Paul Keller

First experimental evidence of alluaudite-like phosphates with high Li-content: the (Na1-xLix)MnFe2(PO4)3 series (x = 0 to 1)

Members of the Na 1-x Li x MnFe 2 (PO 4 ) 3 series, with the alluaudite structure type, were synthesized by solid-state reaction in air. The crystal structure refinement of the NaMnFe 2 (PO 4 ) 3 end-member (space group C2/ c, Z = 4, a = 12.018(2), b = 12.565(3), c = 6.415(1) A, β = 114.33(3)°), a synthetic compound with a chemical composition corresponding to the idealized composition of the Buranga alluaudite, was carried out to R 1 = 0.026. The following cationic distribution was observed: Na + + □ in A(1) and A(2)′ (□ denotes lattice vacancies), Mn 2+ in M(1), Fe 3+ + Fe 2+ in M(2). The A(2)′ site exhibits a distorted gable disphenoid morphology and is found at the (0, y, ¼) (y ≈ 0) position in channel 2 of the alluaudite structure. The crystal structure of Na 0.5 Li 0.5 MnFe 2 (PO 4 ) 3 (space group C2/ c, Z = 4, a = 11.988(2), b = 12.500(3), c = 6.392(1) A, β = 114.67(3)°), refined to R 1 = 0.034, leads to the cationic distribution: Li + + Na + + □ in A(2)′, Na + + □in A(1), Mn 2+ in M(1), Fe 3+ + Fe 2+ in M(2). Thus, the substitution mechanism involved in the replacement of Na by Li in the Na 1-x Li x MnFe 2 (PO 4 ) 3 alluaudite-like compounds corresponds to □ + Na □ Li + □, with x ranging from 0.00 to 0.90.

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.

Phosphate mineral associations in the Cañada pegmatite (Salamanca, Spain): Paragenetic relationships, chemical compositions, and implications for pegmatite evolution

Abstract The Cañada pegmatite (Salamanca, Spain), a Li-P-(Sn-Nb ± Ta)-bearing granitic pegmatite, is intruded partly into a gabbro and partly into a leucogranite. Three phosphate associations have been distinguished, based on paragenesis, texture, and chemistry: (1) ferrisicklerite-magniotriplite-johnsomervilleite association (in the border zone), in which Mg-bearing phosphates and coexisting Fe-Mg silicates are abundant; (2) ferrisicklerite-graftonite association (in the transition zone) characterized by higher values of Fe/(Fe + Mg) than in the border zone; and (3) triphylite-sarcopside association (in an inner zone), which also includes minor montebrasite, ferrocolumbite, and cassiterite, typical of an evolved pegmatite facies. As a result of the decrease of Mg, the Fe/(Fe + Mg) ratios for phosphates, biotite, and tourmaline increase from the border to the inner association (e.g., for ferrisicklerite and graftonite, from 0.67 and 0.85 in the border to 0.94 and 0.98 in the inner association, respectively). This difference is particularly evident for biotite and tourmaline; for example, the Fe/(Fe + Mg) ratios for tourmaline range from 0.59 in the border to 0.86 in the inner zone. These variations seem to reflect contamination of marginal zones of the pegmatite by some type of reaction with the host gabbro. Thus, an evolutionary trend involving inward crystallization from the margins and contamination of fluids from wallrocks into pegmatite-forming melt may be a plausible genetic model. The occurrence of phosphates along with Fe-Mg silicates would indicate that the melt contained on the order of 1.3-2.4 wt% P2O5, based on experimental silicate-phosphate equilibria.

Chemistry, paragenesis and significance of tourmaline in pegmatites of the Southern Tin Belt, central Namibia

Abstract Numerous pegmatite bodies occur in several distinct belts within the Damara orogen of central Namibia. Many of the pegmatites have been mined extensively for Sn, Li, Nb, Ta, Cs, mica, feldspar, and gem tourmaline and gem beryl. Tourmaline is widespread as an accessory and locally abundant mineral in different geological environments within the Damara Orogen. In pegmatites from the Southern Tin Belt (STB), it exhibits diverse habits and colours, from black to dark green, light green, and pink, as well as variations in composition depending on pegmatite type and location within the pegmatite. Electron microprobe analyses of the tourmalines show a significant chemical variation from Fe-rich compositions with moderate Al in the Y sites to Li-rich compositions with high Y Al , and relatively high F. Fe/(Fe+Mg) ratios for black, green, and pink tourmalines vary from 0.65 to 0.95, 0.66 to 0.99, and 0.50 to 0.87, respectively. Na/(Na+Ca) ratios increase from Li-bearing tourmaline (0.83–0.94) to black tourmaline (0.95–0.99). Tourmaline compositions mostly plot along trends involving proton-loss and alkali-defect substitutions from the schorl additive component. These substitutions account for increases of Al in the Y site, and decreases in the X site and OH occupancies at the expense of the schorl component. Amounts of Al (Y) range from 0.23–0.63 in black tourmalines to 0.82–1.44 in green and pink tourmalines. Within individual pegmatites, the abrupt transition from black tourmaline to green tourmaline, and later pink tourmaline, reflects decreasing Fe and increasing Li and Al via the substitution LiAlFe−2. Enrichment of Li in the tourmaline is accompanied by increases of Be, Sn, Rb, Cs, and Pb. Insignificant amounts of Fe3+ in the tourmalines suggest a low oxidation state of the pegmatite-forming fluids. As the precipitation of cassiterite requires an increase in fO2, such conditions may account for the distribution of tourmaline and cassiterite in different pegmatite zones. Field relations, distribution, petrographic features, and tourmaline chemistry provide evidence of two pegmatitic domains in the STB: (1) a western one, characterized by pegmatites with Sn, Nb>Ta, (Li, Be) mineralization and containing Fe–Mn phosphates and black tourmaline, which may be explained in terms of simultaneous crystallization of two petrologenetic units; and (2) an eastern one characterized by pegmatites with Li>Be, (Sn, Nb>Ta) mineralization and elbaite, which formed via fractionation processes and sequential crystallization from the margins inward to the cores.

Micas of the muscovite – lepidolite series from Karibib pegmatites, Namibia

Abstract Micas of the muscovite-lepidolite series are main constituents of the evolved pegmatites from the Okatjimukuju-Kaliombo portion of the Karibib belt, Namibia. The compositional variations shown by the micas from the intermediate zones are mainly controlled by the Li3Al−1□−2 and SiLi2Al−2□−1 substitution schemes, whereas for the micas from the core margins and the replacement bodies, only the first of these two exchange vectors seems to operate. The chemical composition of the micas not only depends on the degree of pegmatite evolution, but also on the position in the internal zonation of the pegmatite. Micas from the core margins and the replacement units are generally richer in F, Li, Rb, Cs and Zn than those from the intermediate zones. In general, the contents of these elements increase with decreasing K/Rb ratio. However, some data departing from this general trend are also observed, which could be related to subsolidus processes. Some pegmatite bodies show a complete internal evolution, developed from the margins to the core zone, which is reflected in the chemical composition of the micas. The regional distribution of pegmatites does not define a zonation, because an overlapping of pegmatites with different degrees of evolution occurs. This could be due to the high level of evolution attained by most of the rare-element pegmatites, and to their topography with respect to a dome structure of the basement.

Ferrorosemaryite, NaFe2+Fe3+Al(PO4)3, a new phosphate mineral from the Rubindi pegmatite, Rwanda

Ferrorosemaryite, ideally □ NaFe2+Fe3+Al(PO4)3, is a new mineral species from the Rubindi pegmatite, Rwanda. It occurs as large idiomorphic grains reaching 3 mm, embedded in scorzalite. Associated phosphate minerals are scorzalite, trolleite, montebrasite, bertossaite, brazilianite, with accessory augelite, triplite and lacroixite. The mineral is transparent and exhibits a dark-green to bronze colour, with a resinous lustre and with a greenish to brownish streak. It is non-fluorescent, brittle, and shows a perfect {010} cleavage and a good {101} cleavage. The estimated Mohs hardness is 4. The calculated density is 3.62 g/cm3. Ferrorosemaryite is biaxial negative, with α = 1.730(5), β = 1.758(7), and γ= 1.775(5) (Λ = 590 nm). Pleochroism is from dark green ( X ) to dark brown ( Z ). The measured 2 V angle is 82(1)°, and the calculated 2 V angle is 75°. A strong dispersion r < v has been observed, but the optical orientation has not been determined. Electron microprobe analyses gave P2O5 46.00, Al2O3 9.12, Fe2O3 21.01, FeO 11.10, MgO 0.19, MnO 7.96, CaO 0.44, Na2O 2.85, K2O 0.01, total 98.68 wt. %. The resulting empirical formula, calculated on the basis of 12 O, is □1.00(Na0.42Mn2+0.28Ca0.04□0.26)Σ1.00(Fe2+0.71Mn2+0.24Fe3+0.05)Σ1.00Fe3+1.00(Al0.82Fe3+0.16Mg0.02)Σ1.00[(P0.99□0.01)O4]3. The single-crystal unit-cell parameters are a = 11.838(1), b = 12.347(1), c = 6.2973(6) A, β = 114.353(6)°, and V = 838.5(1) A3, space group P 21/ n . The eight strongest lines in the powder X-ray diffraction pattern [ d (in A)( I )( hkl )] are: 8.102(30)(110), 6.167(50)(020), 5.382(40)(200), 4.054(45)(220), 3.448(65)(310), 3.011(40)(112), 2.693(75)(400), 2.677(100)(240). Ferrorosemaryite is the Fe2+ analogue of rosemaryite, and belongs to the wyllieite group of minerals. The crystal structure of ferrorosemaryite has been refined, based on single-crystal X-ray diffraction data, to R 1 = 2.43 %. The infrared spectrum is similar to those of alluaudite-type phosphates. The mineral species and name were approved by the Commission on New Minerals and Mineral Names of the International Mineralogical Association (no. 2003–063).

Intercrystalline cation partitioning between minerals of the triplite-zwieselite-magniotriplite and the triphylite-lithiophilite series in granitic pegmatites

Minerals of the triphylite-lithiophilite, Li(Fe, Mn)PO4, and the triplite-zwieselite-magniotriplite series, (Mn, Fe, Mg)2PO4F, occur in the late stage period of pegmatite evolution. Unfortunately, neither are the genetic relationships between these phosphates fully understood nor are thermodynamic data known. Consequently, phosphate associations and assemblages from 8 granitic pegmatites — Clementine II, Rubicon II and III, and Tsaobismund (Namibia); Hagendorf-Süd and Rabenstein (Germany); Valmy (France); Viitaniemi (Finland) — have been tested for compositional zoning and intercrystalline partitioning of main elements by electron microprobe techniques. Although the selected pegmatites display varying degrees of fractionation, and the intergrowth textures indicate different genetic relationships between the phosphates, the plots of mole fractions XFe=Fe/(Fe+Mn+Mg+Ca), XMn=Mn/(Fe+Mn+Mg+Ca), and XMg=Mg/(Fe+Mn+Mg+Ca) can be fitted relatively well with smooth curves in Roozeboom diagrams. Their deviations from symmetrical distribution curves are mainly dependent upon XMg or XCa, and upon non-ideal solutions. Surprisingly small differences between the partition coefficients were detected for intergrowths of different origin. However, the partitioning of shared components among coexisting phases is clearly dependent upon the conditions of formation. Compositional zoning is observed only when both Fe−Mn phosphates are intergrown mutually or with other Fe−Mn−Mg mineral solid-solutios. Thus, the zoning does not seem to be due to continuous crystallization, but to later diffusion processes. The triplite structure has preference for Mn, Mg, and Ca, while Fe prefers minerals of the triphylite series. A quantification of main element fractionation between minerals of the triphylite and the triplite series is possible in the cases where diffusion can be excluded. For the Fe/(Fe+Mn) ratios of core compositions an equation with a high correlation coefficient (R=0.988) was determined: Fe/(Fe+Mn)Tr=[Fe/(Fe+Mn)Li]/{2.737-(1.737)[Fe/(Fe+Mn)Li]} (Tr=triplite series, Li=triphylite series). Consequently, the Fe/(Fe+Mn) ratio of the triplite series can now also be used in the interpretation of pegmatite evolution, just like that of the triphylite series which has been successfully applied in the past.

Ekatite, (Fe3+, Fe2+, Zn)12(OH)6[AsO3]6[AsO3HOSiO3]2, a new mineral from Tsumeb, Namibia, and its crystal structure

Ekatite, ideally (Fe 3+ , Fe 2+ , Zn) 12 (OH) 6 [AsO 3 ] 6 [AsO 3 HOSiO 3 ] 2 , is a new mineral from Tsumeb, Namibia. It is associated with chalcocite and occurs as small sprays of fine needles, up to 2 mm long and less than 0.2 mm in diameter. Ekatite is brownish black, translucent with a brown streak and vitreous lustre. It is brittle without cleavage and its Mohs9 hardness is ∼3. The mineral is non-fluorescent, optically uniaxial positive with indices of refraction ω ∼1.99 and e ∼2.08, pleochroic with ω = dark brownish black and e = medium brown. Microprobe analysis revealed the following chemical composition (in wt%): (total Fe given as) Fe 2 O 3 50.79, ZnO 3.78, As 2 O 3 42.50, and SiO 2 2.10. The empirical formula based on 20 cations and the results of structure determination is: (Fe 3+ 5.95 Fe 2+ 5.14 Zn 0.81 )Σ 11.90 (OH) 6.00 (As 1.01 O 3 ) 6.00 [(As 3 ) 1.43 (HOSiO 3 ) 0.61 ]Σ 2.04. Single crystal X-ray studies give the following parameters: hexagonal, space group P6 3 mc ; a = 12.773(2), c = 5.051(1) A 3 , V = 713.7(4) A 3 with Z = 1; D calc = 4.061 g/cm 3 . The strongest reflections of the powder pattern are [d obs in A, (I vis ) and (hkl hex )]: 11.11 (3) (100), 6.37 (5) (110), 3.220 (10) (211, 220), 2.766 (3) (400), 2.420 (7) (401, 410), 1.867 (3) (402), 1.672 (3) (521), and 1.508 (3) (701, 531). The crystal structure was solved by direct methods and refined to R = 0.043 using 980 observed unique reflections. It contains dimers of face-sharing (Fe 3+ Fe 2+ Zn)O 4 (OH) 2 octahedra, which are linked together by common edges to form infinite double chains. They are connected by corners to form a three-dimensional framework with two types of channels running along the c -axis. The larger is hexagonal and lined by six AsO 3 -groups, in such a way that the lone-pairs of As 3+ point to its centre. Trigonal pyramids of AsO 3 and HOSiO 3 tetrahedra, in random distribution, occupy the smaller triangular channel, with either the lone pairs or the OH oriented along the c -axis. Ekatite is structurally related to phosphoellenbergerite, ellenbergerite and holtedahlite, as well as to several transition-metal tellurites, selenites, phosphites and Zn 7 (OH) 3 (SO 4 )(VO 4 ) 3 . The name is in honour of Dieter Ekat (1935–1996), a Namibian mining engineer.

The crystal structures of zincroselite and gaitite Two natural polymorphs of Ca2Zn[AsO4]2·2H2O from Tsumeb, Namibia

The crystal structures of zincroselite and gaitite, two polymorphs of Ca 2 Zn[AsO 4 ] 2 ·2H 2 O from Tsumeb (Namibia), were determined from single-crystal X-ray intensity data. The type crystal of zincroselite, monoclinic, P 2 1 / c , a = 5.827(1), b = 12.899(3), c = 5.646(1) A and β = 107.69(3)°, Z = 2, was refined with 935 unique reflections to R 1 = 2.61%. The mean bond distances are = 1.690 A, 2 O)> = 2.123 A and 2 O)> = 2.526 A. Gaitite, P 1, a = 5.899(1), b = 6.978(1) and c = 5.755(1) A, α = 97.41(3)°, β = 109.08(3)° and γ= 108.09(3)°, Z = 1, was refined with 914 unique reflections to R 1 = 1.77%. The mean bond distances are = 1.687 A, 2 O)> = 2.102 A and 2 O)> = 2.525 A. For both structures, the positions of all H atoms were isotropically refined and the hydrogen-bonding arrangement identified. The common structural unit of zincroselite and gaitite are [Ca 2 Zn(H 2 O) 2 O 8 (AsO 4 ) 2 ] 16- rods parallel to the c axis. The polymorphism of Ca 2 Zn[AsO 4 ] 2 ·2H 2 O referred to the glide plane c can easily be visualized with different rod-packings. Zincroselite and gaitite belong to the roselite and collinsite structure type, respectively.

Hydrothermal synthesis and crystal structure of Na (Na,Mn )7Mn22 (PO4)18·0.5H2O, a new compound of fillowite structure type

The chemical compound Na(Na,Mn) 7 Mn 22 (PO 4 ) 18 ·0.5H 2 O was synthesized with a Tuttle pressure vessel at 800°C and 1 kbar during experimental investigations on the conditions of fillowite formation. The crystal structure of a single-crystal, space group R 3 (No. 148), a = 15.2741(9), c = 43.334(3) A, Z = 6, was determined from X-ray intensity data and refined up to R 1 = 0.0728 and R 1{FO>4σ(FO)} = 0.0546. The crystal structure of Na(Na,Mn) 7 Mn 22 (PO 4 ) 18 ·0.5H 2 O is similar to the structure of fillowite, Na 6 Ca 3 (Mn,Fe) 21 (PO 4 ) 18. , but a significant difference between the synthetic H 2 O containing compound and fillowite is the replacement of (Na12)O 8 - by Mn(H 2 O) 2 O 5 - polyhedra, which form characteristic six-member rings. The coordination polyhedra around Na12 and Mn12 are so different that simple isomorphous substitution of Na12 by Mn12 is very unlikely, but whole six-member rings either of (Na12)O 8 - or (Mn12)(H 2 O) 2 O 5 polyhedra seem to be distributed statistically over the crystal structure. No indication for ordering, e.g. sheet by sheet, has been observed. Na(Na,Mn) 7 Mn 22 (PO 4 ) 18 ·0.5H 2 O is the first fillowite-like compound for which water is located in the structure from single-crystal study. This result is proved by infrared spectral data and indicates that water can play an essential role in the fillowite structure-type. Structural features of the synthetic compounds Na(Na,Mn) 7 Mn 22 (PO 4 ) 18 ·0.5H 2 O, Na 4 Ca 4 Mg 21 (PO 4 ) 18 and fillowite are compared.