italy, as colorless isometric hexagonal bipyramidal-pinacoidal crystals up to 1 cm in size overgrowing the walls of cavities in a rock sample composed of sanidine, diopside, andradite, leucite and hauyne. The mineral is brittle, with perfect cleavage parallel to {10$ \bar 1 $ \bar 1 0} and imperfect cleavage or parting (?) parallel to {0001}. The Mohs hardness is 5. D_meas = 2.51(1) g/cm³ (by equilibration with heavy liquids). The densities calculated from single-crystal X-ray data and from X-ray powder data are 2.515 g/cm³ and 2.520 g/cm³, respectively. The IR spectrum demonstrates the presence of SO₄²⁻, H₂O, and absence of CO₃²⁻. Biachellaite is uniaxial, positive, ω = 1.512(1), ɛ = 1.514(1). The weight loss on ignition (vacuum, 800°C, 1 h) is 1.6(1)%. The chemical composition determined by electron microprobe is as follows, wt %: 10.06 Na₂O, 5.85 K₂O, 12.13 CaO, 26.17 Al₂O₃, 31.46 SiO₂, 12.71 SO₃, 0.45 Cl, 1.6 H₂O (by TG data), −0.10 −O=Cl₂, total is 100.33. The empirical formula (Z = 15) is (Na_3.76Ca_2.50K_1.44)_Σ7.70(Si_6.06Al_5.94O₂₄)(SO₄)_1.84Cl_0.15(OH)_0.43 · 0.81H₂O. The simplified formula is as follows: (Na,Ca,K)₈(Si₆Al₆O₂₄)(SO₄)₂(OH)_0.5 · H₂O. Biachellaite is trigonal, space group P3, a =12.913(1), c = 79.605(5) ?; V = 11495(1) ?³. The crystal structure of biachellaite is characterized by the 30-layer stacking sequence (ABCABCACACBACBACBCACBACBACBABC)_∞. The tetrahedral framework contains three types of channels composed of cages of four varieties: cancrinite, sodalite, bystrite (losod) and liottite. The strongest lines of the X-ray powder diffraction pattern [d, ? (I, %) (hkl)] are as follows: 11.07 (19) (100, 101), 6.45 (18) (110, 111), 3.720 (100) (2.1.10, 300, 301, 2.0.16, 302), 3.576 (18) (1.0.21, 2.0.17, 306), 3.300 (47) (1.0.23, 2.1.15), 3.220 (16) (2.1.16, 222). The type material of biachellaite has been deposited at the Fersman Mineralogical Museum of the Russian Academy of Sciences, Moscow, Russia, registration number 3642/1.     

Fivegite K4Ca2[AlSi7O17(O2 ? xOHx)][(H2O)2 ? xOH]Cl: A new mineral species from the Khibiny alkaline pluton of the Kola Peninsula in Russia

A new mineral fivegite has been identified in a high-potassium hyperalkaline pegmatite at Mt. Rasvumchorr in the Khibiny alkaline complex of the Kola Peninsula in Russia. This mineral is a product of the hydrothermal alteration of delhayelite (homoaxial pseudomorphs after its crystals up to 2 × 3 × 10 cm in size). Hydrodelhayelite, pectolite, and kalborsite are products of fivegite alteration. The associated minerals are aegirine, potassic feldspar, nepheline, sodalite, magnesiumastrophyllite, lamprophyllite, lomonosovite, shcherbakovite, natisite, lovozerite, tisinalite, ershovite, megacyclite, shlykovite, cryptophyllite, etc. Areas of pure unaltered fivegite are up to 2 mm in width. The mineral is transparent and colorless; its luster is vitreous to pearly. Its Cleavage is perfect (100) and distinct (010). Its Mohs hardness is 4, D(meas) = 2.42(2), and D(calc) = 2.449 g/cm³. Fivegite is optically biaxial positive: α 1.540(1), β 1.542(2), γ 1.544(2), and 2V(meas) 60(10)°. Its orientation is X = a, y = c, and Z = b. Its IR spectrum is given. Its chemical composition (wt %; electron microprobe, H₂O determined by selective sorption) is as follows: 1.44 Na₂O, 19.56 K₂O, 14.01 CaO, 0.13 SrO, 0.03 MnO, 0.14 Fe₂O₃, 6.12 Al₂O₃, 50.68 SiO₂, 0.15 SO₃, 0.14 F, 3.52 Cl, 4.59 H₂O; −O = −0.85(Cl,F)2; total 99.66. The empirical formula based on (Si + Al + Fe) = 8 is H_4.22K_3.44Na_0.39Ca_2.07Sr_0.01Fe_0.01Al_1.00Si_6.99O_21.15F_0.06Cl_0.82(SO₄)_0.02. The simplified formula is K₄Ca₂[AlSi₇O₁₇(O_{2 − x} OH _x ][(H₂O)_{2 − x} OH _x ]Cl (X = 0−2). Fivegite is orthorhombic: Pm2₁ n, a = 24.335(2), b = 7.0375(5), c = 6.5400(6) ?, V = 1120.0(2) ?³, and Z = 2. The strongest reflections of the X-ray powder pattern are as follows (d, ?, (I, %), [hkl]): 3.517(38) [020], 3.239(28) [102], 3.072(100) [121, 701], 3.040(46) [420, 800, 302], 2.943 (47) [112], 2.983(53) [121], 2.880 (24) [212, 402], 1.759(30) [040, 12.2.0]. The crystal structure was studied using a single crystal: R _hkl = 0.0585. The base of fivegite structure is delhayelite-like two-layer terahedral blocks [(Al,Si)₄Si₁₂O₃₄(O_{4 − x} OH _x )] linked by Ca octahedral chains. K⁺ and Cl⁻ are localized in zeolite-like channels within the terahedral blocks, whereas H₂O and OH occur between the blocks. The mineral is named in memory of the Russian geological and mining engineer Mikhail Pavlovich Fiveg (1899–1986), the pioneering explorer of the Khibiny apatite deposits. The type specimen is deposited at the Fersman Mineralogical Museum of the Russian Academy of Sciences in Moscow. The series of transformations is discussed: delhayelite K₄Na₂Ca₂[AlSi₇O₁₉]F₂Cl—fivegite K₄Ca₂[AlSi₇O₁₇(O_{2 − x} OH _x ]Cl—hydrodelhayelite KCa₂[AlSi₇O₁₇(OH)₂](H₂O)_{6 − x} .     

Alloriite, Na5K1.5Ca(Si6Al6O24)(SO4)(OH)0.5 · H2O, a new mineral species of the cancrinite group

Alloriite, a new mineral species, has been found in volcanic ejecta at Mt. Cavalluccio (Campagnano municipality, Roma province, Latium region, Italy) together with sanidine, biotite, andradite, and apatite. The mineral is named in honor of Roberto Allori (b. 1933), an amateur mineralogist and prominent mineral collector who carried out extensive and detailed field mineralogical investigations of volcanoes in the Latium region. Alloriite occurs as short prismatic and tabular crystals up to 1.5 × 2 mm in size. The mineral is colorless, transparent, with a white streak and vitreous luster. Alloriite is not fluorescent and brittle; the Mohs’ hardness is 5. The cleavage is imperfect parallel to {10 0}. The density measured with equilibration in heavy liquids is 2.35g/cm³ and calculated density (D _calc) is 2.358 g/cm³ (on the basis of X-ray single-crystal data) and 2.333 g/cm³ (from X-ray powder data). Alloriite is optically uniaxial, positive, ω = 1.497(2), and ɛ = 1.499(2). The infrared spectrum is given. The chemical composition (electron microprobe, H₂O determined using the Penfield method, CO₂, with selective sorption, wt %) is: 13.55 Na₂O, 6.67 K₂O, 6.23 CaO, 26.45 Al₂O₃, 34.64 SiO₂, 8.92 SO₃, 0.37 Cl, 2.1 H₂O, 0.7 CO₂, 0.08-O = Cl₂, where the total is 99.55. The empirical formula (Z = 1) is Na_19.16K_6.21Ca_4.87(Si_25.26Al_22.74O₉₆)(SO₄)_4.88(CO₃)_0.70Cl_0.46(OH)_0.76 · 4.73H₂O. The simplified formula (taking into account the structural data, Z = 4) is: [Na(H₂O)][Na₄K_1.5(SO₄)] · [Ca(OH,Cl)_0.5](Si₆Al₆O₂₄). The crystal structure has been studied (R = 0.052). Alloriite is trigonal, the space group is P31c; the unit-cell dimensions are a = 12.892(3), c = 21.340(5) ?, and V = 3071.6(15) ?³. The crystal structure of alloriite is based on the same tetrahedral framework as that of afghanite. In contrast to afghanite containing clusters [Ca-Cl]⁺ and chains ...Ca-Cl-Ca-Cl..., the new mineral contains clusters [Na-H₂O]⁺ and chains ...Na-H₂O-Na-H₂O.... The strongest reflections in the X-ray powder diffraction pattern [d, ? (I, %)(hkl)] are: 11.3(70)(100), 4.85(90)(104), 3.76(80)(300), 3.68(70)(301), 3.33(100)(214), and 2.694(70)(314, 008). The type material of alloriite is deposited in the Fersman Mineralogical Museum, Russian Academy of Sciences, Moscow. The registration number is 3459/1. Original Russian Text ? N.V. Chukanov, R.K. Rastsvetaeva, I.V. Pekov, A.E. Zadov, 2007, published in Zapiski Rossiiskogo Mineralogicheskogo Obshchestva, 2007, No. 1, pp. 82–89. A new mineral alloriite and its name were accepted by the Commission on New Minerals and Mineral Names, Russian Mineralogical Society, May 8, 2006. Approved by the Commission on New Minerals and Mineral Names, International Mineralogical Association, August 2, 2006.     

四川冕宁木落稀土矿床稀土矿物中富CO2和H2流体包裹体研究

木落稀土矿床位于四川省冕宁县境内，其成矿与喜山期岩浆碳酸岩有关。通过对矿床中主要稀土矿物氟碳铈矿中流体包裹体的岩相学、包裹体显微测温分析和包裹体成分的LRM分析等，对成矿流体的特征、演化及稀土矿化过程进行了讨论，结果表明与稀土成矿有关的流体为富CO₂和SO₂ 的中高温、高压超临界流体。温度、压力降低和流体不混溶是造成稀土矿物沉淀的主要机制。     

Shlykovite KCa[Si4O9(OH)] · 3H2O and cryptophyllite K2Ca[Si4O10] · 5H2O, new mineral species from the Khibiny alkaline pluton, Kola Peninsula, Russia

New minerals, shlykovite and cryptophyllite, hydrous Ca and K phyllosilicates, have been identified in hyperalkaline pegmatite at Mount Rasvumchorr, Khibiny alkaline pluton, Kola Peninsula, Russia. They are the products of low-temperature hydrothermal activity and are associated with aegirine, potassium feldspar, nepheline, lamprophyllite, eudialyte, lomonosovite, lovozerite, tisinalite, shcherbakovite, shafranovskite, ershovite, and megacyclite. Shlykovite occurs as lamellae up to 0.02 × 0.02 × 0.5 mm in size or fibers up to 0.5 mm in length usually combined in aggregates up to 3 mm in size, crusts, and parallel-columnar veinlets. Cryptophyllite occurs as lamellae up to 0.02 × 0.1 × 0.2 mm in size intergrown with shlykovite being oriented parallel to {001} or chaotically arranged. Separate crystals of the new minerals are transparent and colorless; the aggregates are beige, brownish, light cream, and pale yellowish-grayish. The cleavage is parallel to (001) perfect. The Mohs hardness of shlykovite is 2.5–3. The calculated densities of shlykovite and cryptophyllite are 2.444 and 2.185 g/cm³, respectively. Both minerals are biaxial; shlykovite: 2V _meas = −60(20)°; cryptophyllite: 2V _meas > 70°. The refractive indices are: shlykovite: α = 1.500(3), β = 1.509(2), γ = 1.515(2); cryptophyllite: α = 1.520(2), β = 1.523(2), γ = 1.527(2). The chemical composition of shlykovite determined by an electron microprobe (H₂O determined from total deficiency) is as follows, wt %: 0.68 Na₂O, 11.03 K₂O, 13.70 CaO, 59.86 SiO₂, 14.73 H₂O; the total is 100.00. The empirical formula calculated on the basis of 13 O atoms (OH/H₂O calculated from the charge balance) is (K_0.96Na_0.09)_Σ1.05Ca_1.00Si_4.07O_9.32(OH)_0.68 · 3H₂O. The idealized formula is KCa[Si₄O₉(OH)] · 3H₂O. The chemical composition of cryptophyllite determined by an electron microprobe (H₂O determined from the total deficiency) is as follows, wt %: 1.12 Na₂O, 17.73 K₂O, 11.59 CaO, 0.08 Al₂O₃, 50.24 SiO₂, 19.24 H₂O, the total is 100.00. The empirical formula calculated on the basis of (Si,Al)₄(O,OH)₁₀ (OH/H₂O calculated from the charge balance) is (K_1.80Na_0.17)_Σ1.97Ca_0.99Al_0.01Si_3.99O_9.94(OH)_0.06 · 5.07H₂O. The idealized formula is K₂Ca[Si₄O₁₀] · 5H₂O. The crystal structures of both minerals were solved on single crystals using synchrotron radiation. Shlykovite is monoclinic; the space group is P2₁/n; a = 6.4897(4), b = 6.9969(5), c = 26.714(2)?, β = 94.597(8)°, V = 1209.12(15)?³, Z = 4. Cryptophyllite is monoclinic; the space group is P2₁/n; a = 6.4934(14), b = 6.9919(5), c = 32.087(3)?, β = 94.680(12)°, V= 1451.9(4)?, Z = 4. The strongest lines of the X-ray powder patterns (d, ?-I, [hkl] are: shlykovite 13.33–100[002], 6.67–76[004], 6.47–55[100], 3.469–45[021], 3.068–57[$ \bar 1 $ \bar 1 21], 3.042–45[121], 2.945–62[ 23], 2.912–90[025, 12, 211]; cryptophyllite 16.01–100[002], 7.98–24[004], 6.24–48[101], 3.228–22[$ \bar 1 $ \bar 1 09], 3.197–27[0.0.10], 2.995–47[122], 2.903–84[123, 204, $ \bar 1 $ \bar 1 24, 211], 2.623–20[028, 08, 126]. Shlykovite and cryptophyllite are members of new related structural types. Their structures are based on a two-layer packet consisting of tetrahedral Si layers linked with octahedral Ca chains. Mountainite, shlykovite and cryptophyllite could be combined into the mountainite structural family. Shlykovite is named in memory of Russian geologist V. G. Shlykov (1941–2007); the name cryptophyllite is from the Greek words meaning concealed and leaf that allude to its layered structure (phyllosilicate) in combination with a lamellar habit and intimate intergrowths with visually indistinguishable shlykovite. Type specimens of the minerals are deposited at the Fersman Mineralogical Museum of the Russian Academy of Sciences, Moscow.     

Armouring of well cement in H2S–CO2 saturated brine by calcite coating – Experiments and numerical modelling

The active acid gas (H₂S–CO₂ mixture) injection operations in North America provide practical experience for the operators in charge of industrial scale CO₂ geological storage sites. Potential leakage via wells and their environmental impacts make well construction durability an issue for efficiency/safety of gas geological storage. In such operations, the well cement is in contact with reservoir brines and the injected gas, meaning that gas–water–solid chemical reactions may change the physical properties of the cement and its ability to confine the gas downhole. The cement-forming Calcium silicate hydrates carbonation (by CO₂) and ferrite sulfidation (by H₂S) reactions are expected. The main objective of this study is to determine their consequences on cement mineralogy and transfer ability. Fifteen and 60 days duration batch experiments were performed in which well cement bars were immersed in brine itself caped by a H₂S–CO₂ phase at 500 bar–120 °C. Scanning electron microscopy including observations/analyses and elemental mapping, mineralogical mapping by micro-Raman spectroscopy, X-ray diffraction and water porosimetry were used to characterize the aged cement. Speciation by micro-Raman spectroscopy of brine trapped within synthetic fluid inclusions were also performed. The expected calcium silicate hydrates carbonation and ferrite sulfidation reactions were evidenced. Furthermore, armouring of the cement through the fast creation of a non-porous calcite coating, global porosity decrease of the cement (clogging) and mineral assemblage conservation were demonstrated. The low W/R ratio of the experimental system (allowing the cement to buffer the interstitial and external solution pH at basic values) and mixed species diffusion and chemical reactions are proposed to explain these features. This interpretation is confirmed by reactive transport modelling performed with the HYTEC code. The observed cement armouring, clogging and mineral assemblage conservation suggest that the tested cement has improved transfer properties in the experimental conditions. This work suggests that in both acid gas and CO₂ geological storage, clogging of cement or at least mineral assemblage conservation and slowing of carbonation progress could occur in near-well zones where slight water flow occurs e.g. in the vicinity of caprock shales.     

Eurekadumpite, (Cu,Zn)16(TeO3)2(AsO4)3Cl(OH)18·7H2O, a new supergene mineral species

A new mineral eurekadumpite found at the Centennial Eureka Mine in the Tintic district of Juab County in Utah in the United States occurs in the oxidation zone along with quartz, macalpineite, malachite, Zn-bearing olivenite, goethite, and Mn oxides. Eurekadumpite forms spherulites or rosettes up to 1 mm in size and their clusters and crusts up to 1.5 cm² in cavities. Its individuals are divergent and extremely thin (up to 0.5 mm across and less than 1 μm thick) hexagonal or roundish leaflets. The mineral is deep blue-green or turquoise-colored. Its streaks are light turquoise-colored. Its luster is satiny in aggregates and pearly on individual flakes. Its cleavage is (010) perfect and micalike. Its flakes are flexible but inelastic. Its Mohs hardness is 2.5–3.0, and D(meas) = 3.76(2) and D(calc) = 3.826 g/cm³. The mineral is optically biaxial negative, and α = 1.69(1), β ∼ γ = 1.775(5), and 2V _meas = 10(5)°. Its pleochroism is strong: Y = Z = deep blue-green, and X = light turquoise-colored. Its orientation is X = b. The wavenumbers of the bands in the IR spectrum (cm⁻¹; the strong lines are underlined, and w denotes the weak bands) are 3400, 2990, 1980w, 1628, 1373w, 1077, 1010, 860, 825, 803, 721w, 668, 622, 528, 461. The IR spectrum shows the occurrence of the tellurite (Te⁴⁺,O₃)²⁻ and arsenate (As⁵⁺,O₄)³⁻ anionic groups and H₂O molecules; Cu and Zn cations are combined with OH⁻ groups. The chemical composition of eurekadumpite is as follows (wt %, average of 14 electron-microprobe analyses; H₂O determined using the Alimarin method): 0.04 FeO, 36.07 CuO, 20.92 ZnO, 14.02 TeO₂, 14.97 As₂O₅, 1.45 Cl, 13.1 H₂O, O = Cl₂ −0.33, total 100.24. The empirical formula based on 2 Te atoms is (Cu_10.32Zn_5.85Fe_0.01)_Σ16.18(TeO₃)₂(AsO₄)_2.97[Cl_0.93(OH)_0.07]_Σ1(OH)_18.45 · 7.29H₂O. The idealized formula is (Cu,Zn)₁₆(TeO₃)₂(AsO₄)₃Cl(OH)₁₈ · 7H₂O. Eurekadumpite is monoclinic (pseudohexagonal), and the most probable space groups are P2/m, P2, or Pm. The unit-cell parameters refined from the powder X-ray data are as follows: a = 8.28(3), b = 18.97(2), c = 7.38(2) ?, β = 121.3(6)°, V = 990(6) ?³, and Z = 1. The strongest reflections of the X-ray powder pattern (d, ? (I) [hkl]) are as follows: 18.92(100) [010], 9.45(19) [020], 4.111(13) [[`2]\bar 2 01], 3.777(24) [050, [`2]\bar 2 21, 041], 2.692(15) [[`3]\bar 3 11, 151, [`3]\bar 3 02], 2.524(41)[170, [`2]\bar 2 52, [`1]\bar 1 71], 1.558(22) [[`4]\bar 4 82, [`3]\bar 3 .10.1, 024]. The name of the mineral means, firstly, that it was found in specimens from dumps of the Centennial Eureka Mine. In addition, it could mean found in a dump (the Greek word eureka means I have found it). There is an allusion to the great role that dumps of abandoned mines have played in the discovery of new minerals. Type specimens are deposited at the Fersman Mineralogical Museum of the Russian Academy of Sciences in Moscow, at the Smithsonian National Museum of Natural History in Washington, and at the American Museum of Natural History in New York.     

Nanoinclusions of high-pressure hydrous silicate, Mg3Si4O10(OH)2 · n H2O (10?-phase), in mantle olivine: Mechanisms of formation and transformation

The 10?-phase, Mg₃Si₄O₁₀(OH)₂ · nH₂O, where n = 0.65÷2, belongs to the group of dense hydrous magnesium silicates (DHMS), which were produced in experiments and are regarded as hypothetical mineral carriers for H₂O in the mantle. However, DHMS were almost never observed in nature. The only exception is the finding of the 10?-phase as nanoinclusions in olivines from mantle nodules in kimberlites. The inclusions with sizes of a few ten nanometers have a pseudohexagonal habit and are characterized by the presence of voids free of solids. The 10?-phase fills the equatorial parts of the inclusions, and, in the majority of inclusions, it is replaced by the low-pressure serpentine + talc assemblage. Based on the analysis of electron microscope images, a model was proposed for the solid-state formation of inclusions, the precursory material of which was transformed to the 10?-phase with the liberation of a water fluid. According to this model, the formation of hydrous nanoinclusions and their subsequent autoserpentinization occurred without the influx of H₂O from the external medium through the mobilization of intrinsic hydroxyl-bearing point defects trapped during olivine crystallization. The subsequent autoserpentinization of the inclusions occurred during decompression owing to interaction between the inclusion material and the host olivine matrix. The process was accompanied by the partial exhaustion of the fluid phase and the replacement 10?-phase + H₂O = Serp + Tc. The criterion for the credibility of the model is the conservation of the volume of material during the reaction at P = const and T = const. Original Russian Text ? N.R. Khisina, R. Wirth, 2008, published in Geokhimiya, 2008, No. 4, pp. 355–363.     

Synthetic fluid inclusions XVIII: Experimental determination of the PVTX properties of H2O-CH4 to 500 °C, 3 kbar and XCH4 ?4 mol.%

The pressure-volume-temperature-composition (PVTX) properties of H₂O-CH₄ were determined from the bubble point curve to 500 °C and 3 kbar for compositions ?4 mol.% CH₄ using the synthetic fluid inclusion technique. H₂O-CH₄ inclusions were produced by loading known amounts of Al₃C₄ and H₂O into platinum capsules along with pre-fractured and inclusion-free quartz cores. During heating the Al₃C₄ and H₂O react to produce CH₄, and the H₂O-CH₄ homogeneous mixture was trapped as inclusions during fracture healing at elevated temperature and pressure. The composition of the fluid in the inclusion was confirmed using the weight loss technique after the experiment and by Raman spectroscopic analysis of the inclusions.Homogenization temperatures of the inclusions were determined and the results were used to construct iso-T_h lines, defined as a line connecting the formation temperature and pressure with the homogenization temperature and pressure. The pressure in the inclusion at the homogenization temperature was calculated from the Duan equation of state (EOS). The slope (ΔP/ΔT) of each iso-T_h line was calculated and the results fitted to a polynomial equation using step-wise multiple regression analysis to estimate the slope of the iso-T_h line as a function of the homogenization temperature and composition according to:

(ΔP/ΔT)=a+b·m+c·m⁴+d·(T_h)²+e·m·T_h+f·m·(T_h)⁴,     

Thermodynamic analysis of the system Na2O-K2O-CaO-Al2O3-SiO2-H2O-F2O?1: Stability of fluorine-bearing minerals in felsic igneous suites

Thermodynamic analysis of the system Na₂O-K₂O-CaO-Al₂O₃-SiO₂-H₂O-F₂O_–1 provides phase equilibria and solidus compatibilities of rock-forming silicates and fluorides in evolved granitic systems and associated hydrothermal processes. The interaction of fluorine with aluminosilicate melts and solids corresponds to progressive fluorination of their constituent oxides by the thermodynamic component F₂O_–1. The chemical potential (F₂O_–1) buffered by reaction of the type: MO_n/2 (s)+n/2 [F₂O_–1]=MF_n (s, g) where M=K, Na, Ca, Al, Si, explains the sequential formation of fluorides: carobbiite, villiaumite, fluorite, AlF₃, SiF₄ as well as the common coexistence of alkali- and alkali-earth fluorides with rock-forming aluminosilicates. Formation of fluorine-bearing minerals first starts in peralkaline silica-undersaturated, proceeds in peraluminous silica-oversaturated compositions and causes progressive destabilization of nepheline, albite and quartz, in favour of villiaumite, cryolite, topaz, chiolite. Additionally, it implies the increase of buffered fluorine solubilities in silicate melts or aqueous fluids from peralkaline silica-undersaturated to peraluminous silica-oversaturated environments. Subsolidus equilibria reveal several incompatibilities: (i) topaz is unstable with nepheline or villiaumite; (ii) chiolite is not compatible with albite because it only occurs only at very high F₂O_–1 levels. The stability of topaz, fluorite, cryolite and villiaumite in natural felsic systems is related to their peralkalinity (peraluminosity), calcia and silica activity, and linked by corresponding chemical potentials to rock-forming mineral buffers. Villiaumite is stable in strongly peralkaline and Ca-poor compositions (An_<0.001). Similarly, cryolite stability requires coexistence with nearly-pure albite (An_<2). Granitic rocks with Ca-bearing plagioclase (An_>5) saturate with topaz or fluorite. Crystallization of topaz is restricted to peraluminous conditions, consistent with the presence of Li-micas or anhydrous aluminosilicates (cordierite, garnet, andalusite). Fluorite is predicted to be stable in peraluminous biotite granites, amphibole-, clinopyroxene- or titanite-bearing calc-alkaline suites as well as in peralkaline granitic and syenitic rocks. Fluorine concentrations in felsic melts buffered by the coexistence of F-bearing minerals and feldspars increase from peralkaline through metaluminous to mildly peraluminous compositions. At low-temperature conditions, the hydrothermal evolution of peraluminous granitic and greisen systems is controlled by white mica-feldspar-fluoride equilibria. With decreasing temperature, topaz gradually breaks down via: (i) (OH)F_–1 substitution and fluorine transfer to fluorite by decalcification of plagioclase below 600 °C, (ii) formation of muscovite and additional fluorite at 475–315 °C, and (iii) formation of paragonite and cryolite, consuming F-rich topaz and albite below 315 °C. These equilibria explain the absence of magmatic fluorite in Ca-bearing topaz granitic rocks; its abundance in hydrothermal rocks is due to: (i) closed-system defluorination of topaz, (ii) open-system decalcification of plagioclase or (iii) hydrolytic alteration. These results provide a complete framework for the investigation of fluorine-bearing mineral stabilities in felsic igneous suites.Electronic Supplementary Material Supplementary material is available in the online version of this article at . A link in the frame on the left on that page takes you directly to the supplementary material.Editorial responsibility: T.L. Grove     

Hydrous olivine (Mg1? y? Fe2+y)2? x ?v x ? SiO4H2 x – a new DHMS phase of variable composition observed as nanometer-sized precipitations in mantle olivine

 An olivine grain from a peridotite nodule 9206 (Udachnaya kimberlite, Siberia) was investigated by TEM methods including AEM, HRTEM, SAED and EELS techniques. A previous study of the 9206 olivine sample revealed OH absorption bands in the IR spectrum and abundant nanometer-sized OH-bearing inclusions, of hexagonal-like or lamellar shape. Inclusions, which are several hundred nm in size, consist of 10 ? phase, talc and serpentine (chrysotile and lizardite). The lamellar (LI) and hexagon-like small inclusions of several ten nm in size (SI) are the topic of the present paper. AEM investigations of the inclusions reveal Mg, Fe and Si as cations only. The Mg/Si and Fe/Si atomic ratios are lower in the inclusions than in the host olivine. The Si concentration in the olivine host and both lamellar inclusions and small inclusions is the same. A pre-peak at 528eV was observed in EEL spectra of LI and SI, which is attributed to OH⁻ or Fe³⁺. From these data it is concluded that there is a OH⁻- or Fe³⁺-bearing cation-deficient olivine-like phase present. HRTEM lattice fringe images of LI and SI exhibit modulated band-like contrasts, which are superimposed onto the olivine lattice. Diffraction patterns (Fourier-transforms) of the HREM images as well as SAED patterns show that the band-like contrasts in HRTEM images of the inclusions are caused by periodic modulations of the olivine lattice. Three kinds of superperiodicity in the olivine structure such as 2a, 3a and 3c, were observed in SAED patterns. The corresponding olivine supercells labelled here as Hy-2a, Hy-3a and Hy-3c were derived. The M1-vacancies located in the (100) and (001) octahedral layers of the olivine lattice are suggested to form ordered arrays of planar defects (PD), which cause the band-like contrasts in HRTEM patterns as well as the superperiodicity in the SAED patterns. The vacancy concentrations as well as the chemical composition of Hy-2a, Hy-3a and Hy-3c olivine supercells were calculated using crystal chemical approaches, assuming either {(OH)^< _O−V^" _Me−(OH)^< _O}^↔, or {F e ^< _Fe −H _Me ^′}^↔ or {2F e ^< _Fe −V _Me ^"}^↔ point defect associates. The calculated theoretical compositions Mg_1.615Fe⁺² _0.135v_0.25SiO₄H_0.5 (Hy-2a) and Mg_1.54Fe²⁺ _0.12v_0.33SiO₄H_0.66 (Hy-3a and Hy-3c) are in a good agreement with the AEM data on inclusions. Hy-2a, Hy-3a and Hy-3c are considered to be a hydrous olivine with the extended chemical formula (Mg_1-yFe²⁺ _y)_2−xv_xSiO₄H_2x. The crystal structure of hydrous olivine is proposed to be a modular olivine structure with Mg-vacant modules. The crystal chemical formula of hydrous olivines in terms of a modular structure can be written as [MgSiO₄H₂] · 3[Mg_1.82Fe_0.18SiO₄] for Hy-2a, [MgSiO₄H₂] · 2[Mg_1.82Fe_0.18SiO₄] for Hy-3a and Hy-3c. Hydrous olivine is suggested to be exsolved from the olivine 9206, which has been initially saturated by OH-bearing point defects. The olivine 9206 hydration as well as the following exsolution of hydrous olivine inclusions is suggested to occur at high pressure-high temperature conditions of the upper mantle. Received: 15 January 2001 / Accepted: 2 July 2001     

Burovaite-Ca, (Na,K)4Ca2(Ti,Nb)8[Si4O12]4(OH,O)8·12H2O, a new labuntsovite-group mineral species and its place in low-temperature mineral formation in pegmatites of the Khibiny pluton, Kola Peninsula, Russia

This paper presents data on burovaite-Ca, the first Ti-dominant member of the labuntsovite group with a calcium D-octahedron. The idealized formula of burovaite-Ca is (K,Na)₄Ca₂(Ti,Nb)₈[Si₄O₁₂]₄(OH,O)₈ · 12H₂O. The mineral has been found in the hydrothermal zone of aegirine-microcline pegmatite located in khibinite at Mt. Khibinpakhkchorr, the Khibiny pluton, Kola Peninsula, Russia. Radiaxial intergrowths of burovaite-Ca and labuntsovite-Mn associated with lemmleynite-Ba, analcime, and apophyllite have been identified in caverns within microcline. The mean composition of the mineral is as follows, wt %: 3.72 Na₂O, 2.76 K₂O, 4.22 CaO, 0.47 SrO, 0.23 BaO, 0.01 MnO, 0.30 Fe₂O₃, 0.14 Al₂O₃, 42.02 SiO₂, 17.30 TiO₂, 15.21 Nb₂O₅, 12.60 H₂O (measured); the total is 98.98. Its empirical formula has been calculated on the basis of [(Si,Al)₁₆O₄₈]: {(Na_3.10K_1.07Ca_0.37Sr_0.04Ba_0.04)_4.62}(Ca_1.28Zn_0.01)_1.29(Ti_4.97Nb_2.56Fe_0.08Ta_0.02)_7.63(Si_15.93Al_0.07)₁₆O₄₈(OH_6.70O_0.93)_7.63 · 12H₂O. The strongest lines in the X-ray powder diffraction pattern of burovaite-Ca (I-d ?] are as follows: 70–7.08, 40–6.39, 40–4.97, 30–3.92, 40–3.57, 100–3.25, 70–3.11, 50–2.61, 70–2.49, 40–2.15, 50–2.05, 70–1.712, 70–1.577, and 70–1.444. The structure of burovaite-Ca was solved by A.A. Zolotarev, Jr. The mineral is monoclinic, space group C2/m. The unit-cell dimensions are a = 14.529(3), b = 14.203(3), c = 7.899(1), β = 117.37(1)°, V = 1447.57 ?₃. Burovaite-Ca is an isostructural Ti-dominant analogue of karupm?llerite-Ca and gjerdingenite-Ca. Two stages of mineral formation—pegmatite proper and hydrothermal—have been recognized in the host pegmatite. The hydrothermal stage included K-Ba-Na, Na-K-Ca, and Na-Sr substages. Burovaite-Ca is related to the intermediate Na-K-Ca substage. At the first substage, labuntsovite-Mn and lemmleynite-Ba were formed, and tsepinite-Na, paratsepinite-Nd, and tsepinite-Sr were formed at the final substage. Thus, the sequence of crystallization of labuntsovite-group minerals is characterized by the replacement of the potassium regime by the sodium regime of alkaline solutions in the evolved host pegmatite.     

Synthesis, crystal structure and crystal chemistry of ferri-clinoholmquistite, ?Li 2Mg 3Fe 3+ 2Si 8O 22(OH) 2

This work reports the synthesis of ferri-clinoholmquistite, nominally _Li2(Mg₃Fe³⁺₂)Si₈O₂₂(OH)₂, at varying f_O2 conditions. Amphibole compositions were characterized by X-ray (powder and single-crystal) diffraction, microchemical (EMPA) and spectroscopic (FTIR, Mössbauer and Raman) techniques. Under reducing conditions ( NNO+1, where NNO = Nickel–Nickel oxide buffer), the amphibole yield is very high (>90%), but its composition, and in particular the FeO/Fe₂O₃ ratio, departs significantly from the nominal one. Under oxidizing conditions ( NNO+1.5), the amphibole yield is much lower (<60%, with Li-pyroxene abundant), but its composition is close to the ideal stoichiometry. The exchange vector of relevance for the studied system is ^M2(Mg,Fe²⁺) ^M4(Mg,Fe²⁺) ^M2Fe³⁺_–1 ^M4Li_–1, which is still rather unexplored in natural systems. Amphibole crystals of suitable size for structure refinement were obtained only at 800 °C, 0.4 GPa and NNO conditions (sample 152), and have C2/m symmetry. The X-ray powder patterns for all other samples were indexed in the same symmetry; the amphibole closest to ideal composition has a = 9.428(1) Å, b = 17.878(3) Å, c = 5.282(1) Å, = 102.06(2)°, V = 870.8(3) ų. Mössbauer spectra show that Fe³⁺ is strongly ordered at M2 in all samples, whereas Fe²⁺ is disordered over the B and C sites. FTIR analysis shows that the amount of ^CFe²⁺ increases for increasingly reducing conditions. FTIR data also provide strong evidence for slight but significant amounts of Li at the A sites.     

Experimental determination of the hydrothermal solubility of ReS2 and the Re–ReO2 buffer assemblage and transport of rhenium under supercritical conditions

To understand the aqueous species important for transport of rhenium under supercritical conditions, we conducted a series of solubility experiments on the Re–ReO₂ buffer assemblage and ReS₂. In these experiments, pH was buffered by the K–feldspar–muscovite–quartz assemblage; in sulfur-free systems was buffered by the Re–ReO₂ assemblage; and and in sulfur-containing systems were buffered by the magnetite–pyrite–pyrrhotite assemblage. Our experimental studies indicate that the species ReCl₄ ⁰ is dominant at 400°C in slightly acidic to near-neutral, and chloride-rich (total chloride concentrations ranging from 0.5 to 1.0 M) environments, and ReCl₃ ⁺ may predominate at 500°C in a solution with total chloride concentrations ranging from 0.5 to 1.5 M. The results also demonstrate that the solubility of ReS₂ is about two orders of magnitude less than that of ReO₂. This finding not only suggests that ReS₂ (or a ReS₂ component in molybdenite) is the solubility-controlling phase in sulfur-containing, reducing environments but also implies that a mixing process involving an oxidized, rhenium-containing solution and a solution with reduced sulfur is one of the most effective mechanisms for deposition of rhenium. In analogy with Re, TcS₂ may be the stable Tc-bearing phase in deep geological repositories of radioactive wastes.     

The effect of oxygen vacancies and aluminium substitution on the high-pressure properties of brownmillerite-structured Ca2Fe2?xAlxO5

The structural evolution with pressure and the equations of state of three members of the brownmillerite solid solution, Ca₂(Fe_2−x Al _x )O₅, have been determined by single-crystal X-ray diffraction up to a maximum pressure of 9.73 GPa. The compositions of the samples were x = 0.00 and x = 0.37 (with Pnma symmetry) and x = 0.55 (with I2mb symmetry). No phase transitions were observed in the experiments. The equation of state parameters determined from the pressure-volume data are K _0T = 128.0 (7) GPa, K′₀ = 5.8 (3) for the sample with x = 0.00, K _0T = 131 (2) GPa, K′₀ = 5.5 (4) for x = 0.37, and K _0T = 137.5 (6) GPa, K′₀ = 4 for x = 0.55. The bulk modulus therefore increases with Al content, being 11% higher in the x = 0.55 sample than in the Al-free sample. The unit-cell compression is anisotropic, with the c-axis being stiffer than a or b, and the anisotropy increases with increasing Al content of the structure. The structural response to pressure of all samples is similar. The (Al,Fe)O₄ tetrahedra and the (Al,Fe)O₆ octahedra undergo approximately isotropic compression. There is an increase in the twists of the chains of corner-sharing (Al,Fe)O₄ tetrahedra, and an increase in the tilts of the (Al,Fe)O₆ octahedra, because these framework polyhedra are stiffer than the Ca–O bonds to the extra-framework Ca site. The alignment of the two shortest Ca–O bonds sub-parallel to [001] accounts for the relative stiffness of the c-axis and thus the elastic anisotropy. Electronic supplementary material  The online version of this article (doi:) contains supplementary material, which is available to authorized users.