André Mathieu Fransolet

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.

Sudoite, di/trioctahedral chlorite: A stable low-temperature phase in the system MgO-Al2O3-SiO2-H2O

Sudoite, ideally (Mg2Al3)[AlSi3O10](OH)8, was synthesized in small quantities from a number of starting materials using seeds of the natural mineral. Because its powder X-ray diffraction pattern is very similar to that of normal, trioctahedral chlorite, a technique based on relative intensities of 001-peaks of the chlorite-type phases was used, in addition to the standard X-ray method, to determine growth or breakdown of sudoite. Seeded runs indicate that sudoite is more stable than at least five alternative mineral assemblages in the system MgO-Al2O3-SiO-H2O below about 370°–390° C at water pressures up to at least 7 kbar. At higher temperatures sudoite decomposes into assemblages of normal chlorite with an Al2SiO5-phase and either quartz or pyrophyllite. However, the exact locations of the univariant breakdown curves could not be determined due to very low reaction rates. Schreinemakers analyses indicate that the assemblage sudoite+quartz represents the low-temperature equivalent of the common pair chlorite+pyrophyllite, and that sudoite+quartz is limited to water pressures below about 7 kbar because of its reaction to form the high-pressure phase Mg-carpholite; however, in the absence of quartz, the stability fields of sudoite and of Mg-carpholite overlap at pressures above 7 kbar.These stability data are in general agreement with two well-documented sudoite occurrences in quartz veins cutting highly oxidized, low-pressure manganiferous metapelites, and with one occurrence in a silica-deficient high-pressure metamorphic metabauxite. Sudoites may be more common in low-grade metamorphic rocks than known thus far, but they may not be stable under surface conditions.

An X-ray Rietveld, infrared, and Mössbauer spectral study of the NaMn(Fe1−xInx)2(PO4)3 alluaudite-type solid solution

Abstract Several compounds of the NaMn(Fe1-xInx)2(PO4)3 solid solution were synthesized by solid state reaction in air; pure alluaudite-like compounds were obtained for x = 0.00 to 1.00. X-ray Rietveld refinements indicate the presence of Na+ at the A1 and A2’ sites, Mn2+ at the M1 site, and Fe2+, Fe3+, and In3+ at the M2 site. The presence of small amounts of In3+ at the M1 site, and Mn2+ at the M2 site, indicates a partially disordered distribution between these cations. A good correlation was also established between the M1-M2 bond distance and the β angle of the alluaudite-like compounds. The disordered distribution of Fe2+, Fe3+, and In3+ at the M2 site is confirmed by the broadness of the infrared absorption bands. The Mossbauer spectra, measured between 90 and 295 K, were analyzed in terms of a model that takes into account the next-nearest neighbor interactions around the M2 crystallographic site. In all cases these spectra reveal the unexpected presence of small amounts of Fe2+ at the M2 site, an amount that decreases as the In3+ content increases. The Fe2+ and Fe3+ isomer shifts are typical of the alluaudite structure and vary with temperature, as expected from a second-order Doppler shift. The derived iron vibrating masses and Mossbauer lattice temperatures are within the expected range of values for iron cations in an octahedral environment. The Fe2+ and Fe3+ quadrupole splittings are also typical of the alluaudite structure and the temperature dependence of the Fe2+ quadrupole splitting was fit with the model of Ingalls (1964), which yielded a ground state orbital splitting of ca. 380 to 570 cm-1 for the Fe2+ sites.

Mossbauer spectral evidence for next-nearest neighbor interactions within the alluaudite structure of Na1-xLixMnFe2(PO4)(3)

Abstract The Mossbauer spectra of the Na 1− x Li x MnFe 2 (PO 4 ) 3 compounds, with x =0.00, 0.25, 0.50, and 0.75, have been measured between 90 and 295 K and analyzed in terms of a model which takes into account the next-nearest neighbor interactions within the alluaudite structure. Surprisingly, the spectra reveal an unexpected presence of iron(II) in these compounds; the amount of iron(II) is observed to decrease from 19 to 15 atomic percent of the total iron content with increasing x . The temperature dependence of the Fe 2+ and Fe 3+ isomer shifts agrees with that expected from a second-order Doppler shift and the resulting iron vibrating masses and Mossbauer lattice temperatures are within the expected range of values for iron cations in an octahedral environment. The temperature dependence of the Fe 2+ quadrupole splitting has been fit with the Ingalls model and the results yield a ground state orbital splitting of ca. 500xa0cm −1 for the iron(II) sites. The compositional dependence of the isomer shifts and Fe 2+ content can be understood in terms of a decrease in the unit-cell volume with increasing substitution of sodium by lithium, a substitution which does not influence the observed quadrupole splittings.

A structural, infrared, and Mössbauer spectral study of rosemaryite, NaMnFe3+Al(PO4)3

Rosemaryite, ideally NaMnFe 3+ Al(PO 4 ) 3 , has been collected in the Buranga pegmatite, Rwanda. A single-crystal structure refinement was performed to R 1 = 4.01 %, in the P 2 1 / n space group, with a = 12.001(2), b = 12.396(1), c = 6.329(1) A, β = 114.48(1)°, Vol. = 856.9(2) A 3 , Z = 4. The crystal structure and cation distributions are similar to those of ferrorosemaryite, NaFe 2+ Fe 3+ Al(PO 4 ) 3 , and qingheiite, Na 2 MnMgAl(PO 4 ) 3 , but aluminium predominantly occurs in the M(2a) site, not in the M(2b) site as observed in ferrowyllieite, Na 2 Fe 2+ 2 Al(PO 4 ) 3 . The topologies of the X(1a) and X(1b) crystallographic sites are identical to those found in ferrorosemaryite, and correspond to a distorted octahedron and to a distorted cube, respectively. The [7+1]-coordinated X(2) site is a very distorted gable disphenoid, similar to the A(2)’ site of the alluaudite structure. Mossbauer spectra have been obtained from 4.2 to 295 K, and fitted with a model including two Fe 3+ and two Fe 2+ doublets. The Fe 2+ component corresponding to 2/3 of the Fe 2+ spectral area and having a smaller quadrupole splitting of 2.63 mm/s at 15 K, is assigned to the Fe 2+ on the M(2a) site, and the Fe 2+ component with the larger quadrupole splitting of 3.17 mm/s at 15 K, is assigned to the Fe 2+ on the M(1) site. Fe 3+ is located only at the M(2a) and M(2b) sites, and the Fe 3+ component corresponding to 3/4 of the Fe 3+ and exhibiting the larger quadrupole spitting of 0.77 mm/s at 15 K, is most likely associated with Fe 3+ on the M(2b) site. The infrared spectrum of rosemaryite shows absorption bands at 3450 and 1624 cm −1 , bands that arise from the vibrational modes of H 2 O and confirm the presence of water in the channels of the wyllieite structure. A comparison of both the Mossbauer spectra and structural data of rosemaryite with those of other phosphates of the alluaudite and wyllieite groups, is also presented.

An 57Fe Mössbauer spectral study of vermiculitization in the Palabora Complex, Republic of South Africa

Abstract Two phlogopite, two mixed-layer phlogopite-vermiculite, and two vermiculite samples collected from the Palabora Complex of South Africa have been investigated at 295 K by X-ray diffraction, chemical analysis, and Mössbauer spectroscopy. In addition the temperature dependence of the Mössbauer spectra has been measured between 95 and 295 K for one phlogopite and one mixed-layer sample. The results of the chemical analyses and the Mössbauer spectra improve our knowledge of the vermiculitization process in the Palabora Complex. Both techniques indicate oxidation of the Fe ions during the sequence: phlogopite → mixed-layer → vermiculite. Further, the Mössbauer spectra indicate that Fe oxidation occurs mainly in the octahedral sites and suggest that migration and oxidation of the Fe2+ ions from the octahedral sites to the tetrahedral sites may occur during the transformation of phlogopite into a mixed-layer phase. Finally, the vermiculitization process involves both Fe oxidation and loss of K with a concomitant increase in the Mg content.

A miscibility gap in trioctahedral Mn-Mg-Fe chlorites: Evidence from the Lienne Valley manganese deposit, Ardennes, Belgium

Low-temperature veinlets crosscutting low-grade manganiferous ironstones of Ordovician age contain four texturally distinct types of chlorites with nearly constant Al/Si-ratios that form two separate populations regarding their Mn/Mg/Fe-ratios: One with low iron contents (<1.5 w.% FeO) and molar Mg/Mn-ratios just below unity (magnesian pennantites), the other with higher iron (7–11 w.% FeO) and Mg/Mn≳4 (manganoan clinochlores). The two populations, which can be distinguished readily by their characteristic optical elongation and dispersion colors, are intimately intergrown and have formed partly during consecutive stages of a chlorite crystallization sequence, partly by simultaneous growth and possibly even as exsolution products of a pre-existing homogeneous chlorite phase of intermediate composition. These findings indicate a miscibility gap in the chlorite solid solutions beginning along the binary Mg-Mn series and extending into the ternary system. There may be a solvus relationship with the miscibility gap closing at higher temperatures (400° C?). One very intensely colored chlorite type of the pennantite population may contain Fe3+ or Mn3+ or both.Additional minerals in the veinlets are spessartine, kutnahorite, quartz, and an allanite-piemontite phase. Crystallization began near the centers of the present veins with Mn-rich minerals and continued towards their edges and into the extremely thin ends of the developing fractures with the deposition of the more Fe-rich chlorites that are in equilibrium with the ironstone.

A comparative X-ray diffraction, Mössbauer and NMR spectroscopic study of the vermiculites from Béni Bousera, Morocco and Palabora, Republic of South Africa

Abstract Five vermiculite samples collected from Béni Bousera, Morocco and four from Palabora, South Africa were investigated by X-ray diffraction, chemical analysis, 57Fe Mössbauer spectroscopy, and 27Al magic angle spinning nuclear magnetic resonance. The X-ray diffraction studies indicate that all vermiculites have very similar crystallographic parameters. The chemical analyses and the NMR spectra indicate that the Béni Bousera vermiculites contain Al3+ cations in both octahedral and tetrahedral sheets and the Palabora vermiculites contain Al3+ in the tetrahedral sheet. The Mössbauer spectra indicate that the Béni Bousera vermiculites contain more Fe2+ cations than the Palabora vermiculites and do not contain tetrahedral Fe3+ cations. The different cation compositions and distribution in the two sets of vermiculites may result from different parent minerals, i.e. chlorite in the case of Béni Bousera and phlogopite in the case of Palabora, and different genetic processes, i.e. weathering in Béni Bousera and hydrothermal alteration in Palabora.

Ti Substitution Mechanisms in Phlogopites from the Suwalki Massif-type Anorthosite, NE Poland

Intercumulus titanian phlogopite occurs in leuco- and gabbro-noritic cumulates from the Suwalki anorthosite massif, NE Poland. The degree of Ti enrichment in the micas ranges from 2.59 to 9.41 wt.% TiO2. The chemical composition is highly variable for several other elements: Al 2O3 (13.07-16.75 wt.%), K2O (7.90-10.16 wt.%), FeOtot (6.92-16.69 wt.%), Fe2O3 (0.82-2.95 wt.%), and MgO (9.86-19.54 wt.%), with a Mg/(Fe + Mg) ratio ranging from 0.47 to 0.83. Substitution mechanisms for Ti are proposed, which suggest the presence of exchange vectors involving octahedral and tetrahedral cations. In samples characterized by low Ti contents (0.147-0.239 Ti a.p.f.u.), the Ti incorporation mechanism is: [6]Ti4+ + [6]□ = 2( [6]Mg2+, [6]Fe2+, [6]Mn2+), where [6]□ corresponds to a vacancy in octahedral coordination (Ti-vacancy). In the two groups with intermediate (0.164-0.326 Ti a.p.f.u.) and high Ti contents (0.477-0.532 Ti a.p.f.u.), the Ti substitution mechanism corresponds to the reaction: [6]Ti4+ + 2([4]A13+, [4]Fe3+) = ([6]Mg2+, [6]Fe2+, [6]Mn2+) + 2 [4]Si4+ (Ti-Tschermak). The Mossbauer spectral investigation shows that 0.046-0.167 a.p.f.u. Fe3+ occur on the octahedral sites of the structure. The substitution mechanism responsible for the incorporation of Fe3+ in phlogopites from Suwalki is 3( [6]Mg2+, [6]Fe2+) = 2( [6]Al3+, [6]Fe3+) + [6](M3+-vacancy). The use of the Ti content of phlogopite as geothermometer reveals crystallization temperatures from 729 ± 15 to 874 ± 15 °C for the phlogopites.

The solid solution (Fe0.81Al0.19)(H2PO4)3 with a strong hydrogen bond

Single crystals of the solid solution iron aluminium tris(dihydrogenphosphate), (Fe(0.81)Al(0.19))(H(2)PO(4))(3), have been prepared under hydrothermal conditions. The compound is a new monoclinic variety (gamma-form) of iron aluminium phosphate (Fe,Al)(H(2)PO(4))(3). The structure is based on a two-dimensional framework of distorted corner-sharing MO(6) (M = Fe, Al) polyhedra sharing corners with PO(4) tetrahedra. Strong hydrogen bonds between the OH groups of the H(2)PO(4) tetrahedra and the O atoms help to consolidate the crystal structure.