Phase relations in the MgO–P2O5–H2O system and the stability of phosphoellenbergerite: petrological implications

The polymorphic relations for Mg₃(PO₄)₂ and Mg₂PO₄OH have been determined by reversed experiments in the temperature-pressure (T-P) range 500–1100 °C, 2–30 kbar. The phase transition between the low-pressure phase farringtonite and Mg₃(PO₄)₂-II, the Mg analogue of sarcopside, is very pressure dependent and was tightly bracketed between 625 °C, 7 kbar and 850 °C, 9 kbar. The high-temperature, high-pressure polymorph, Mg₃(PO₄)₂-III, is stable above 1050 °C at 10 kbar and above 900 °C at 30 kbar. The low-pressure stability of farringtonite is in keeping with its occurrence in meteorites. The presence of iron stabilizes the sarcopside-type phase towards lower P. From the five Mg₂PO₄OH polymorphs only althausite, holtedahlite, β-Mg₂PO₄OH (the hydroxyl analogue of wagnerite) and ɛ-Mg₂PO₄OH were encountered. Relatively speaking, holtedahlite is the low-temperature phase (<600 °C), ɛ-Mg₂PO₄OH the high-temperature, low-pressure phase and β-Mg₂PO₄OH the high-temperature, high-pressure phase, with an intervening stability field for althausite which extends from about 3 kbar at 500 °C to about 12 kbar at 800 °C. Althausite and holtedahlite are to be expected in F-free natural systems under most geological conditions; however, wagnerite is the most common Mg-phosphate mineral, implying that fluorine has a major effect in stabilizing the wagnerite structure. Coexisting althausite and holtedahlite from Modum, S. Norway, show that minor fluorine is strongly partitioned into althausite (K_D ^F/OH≈ 4) and that holtedahlite may incorporate up to 4 wt% SiO₂. Synthetic phosphoellenbergerite has a composition close to (Mg_0.9□_0.1)₂Mg₁₂P₈O₃₈H_8.4. It is a high-pressure phase, which breaks down to Mg₂PO₄OH + Mg₃(PO₄)₂ + H₂O below 8.5 kbar at 650 °C, 22.5 kbar at 900 °C and 30 kbar at 975 °C. The stability field of the phosphate end-member of the ellenbergerite series extends therefore to much lower P and higher T than that of the silicate end-members (stable above 27 kbar and below ca. 725 °C). Thus the Si/P ratio of intermediate members of the series has a great barometric potential, especially in the Si-buffering assemblage with clinochlore + talc + kyanite + rutile + H₂O. Application to zoned ellenbergerite crystals included in the Dora-Maira pyrope megablasts, western Alps, reveals that growth zoning is preserved at T as high as 700–725 °C. However, the record of attainment of the highest T and/or of decreasing P through P-rich rims (1 to 2 Si pfu) is only possible in the presence of an additional phosphate phase (OH-bearing or even OH-dominant wagnerite in these rocks), otherwise the trace amounts of P in the system remain sequestered in the core of Si-rich crystals (5 to 8 Si pfu) and can no longer react. Received: 7 April 1995 / Accepted: 12 November 1997     

A structural,magnetic, and Mössbauer spectral study of several Na–Mn–Fe-bearing alluaudites

The synthesis and the chemical, structural, magnetic, and Mössbauer spectral characterization of three synthetic alluaudites, Na₂Mn₂Fe(PO₄)₃, NaMn Fe₂(PO₄)₃ and Na₂MnFe^IIFe^III(PO₄)₃, and a natural sample with the nominal composition of NaMn Fe₂(PO₄)₃, collected in the Buranga pegmatite, Rwanda, are reported. All four compounds have the expected alluaudite monoclinic C2/c structure with the general formula [A(2)A(2)][A(1)A(1)A(1)₂]M(1)M(2)₂(PO₄)₃ in which manganese(II) is on the M(1) site and manganese(II), iron(III) and, in some cases, iron(II) on the M(2) site. The X-ray structure of Na₂Mn₂Fe(PO₄)₃ also indicates a partially disordered distribution of Na^I and Mn^II on the M(1) and A(1) crystallographic sites. All four compounds are paramagnetic above 40 K and antiferromagnetically ordered below. Above 40 K the effective magnetic moments of NaMnFe₂(PO₄)₃ and Na₂MnFe^IIFe^III(PO₄)₃ are those expected of high-spin manganese(II) and iron(III) with the ⁶A_1g electronic ground state and high-spin iron(II) with the ⁵T_2g electronic ground state. In contrast, the effective magnetic moment of Na₂Mn₂Fe(PO₄)₃ is lower than expected as a result of enhanced antiferromagnetic exchange coupling by the manganese(II) on the M(2) site. The Mössbauer spectra of all four compounds have been measured from 4.2 to 295 K and have been found to be magnetically ordered below 40 K for Na₂Mn₂Fe(PO₄)₃ and 35 K for the remaining compounds. The Mössbauer spectra of Na₂Mn₂Fe(PO₄)₃ exhibit the two expected iron(III) quadrupole doublets and/or magnetic sextets expected for a random distribution of manganese(II) and iron(III) ions on the M(2) site. Further, the Mössbauer spectra of Na₂MnFe^IIFe^III(PO₄)₃ exhibit the two iron(II) and two iron(III) quadrupole doublets and/or magnetic sextets expected for a random distribution of iron(II) and iron(III) on the M(2) site. Surprisingly, the synthetic and natural samples of NaMnFe₂(PO₄)₃ have 19 and 10% of iron(II) on the M(2) site; apparently the presence of some iron(II) stabilizes the alluaudite structure through the reduction of iron(III)–iron(III) repulsion. The temperature dependence of the iron(II) quadrupole splitting yields a 440 to 600 cm^–1 low-symmetry component to the octahedral crystal field splitting at the M(2) site. The iron(II) and iron(III) hyperfine fields observed at 4.2 K are consistent with the presence of antiferromagnetic ordering at low temperatures in all four compounds.     

P-T stability of the lazulite-scorzalite solid-solution series

Summary The stability of members of the lazulite-scorzalite solid-solution series, (Mg,Fe)Al₂ (OH)₂(PO₄)₂, was investigated as a function of T (505 to 675 °C), P (0.1 to 0.3 GPa) and Fe/Mg ratio in hydrothermal synthesis experiments. The oxygen fugacity was controlled by means of the Ni/NiO buffer. It was found that starting from end-member lazulite the stability of the solid-solution members strongly decreases with increasing content of scorzalite component. At 0.2 GPa pure lazulite decomposes at about 660 °C whereas at the same pressure a solid-solution with 80% of lazulite component is only stable up to 590 °C under the oxygen fugacity of the Ni/NiO buffer. The members of the lazulite-scorzalite solid-solution series with limiting composition coexist with an Fe-richer member of the (Mg,Fe)Al(PO₄)O series and berlinite. The mixing behaviour of both the lazulite-scorzalite and the (Mg,Fe)Al(PO₄)O solid-solution series disregarding small amounts of Fe³⁺ is interpreted in terms of a model on the basis of a simple mixture for the lazulite-scorzalite system and of an ideal mixture for the (Mg,Fe)Al(PO₄)O series. With this model the interaction parameter which expresses the non-ideality of the lazulite-scorzalite solid-solution series amounts to . Zusammenfassung P-T Stabilit?t von Lazulith-Scorzalith Mischkristallen Die Stabilit?t der Glieder der Lazulith-Scorzalith Mischkristallreihe, (Mg, Fe)Al₂(OH)₂(PO₄)₂ wurde als Funktion der Temperatur (505 bis 675 °C), des Druckes (0.1 bis 0.3 GPa) und des Fe/Mg-Verh?ltnisses in hydrothermalen Syntheseversuchen untersucht. Die Sauerstoffugazit?t wurde mittels eines Ni/NiO-Puffer kontrolliert. Es konnte festgestellt werden, da? ausgehend vom Lazulith-Endglied die Stabilit?t der Mischkristalle mit zunehmendem Scorzalith-Gehalt stark abnimmt. Reiner Lazulith, MgAl₂(OH)₂(PO₄)₂ zerf?llt unter 0.2 GPa bei 660 °C, w?hrend ein Mischkristall mit 80 mol% Gehalt an Lazulith-Komponente nur bis 590 °C unter der Sauerstoffugazit?t des Ni/NiO-Puffers stabil ist. Hierbei koexistieren die Lazulith-Scorzalith Mischkristalle mit Grenzzusammensetzung mit eisenreicheren Mischphasen des Systems (Mg,Fe)Al(PO₄)₂O und Berlinit. Das Mischungsverhalten sowohl der Lazulith-Scorzalith- als auch der (Mg,Fe)Al(PO₄)₂O-Reihe wurde mit Hilfe eines quantitativen Modelles auf der Basis einer symmetrischen Mischung für Lazulith-Scorzalith und einer idealen Mischung für das System (Mg,Fe)Al(PO₄)₂O interpretiert. Mit Hilfe dieses Modelles wurde der Wechelwirkungsparameter , der die Nichtidealit?t der Lazulith-Scorzalith Mischreihe ausdrückt zu bestimmt. Received August 26, 1998; revised version accepted July 30, 1999     

Refinement of the crystal structure of bonshtedtite, Na3Fe(PO4)(CO3)

The crystal structure of bonshtedtite, Na₃Fe(PO₄)(CO₃) (monoclinic, P2₁/m, a = 5.137(4), b = 6.644(4), c = 8.908(6) Å, β = 90.554(14)°, V = 304.0(4) ų, Z = 2) has been refined to R ₁ = 0.041 on the basis of 1314 unique reflections. The structure is similar to that of other minerals of the bradleyite group. It is based on the [Fe(PO₄)(CO₃)]^3? layers oriented parallel to (001). The layers are formed by corner-sharing PO₄ tetrahedra and FeO₄(CO₃) complexes, where FeO₆ tetrahedra and CO₃ triangles are edge-shared. The topology of the octa-tetrahedral layer in bonshtedtite is similar to that of the autunite-group minerals, but it differs from the latter in terms of local topological properties.     

The crystal structure of Ca5(PO4)2SiO4 (Silico-Carnotite)

Summary The crystal structure of Ca₅(PO₄)₂SiO₄ (silico-carnotite) has been determined from 3358 x-ray diffraction data collected by a counter method and has been refined toR _w =0.038,R=0.045, in space group Pnma. The unit cell parameters area=6.737 (1) Å,b=15.508 (2) Å andc=10.132 (1) Å at 24°C;Z=4. The observed density is 3.06 and the calculated density is 3.03 g · cm^–3. The crystal contains about 2.5% V₂O₅ as an impurity. The bond lengths within the tetrahedral anions suggest that substitution or disorder of PO₄ ^3–, SiO₄ ^4– and possibly VO₄ ^3– occurs among the anion sites. The structure has some relationship to that of Ca₅(PO₄)₃OH, the predominant inorganic phase in the human body, but suggests that the Ca₅(PO₄)₃OH type structure may not be stable without some of the OH positions being filled. Ca₅(PO₄)₂SiO₄ is more closely related to K₃Na(SO₄)₂ (glaserite) if it is considered that there are systematic cation vacancies in Ca₅(PO₄)₂SiO₄.This type of structure is consistent with the view that cation vacancies in the glaserite-type structure account for solid solutions between Ca₂SiO₄ and Ca₃(PO₄)₂ and between Ca₃(PO₄)₂ and CaNaPO₄.

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Die Kristallstruktur vonCa ₅(PO ₄)₂ SiO ₄ (Silicocarnotit)

Zusammenfassung Die Kristallstruktur von Ca₅(PO₄)₂SiO₄ (Silicocarnotit) wurde aus 3358 Röntgendiffraktometer-Daten bestimmt und in Raumgruppe Pnma aufR _w =0,038,R=0,045 verfeinert. Die Gitterkonstanten (bei 24° C) sind:a=6,737 (1) Å,b=15,508 (2) Å undc=10,132 (1) Å,Z=4; D_obs.=3,06 g · cm^–3, D_exp.=3,03 g · cm^–3. Der Kristall enthält etwa 2,5% V₂O₅ als Verunreinigung. Die Bindungslängen in den tetraedrischen Anionen legen nahe, daß unter den Anionenplätzen gegenseitige Vertretung oder Unordnung von PO₄ ^3–, SiO₄ ^4– und möglicherweise VO₄ ^3– auftritt. Die Struktur zeigt einige Verwandtschaft zu der von Ca₅(PO₄)₃OH, der wichtigsten anorganischen Substanz im menschlichen Körper, weist aber darauf hin, daß eine Struktur vom Ca₅(PO₄)₃OH-Typ ohne Besetzung eines Teiles der OH-Position nicht stabil ist. Ca₅(PO₄)₂SiO₄ zeigt engere Beziehungen zu K₃Na(SO₄)₂ (Glaserit), wenn man berücksichtigt, daß in Ca₅(PO₄)₃SiO₄ systematische Kationen-Leerstellen sind. Dieser Strukturtyp ist mit derAuffassung in Übereinstimmung, daß Kationenleerstellen für die festen Lösungen zwischen Ca₂SiO₄ und Ca₃(PO₄)₂ und zwischen Ca₃(PO₄)₂ und CaNaPO₄ verantwortlich sind.

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With 9 Figures     

Structure and crystal chemistry of a dense polymorph of tricalcium phosphate Ca3 (PO4)2: A host to accommodate large lithophile elements in the earth's mantle

A phase of Ca₃ (PO₄)₂, synthesized at 12GPa and 2300° C, is structurally analyzed by the single crystal X-ray diffraction method. This Ca₃(PO₄)₂ is found to be a dense polymorph of tricalcium phosphate isostructural with Ba₃ (PO₄)₂ and named γ-Ca₃ (PO₄)₂. In the structure of Ca₃ (PO₄)₂, a phosphorus atom is tetrahedrally coordinated by oxygen atoms and calcium atoms occupy two types of large metal sites. The Ca(1) site has twelve oxygen neighbours with the mean bond length of 2.739 Å while the other Ca(2) site is coordinated by ten oxygen atoms with the mean Ca-O distance of 2.588Å. The structure is characterized by the translationally interconnected polyhedral sequence PO₄-Ca(2)O₁₀-Ca (1)O₁₂-Ca (2)O₁₀-PO₄ in the direction of the c axis. This dense phase of Ca₃(PO₄)₂ with two large metal sites may be an important host for very large lithophile elements in the deep upper mantle of the earth.     

Lead orthophosphates—III. Stabilities of fluoropyromorphite and bromopyromorphite at 25°C

The reactions of secondary lead orthophosphate with approximately 10^?1 M sodium fluoride and sodium bromide solutions have been investigated at 25°C. Interpretation of the solubility data resulted in solubility product constants for fluoropyromorphite and bromopyromorphite of 10^?71.6 and 10^?78.1, respectively. According to these constants, the stability sequence for lead pyromorphites is Pb₅(PO₄)₃Cl > Pb₅(PO₄)₃Br > Pb₅(PO₄)₃OH > Pb₅(PO₄)₃F. The derived free energy data have been used to evaluate the respective stabilities of fluoro-pyromorphite and bromopyromorphite within the systems PbF₂-PbO-P₂O₅-H₂O and PbBr₂-PbO-P₂O₅-H₂O and to predict the equilibrium behavior of the Pb₅(PO₄)₃F-Pb₅(PO₄)₃OH solid solution under aqueous conditions.     

Mineral phases formed in anoxic sediments by microbial decomposition of organic matter

Microbial decomposition of organic matter in recent sediments of the Landsort Deep—an anoxic basin of the central Baltic Sea—resulted in the formation of a characteristic assemblage of authigenic mineral precipitates of carbonates, sulfides. phosphates and amorphous silica, The dominant crystalline phases are a mixed Mn-carbonate [(Mn_0.85Ca_0.10Mg_0.05)CO₃]. Mn-sulfide [MnS] and Fecarbonate [FeCO₃]. Amorphous Fe-sulfide [FeS]. Mn-phosphate [Mn₃(PO₄)₂] and a mixed Fe-Ca-phosphate [(Fe_0.86Ca_0.14)₃(PO₄)₂] were identified by their chemical compositions only. The variability in composition of these solid phases and their mode of occurrence as a co-existing assemblage constrains the conditions and solution composition from which they precipitated. Estimates of activities for dissolved Fe. Mn. PO₄, CO₃ and S in equilibrium with such an assemblage are close to those found in recent anoxic interstitial water-sediment systems. It is important to have detailed knowledge of the composition and stability conditions of these solid precipitates in order to refine stoichiometric models of interstitial nutrient regeneration in anoxic sediments.     

OH in zoned amphiboles of eclogite from the western Tianshan,NW-China

Chemically-zoned amphibole porphyroblast grains in an eclogite (sample ws24-7) from the western Tianshan (NW-China) have been analyzed by electron microprobe (EMP), micro Fourier-transform infrared (micro-FTIR) and micro-Raman spectroscopy in the OH-stretching region. The EMP data reveal zoned amphibole compositions clustering around two predominant compositions: a glaucophane end-member ( ^B Na₂ ^C M²⁺ ₃ M³⁺ ₂ ^T Si₈(OH)₂) in the cores, whereas the mantle to rim of the samples has an intermediate amphibole composition ( ^A _0.5 ^B Ca_1.5Na_0.5 ^C M ²⁺ _4.5 M _0.5^{3+ T} Si_7.5Al_0.5(OH)₂) (A = Na and/or K; M ²⁺ = Mg and Fe²⁺; M ³⁺ = Fe³⁺ and/or Al) between winchite (and ferro-winchite) and katophorite (and Mg-katophorite). Furthermore, we observed complicated FTIR and Raman spectra with OH-stretching absorption bands varying systematically from core to rim. The FTIR/Raman spectra of the core amphibole show three lower-frequency components (at 3,633, 3,649–3,651 and 3,660–3,663 cm⁻¹) which can be attributed to a local O(3)-H dipole surrounded by ^{M(1) M(3)}Mg₃, ^{M(1) M(3)}Mg₂Fe²⁺ and ^{M(1) M(3)} Fe²⁺ ₃, respectively, an empty A site and ^T Si₈ environments. On the other hand, bands at higher frequencies (3,672–3,673, 3,691–3,697 and 3,708 cm⁻¹) are observable in the rims of the amphiboles, and they indicate the presence of an occupied A site. The FTIR and Raman data from the OH-stretching region allow us to calculate the site occupancy of the A, M(1)–M(3), T sites with confidence when combined with EPM data. By contrast M(2)- and M(4) site occupancies are more difficult to evaluate. We use these samples to highlight on the opportunities and limitations of FTIR OH-stretching spectroscopy applied to natural high pressure amphibole phases. The much more detailed cation site occupancy of the zoned amphibole from the western Tianshan have been obtained by comparing data from micro-chemical and FTIR and/or Raman in the OH-stretching data. We find the following characteristic substitutions Si(T-site) (Mg, Fe)[M(1)–M(3)-site] → Al(T-site) Al[M(1)–M(3)-site] (tschermakite), Ca(M4-site)□ (A-site) → Na(M4-site) Na + K(A-site) (richterite), and Ca(M4-site) (Mg, Fe) [M(1)–M(3)-site] → Na(M4-site) Al[M(1)–M(3)-site] (glaucophane) from the configurations observed during metamorphism.     

Effects of cations and anions on iron and manganese sorption and desorption capacity in calcareous soils from Iran

This study investigated the effect of cations and anions on the sorption and desorption of iron (Fe) and manganese (Mn) in six surface calcareous soil samples from Western Iran. Six 10 mM electrolyte background solutions were used in the study, i.e., KCl, KNO₃, KH₂PO₄, Ca(NO₃)₂, NaNO₃, and NH₄NO₃. NH₄NO₃ and NaNO₃ increased the soil retention of Fe and Mn, whereas Ca(NO₃)₂ decreased the soil retention of Fe and Mn. Iron and Mn sorption was decreased by NO₃ ^? compared with H₂PO₄ ^? or Cl^?. The Freundlich equation adequately described Fe and Mn adsorption, with all background electrolytes. The Freundlich distribution coefficient (K _F) decreased in the order H₂PO₄ ^? > Cl^? > NO₃ ^? for Mn and H₂PO₄ ^? > NO₃ ^? > Cl^? for Fe. The highest sorption reversibility was for Fe and Mn in competition with a Ca²⁺ background, indicating the high mobility of these two cations. A MINTEQ speciation solubility model showed that Fe and Mn speciation was considerably affected by the electrolyte background used. Saturation indices indicated that all ion background solutions were saturated with respect to siderite and vivianite at low and high Fe concentrations. All ion background solutions were saturated with respect to MnCO_3(am), MnHPO₄, and rhodochrosite at low and high Mn concentrations. The hysteresis indices (HI) obtained for the different ion backgrounds were regressed on soil properties indicating that silt, clay, sand, and electrical conductivity (EC) were the most important soil properties influencing Fe adsorption, while cation exchange capacity (CEC), organic matter (OM), and Mn-DTPA affected Mn adsorption in these soils.     

Strukturelle Untersuchungen an Arsenbrackebuschit

Zusammenfassung Röntgenographische Untersuchungen an Einkristallen von Arsenbrackebuschit, Pb₂(Fe, Zn)(OH, OH₂) (AsO₄)₂ (mit FeZn21), ergaben die RaumgruppeP2₁/m mita ₀=7,763(1) Å,b ₀=6.046(1) Å,c ₀=9.022(1)Å, =112,5(1)°,V=391,2(1) ų,Z=2 und _x =6,54 g/cm³. Dreidimensionale Fouriersynthesen und Verfeinerungen nach der Methode der kleinsten Quadrate bis zu einemR-Wert von 0,073 zeigten, daß das neue Mineral strukturell einer Gruppe von Blei-Mineralen der allgemeinen Formel Pb₂ Me(Z) (XO₄) (YO₄) — mitMe=Cu²⁺, Mn²⁺, Zn²⁺, Fe³⁺;X=S, Cr, V, As;Y=P, As, V;Z=OH, OH₂ — zuzuordnen ist. Vertreter dieser Gruppe sind z. B. Tsumebit Pb₂Cu(OH) (SO₄) (PO₄), Vauquelinit Pb₂Cu(OH) (CrO₄) (PO₄) und auch Brackebuschit Pb₂(Mn, Fe) (OH₂) (VO₄)₂. Strukturelle Verwandtschaft besteht mit Tsumcorit Pb(Zn, Fe)₂(OH, OH₂)₂(AsO₄)₂, einem weiteren Blei-Arsenat der gleichen Lagerstätte.

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Structural investigation of arsenbrackebuschite

Summary X-ray single crystal work on arsenbrackebuschite, Pb₂(Fe, Zn) (OH, OH₂) (AsO₄)₂ (with FeZn21), gave space groupP2₁/m witha ₀=7.763(1),b ₀=6.046(1),c ₀=9.022(1) Å, =112.5(1)°,V=391.2(1) ų,Z=2 and _x =6,54 g/cm³. 3-dimensional Fourier syntheses and least-squares refinement (finalR=0.073) showed that the new mineral belongs to a group of lead minerals with the general formula Pb₂ Me(Z) (XO₄) (YO₄)Me=Cu²⁺, Mn²⁺, Zn²⁺, Fe²⁺, Fe³⁺;X=S, Cr, V, As; Y=P, As, V;Z=OH, OH₂. Members of this group, are for example tsumebite, Pb₂Cu(OH) (SO₄)(PO₄), vauquelinite, Pb₂Cu(OH) (CrO₄) (PO₄), and brackebuschite, Pb₂ (Mn, Fe) (OH₂) (VO₄)₂. A structural relationship exists to tsumcorite, Pb(Zn, Fe)₂(OH, OH₂)₂ (AsO₄)₂, another lead-arsenate from Tsumeb.

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Mit 2 Abbildungen     

The phoshate mineral associations of the Tsaobismund pegmatite,Namibia

A detailed mineralogical investigation using the classical methods of identification by X-ray diffraction and by optical properties in thin sections, has revealed thirty one phosphate minerals occurring in the Tsaobismund pegmatite. This investigation is complemented by wet chemical and, mainly, electron microprobe analyses performed on the phosphates known to be typomorphic or considered to be relevant to the hydrothermal alteration. Additionally, microprobe analyses are also given for garnet, gahnite, and ferrocolumbite associated with the phosphates. On the basis of their chemical composition, particularly in terms of their Fe, Mn, and Mg contents, three types of triphylites are distinguished. Triphylite 1 only occurs as a primary phase, triphylite 2 shows exsolution lamellae of sarcopside, and triphylite 3 is partly replaced by a fluorophosphate of the triplite-zwieselite series. These minerals constitute three generations of the parent phases, which were progressively transformed by metasomatic processes, hydrothermal alteration, and by weathering, to give finally three types of complex associations. The Li(Fe,Mn)PO₄ minerals appear to be more sensitive to such transformations than those of the (Fe,Mn)₂PO₄F series. Four main stages of hydrothermal alteration processes have been recognized in the Tsaobismund pegmatite: (i) the Mason-Quensel sequence results from a progressive oxidation of Fe and Mn, and a concomitant Li-leaching of triphylite yielding ferrisicklerite and heterosite, successively; (ii) the metasomatic exchange of Na for Li produces alluaudite; in the present case, the formation of hagendorfite from triphylite 2 is considered to be earlier than the generation of alluaudite-Na occurring in the three associations; (iii) the hydration phase mainly transforms the parent Li(Fe,Mn)PO₄ phase into grey hureaulite, associated with barbosalite and tavorite; (iv) the formation of fluorapatite, not particularly widespread, replaces alluaudite-Na, as well as zwieselite s.l. The following crystallization sequence of the initially formed phosphate minerals is proposed: triphylite 1 triphylite 2 + sarcopside (associated with garnet) triphylite 3 + zwieselite s.l. The most prominent feature of this succession is the increase in the Mg and Zn contents in the composition of the phosphates, as well as the decrease in their Li contents. The variations of the Fe/Mn ratios in this sequence are discussed. The succession triphylite-zwieselite within weakly differentiated and Li-poor pegmatites is of general significance.     

Topotactic formation of ferrisicklerite from natural triphylite under hydrothermal conditions

The topotactic oxidation and delithiation reaction from triphylite, Li(Fe,Mn)PO₄, leading to ferrisicklerite, Li_<1(Fe³⁺,Mn²⁺)PO₄, was investigated under hydrothermal conditions. A cuboid cut from a triphylite single-crystal (Palermo Mine, New Hampshire, USA) with the composition Li_0.93(3)(Fe²⁺ _0.733(6),Fe³⁺ _0.015(1),Mn²⁺ _0.210(4),Mg_0.063(2))_1.021(8)P_1.00(2)O₄ in addition with ground bulk material were treated with KMnO₄ and 30 % H₂O₂(aq) as oxidizing agent in a 0.1 N hydrochloric acid solution in the temperature range between 60 and 200 °C. At 120 °C a rim of 0.1 mm thickness of ferrisicklerite had formed around the core of unreacted triphylite. The sharp reaction boundary was clearly visible, due to the reddish brown absorption colors of ferrisicklerite, compared to colorless triphylite. Using single-crystal X-ray diffraction (XRD), secondary ion mass spectrometry (SIMS), electron probe micro-analysis (EPMA) and ⁵⁷Fe-Mössbauer spectroscopy the product ferrisicklerite was characterized and its composition determined as Li_0.30(7)(Fe²⁺ _0.049(1)Fe³⁺ _0.65(2)Mn²⁺ _0.218(5)Mg_0.062(2))_0.98(1)P_1.01(3)O₄, with unit cell parameters a?=?4.795(1), b?=?9.992(4), and c?=?5.886(2) Å. EPMA investigations across the reaction boundary showed no changes in the concentrations of Fe, Mn, Mg, and P. In contrast, SIMS measurements clearly proved the delithiated state of the ferrisicklerite product. Polarization microscopy revealed that the orientation of the ferrisicklerite rim was the same as that of the original triphylite single-crystal, confirming the strictly topotactic character of the reaction.     

Natural high-pressure polymorph of merrillite in the shock veins of the Suizhou meteorite

A new high-pressure polymorph of merrillite with the structure of trigonal γ-Ca₃(PO₄)₂ was found in the shock-produced veins of the Suizhou meteorite, where it coexists with ringwoodite, majorite, NaAlSi₃O₈-hollandite, and majorite-pyrope garnet. The crystallographic nature of this natural γ-Ca₃(PO₄)₂ phase was characterized by Raman spectroscopy and X-ray diffraction, and all data compare favorably to the same data obtained from γ-Ca₃(PO₄)₂ synthesized at 14 GPa and 1400°C. The cell parameters of this new high-pressure mineral are a = 5.258(1) angstroms and c = 18.727(3) angstroms, space group R-3m, and density = 3.447 (g/cm³), where the number in parentheses are standard deviations in the last significant digits. The natural occurrence of the γ-Ca₃(PO₄)₂ phase together with other high-pressure minerals constrains the pressure of the shock veins at about 23 GPa. The Suizhou meteorite provides the first naturally occurring example of γ-Ca₃(PO₄)₂ polymorph.     

The crystal structure of natural and synthetic holtedahlite

Summary The crystal structure of synthetic holtedahlite, Mg₁₂(PO₃OH, PO4)(PO₄)₅(OH,O)₆,P31m,a = 11.186(3),c = 4.977(1) Å,Z = 1, has been refined toR = 0.033 for 718 observed reflections. Natural holtedahlite, Mg₁₂(PO₃OH, CO₃)(PO₄)₅(OH, O)₆,a = 11.203(3),c = 4.977(1) Å, was refined toR = 0.031 for 1202 observed reflections. The structure contains pairs of face-sharing Mg-octahedra linked by edge-sharing to form double chains alongc. Hydrogen phosphate groups on three-fold axes are partially replaced by carbonate groups in natural holtedahlite. Structural similarities with ellenbergerite are pointed out.

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Die Kristallstruktur von natürlichem und synthetischem Holtedahlit

Zusammenfassung Die Kristallstruktur von synthetischem Holtedahlit, Mg₁₂(PO₃OH,PO₄)(PO₄)₅(OH,O)₆,P31m,a = 11,186(3),c = 4,977(1) Å,Z = 1, wurde für 718 beobachtete Reflexe aufR = 0,033 verfeinert. Natürlicher Holtedahlit, Mg₁₂(PO₃OH, CO₃)(PO₄)₅(OH, O)6,a = 11,203(3),c = 4,977(1) A wurde für 1202 beobachtete Reflexe aufR = 0,031 verfeinert. Die Struktur enthält Paare von über eine Fläche verknüpften Mg-Oktaedern, die weiter durch Kantenverknüpfung Doppelketten nachc bilden. Saure Phosphatgruppen auf dreizähligen Achsen sind im natürlichen Holtedahlit teilweise durch Karbonatgruppen ersetzt. Strukturelle Ähnlichkeiten zu Ellenbergerit werden aufgezeigt.

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With 4 Figures     

Biotransformation of two-line silica-ferrihydrite by a dissimilatory Fe(III)-reducing bacterium: formation of carbonate green rust in the presence of phosphate

The reductive biotransformation of two Si-ferrihydrite coprecipitates (1 and 5 mole % Si) by Shewanella putrefaciens, strain CN32, was investigated in 1,4-piperazinediethanesulfonic acid-buffered media (pH ∼7) with lactate as the electron donor. Anthraquinone-2,6-disulfonate, an electron shuttle, was present in the media. Experiments were performed without and with PO₄³⁻ (P) (1 to 20 mmol/L) in media containing 50 mmol/L Fe. Our objectives were to define the combined effects of SiO₄⁴⁻ (Si) and P on the bioreducibility and biomineralization of ferrihydrites under anoxic conditions. Iron reduction was measured as a function of time, solids were characterized by powder X-ray diffraction and Mössbauer spectroscopy, and aqueous solutions were analyzed for Si, P, Cl⁻ and inorganic carbon. Both of the ferrihydrites were rapidly reduced regardless of the Si and P content. Si concentration had no effect on the reduction rate or mineralization products. Magnetite was formed in the absence of P whereas carbonate green rust GR(CO₃²⁻) ([Fe_(6−x)^IIFe^III_x(OH)₁₂]^x+(CO₃²⁻)_0.5x · yH₂O) and vivianite [Fe₃(PO₄)₂ · 8H₂O], were formed when P was present. GR(CO₃²⁻) dominated as a mineral product in samples with <4 mmol/L P. The Fe(II)/Fe(III) ratio of GR(CO₃²⁻) varied with P concentration; the ratio was 2 in 1 mmol/L P and approached 1 with 4- and 10 mmol/L P. Green rust appeared to form by solid-state transformation of ferrihydrite. Media P and Si concentration dictated the mechanism of transformation. In the 1 mole % Si coprecipitate with 1 mmol/L P, an intermediate Fe(II)/Fe(III) phase with structural Fe(II) slowly transformed to GR with time. In contrast, when ferrihydrite contained more Si (5 mole %) and/or contained higher P (4 mmol/L), sorbed Fe(II) and residual ferrihydrite together transformed to GR. Despite similar chemistries, P was shown to have a profound effect on extent of ferrihydrite reduction and biotransformations while that of Si was minimal.     

Crystallographic studies of some olivine-related (Ni,Mg)3(PO4)2 solid solutions

Olivine-related (Ni, Mg)₃(PO₄)₂ solid solutions were prepared and equilibrated at 1070 K. Accurate monoclinic unit cell dimensions were determined from Guinier-Hägg photographic data. Structural refinements based on the X-ray profile-fitting technique after Rietveld were carried out for pure nickel (II) orthophosphate and for three Ni/Mg solid solutions. (Ni_1-x Mg _x )₃(PO₄)₂ phases with 0.40≦x≦0.60 are probably isostructural with Ni₃(PO₄)₂ (P2₁/a) while phases with low magnesium contents (<27 atom % Mg) deviate structurally from Ni₃(PO₄)₂. The results also show that Ni²⁺ is partially ordered at the octahedralM(1) sites, withK _D (Ni, Mg)=4.0±0.2     

Switzerite: Its chemical formula and crystal structure

Summary Switzerite has the following schematical chemical formula Mn ₄ ^2+(VI) (Me ^3+,2+,1+,)^(VI) (Me ^3+,2+,1+,)^(V) (PO₄)₄. 8 H₂O, whereMe is mainly iron; the mineral is monoclinic, space groupP2₁/c, Z=4; lattice parameters area=8.496,b=13.173,c=17.214 Å, -96.65°. The atomic arrangement was determined by direct methods and refined by least-squares method. FinalR index is 0.077 for 3038 observed reflections. The crystal structure of switzerite can be described as built up by octahedral sheets parallel to (001), with formula [Mn₄O₁₀(H₂O)₄]₂.Me coordination bipyramidal dimers link these units in thec direction whileMe coordination octahedra stick out from the sheets to which they are connected through a vertex. The atomic arrangement of switzerite is compared with that in ludlamite, Fe₃(PO₄)₂·4H₂O, and in whitmoreite, Fe²⁺Fe ₂ ³⁺ (OH)₂(PO₄)₂·4 H₂O. The only analogy in all these structures is the presence of octahedral slabs exhibiting, however, different shapes.

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Switzerit: Chemische Formel und Kristallstruktur

Zusammenfassung Switzerit hat die schematische chemische Formel Mn ₄ ^2+(VI) (Me ^3+,2+,1+,)^(VI) (Me ^3+,2+,1+,)^(V) (PO₄)₄·8 H₂O, wobeiMe hauptsächlich Eisen ist. Das Mineral ist monoklin, RaumgruppeP2₁/c,Z=4; Gitterkonstanten:a=8,496,b=13,173,c=17,214 Å, =96,65°. Die Atomanordnung wurde mit direkten Methoden bestimmt und nach der Methode der kleinsten Quadrate verfeinert. Es wurde für 3038 beobachtete ReflexeR=0,077 erreicht. Man kann die Kristallstruktur des Switzerits als aus Oktaederschichten der Formel [Mn₄O₁₀(H₂O)₄]₂, die parallel zu (001) liegen, beschreiben. Dimere aus trigonalen Dipyramiden umMe verbinden diese Einheiten in Richtung derc-Achse, während Koordinationsoktaeder umMe aus diesen Schichten, an die sie über eine Ecke verknüpft sind, hervorragen. Die Atomanordnung des Switzerits wird mit denen des Ludlamits, Fe₃(PO₄)₂·4 H₂O und des Whitmoreits, Fe²⁺Fe ₂ ³⁺ (OH)₂(PO₄)₂·4 H₂O verglichen. Die einzige Analogie zwischen allen diesen Strukturen ist die Anwesenheit von Oktaederschichten, die aber verschiedene Gestalt haben.

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With 3 Figures     

Role of Fe(II) and phosphate in arsenic uptake by coprecipitation

Natural attenuation of arsenic by simple adsorption on oxyhydroxides may be limited due to competing oxyanions, but uptake by coprecipitation may locally sequester arsenic. We have systematically investigated the mechanism and mode (adsorption versus coprecipitation) of arsenic uptake in the presence of carbonate and phosphate, from solutions of inorganic composition similar to many groundwaters. Efficient arsenic removal, >95% As(V) and ∼55% in initial As(III) systems, occurred over 24 h at pHs 5.5-6.5 when Fe(II) and hydroxylapatite (Ca₅(PO₄)₃OH, HAP) “seed” crystals were added to solutions that had been previously reacted with HAP, atmospheric CO_2(g) and O_2(g). Arsenic adsorption was insignificant (<10%) on HAP without Fe(II). Greater uptake in the As(III) system in the presence of Fe(II) was interpreted as due to faster As(III) to As(V) oxidation by molecular oxygen in a putative pathway involving Fe(IV) and As(IV) intermediate species. HAP acts as a pH buffer that allows faster Fe(II) oxidation. Solution analyses coupled with high-resolution transmission electron microscopy (HRTEM), X-ray Energy-Dispersive Spectroscopy (EDS), and X-Ray Absorption Spectroscopy (XAS) indicated the precipitation of sub-spherical particles of an amorphous, chemically-mixed, nanophase, Fe^III[(OH)₃(PO₄)(As^VO₄)]·nH₂O or Fe^III[(OH)₃( PO₄)(As^VO₄)(As^IIIO₃)_minor]·nH₂O, where As^IIIO₃ is a minor component.The mode of As uptake was further investigated in binary coprecipitation (Fe(II) + As(III) or P), and ternary coprecipitation and adsorption experiments (Fe(II) + As(III) + P) at variable As/Fe, P/Fe and As/P/Fe ratios. Foil-like, poorly crystalline, nanoparticles of Fe^III(OH)₃ and sub-spherical, amorphous, chemically-mixed, metastable nanoparticles of Fe^III[(OH)₃, PO₄]·nH₂O coexisted at lower P/Fe ratios than predicted by bulk solubilities of strengite (FePO₄·2H₂O) and goethite (FeOOH). Uptake of As and P in these systems decreased as binary coprecipitation > ternary coprecipitation > ternary adsorption.Significantly, the chemically-mixed, ferric oxyhydroxide-phosphate-arsenate nanophases found here are very similar to those found in the natural environment at slightly acidic to circum-neutral pHs in sub-oxic to oxic systems, such phases may naturally attenuate As mobility in the environment, but it is important to recognize that our system and the natural environment are kinetically evolving, and the ultimate environmental fate of As will depend on the long-term stability and potential phase transformations of these mixed nanophases. Our results also underscore the importance of using sufficiently complex, yet systematically designed, model systems to accurately represent the natural environment.     

The crystal structure of arsentsumebite, Pb2Cu[(As, S)O4]2(OH)

Summary The crystal structure of arsentsumebite, ideally, Pb₂Cu[(As, S)O₄]₂(OH), monoclinic, space group P2₁/m, a = 7.804(8), b = 5.890(6), c = 8.964(8) ?, β = 112.29(6)°, V = 381.2 ?³, Z = 2, d_calc. = 6.481 has been refined to R = 0.053 for 898 unique reflections with I> 2σ(I). Arsentsumebite belongs to the brackebuschite group of lead minerals with the general formula Pb₂ Me(XO₄)₂(Z) where Me = Cu²⁺, Mn²⁺, Zn²⁺, Fe²⁺, Fe³⁺; X = S, Cr, V, As, P; Z = OH, H₂O. Members of this group include tsumebite, Pb₂Cu(SO₄)(PO₄)(OH), vauquelinite, Pb₂Cu(CrO₄)(PO₄)(OH), brackebuschite, Pb₂ (Mn, Fe)(VO₄)₂(OH), arsenbracke buschite, Pb₂(Fe, Zn)(AsO₄)₂(OH, H₂O), fornacite, Pb₂Cu(AsO₄)(CrO₄)(OH), and feinglosite, Pb₂(Zn, Fe)[(As, S)O₄]₂(H₂O). Arsentsumebite and all other group members contain M = M–T chains where M = M means edge-sharing between MO₆ octahedra and M–T represents corner sharing between octahedra and XO₄ tetrahedra. A structural relationship exists to tsumcorite, Pb(Zn, Fe)₂(AsO₄)₂ (OH, H₂O)₂ and tsumcorite-group minerals Me(1)Me(2)₂(XO₄)₂(OH, H₂O)₂. Received June 24, 2000; revised version accepted February 8, 2001