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The Stability of Primary Alluaudites in Granitic Pegmatites: an Experimental Investigation of the Na2(Mn2–2Xfe1+2X)(PO4)3 System

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The Stability of Primary Alluaudites in Granitic Pegmatites: an Experimental Investigation of the Na2(Mn2–2Xfe1+2X)(PO4)3 System Contrib Mineral Petrol (2006) 152:399–419 DOI 10.1007/s00410-006-0115-2 ORIGINAL PAPER The stability of primary alluaudites in granitic pegmatites: an experimental investigation of the Na2(Mn2–2xFe1+2x)(PO4)3 system Fre´de´ric Hatert Æ Andre´-Mathieu Fransolet Æ Walter V. Maresch Received: 6 January 2006 / Accepted: 5 June 2006 / Published online: 8 August 2006 Ó Springer-Verlag 2006 Abstract In order to assess the geothermometric Keywords Primary alluaudites Æ Na–Mn–Fe2+–Fe3+ potential of the Na2(Mn2–2xFe1+2x)(PO4)3 system phosphates Æ Phase relations Æ Petrological (x = 0–1), which represents the compositions of natural implications Æ Pegmatites weakly oxidized alluaudites, we performed hydrother- mal experiments between 400 and 800°C, at 1 kbar, under an oxygen fugacity (f(O2)) controlled by the Introduction Ni–NiO (NNO), Fe2O3–Fe3O4 (HM), Cu2O–CuO (CT), and Fe–Fe3O4 (MI) buffers. When f(O2)is The alluaudite mineral group consists of Na–Mn–Fe- controlled by NNO, single-phase alluaudites crystallize bearing phosphates which are known to occur in gra- at 400 and 500°C, whereas the association alluaudite + nitic pegmatites, particularly in the beryl-columbite- maric´ite appears between 500 and 700°C. The limit phosphate subtype of the rare-element pegmatites, between these two fields corresponds to the maximum according to the classification of Cˇ erny´ (1991). In temperature that can be reached by alluaudites in granitic pegmatites, alluaudite-group minerals exhibit 2+ 3+ granitic pegmatites, because maric´ite has never been chemical compositions ranging from Na2Mn(Fe Fe ) 3+ 2+ 2+ observed in these geological environments. Because (PO4)3 to hNaMnFe2 (PO4)3, with Mn or some Ca alluaudites are very sensitive to variations of oxygen replacing Na+,Fe2+ replacing Mn2+, and some Mg2+ or 2+ 3+ 2+ fugacity, the field of hagendorfite, Na2MnFe Fe Mn replacing iron, where h represents a lattice (PO4)3, has been positioned in the f(O2)–T diagram, vacancy. and provides a tool that can be used to estimate the According to Moore (1971), alluaudites are produced oxygen fugacity conditions that prevailed in granitic from primary phosphates of the triphylite–lithiophilite 2+ pegmatites during the crystallization of this phosphate. series, LiFe (PO4)–LiMn(PO4), by oxidation coupled with a Li fi Na metasomatic exchange process. Alluaudite-group minerals, which are generally very fine grained compared to minerals of the triphylite– lithiophilite series, were consequently considered to be of secondary origin. Several occurrences of such sec- Communicated by J. Hoefs ondary alluaudites, produced from triphylite–lithiophilite or from their oxidation products ferrisicklerite–sicklerite, F. Hatert (&) Æ A.-M. Fransolet 3+ 2+ 2+ 3+ Li1–x(Fe ,Mn )(PO4)–Li1–x(Mn ,Fe )(PO4), and Laboratoire de Mine´ralogie, De´partement de Ge´ologie, 3+ 3+ 3+ 3+ Baˆtiment B18, Universite´ de Lie`ge, heterosite–purpurite, (Fe ,Mn )(PO4)–(Mn ,Fe ) 4000 Sart-Tilman, Belgium (PO4), were reported by Huvelin et al. (1972), Fransolet e-mail: [email protected] (1975), Fontan et al. (1976), Fontan (1978), Boury (1981), Lahti (1981), Fransolet et al. (1985, 1986), Keller W. V. Maresch Institut fu¨ r Geologie, Mineralogie und Geophysik, and Von Knorring (1989), Roda et al. (1996), and Roda Ruhr-Universita¨t Bochum, 44780 Bochum, Germany Robles et al. (1998). 123 400 Contrib Mineral Petrol (2006) 152:399–419 The existence of primary alluaudites was first men- of granitic pegmatites (London et al. 1999, 2001). With tioned by Quensel (1957), who considered that this goal in mind, we decided to assess the geother- hu¨ hnerkobelite and varulite crystallized between 400 mometric potential of the Na2(Mn2–2xFe1+2x)(PO4)3 and 600°C, during the first stages of pegmatite evolu- system (x = 0–1), which models the compositions of tion [hu¨ hnerkobelite is not a valid species any more, natural, weakly oxidized, primary alluaudites. For this and corresponds to alluaudite or ferroalluaudite, purpose, we performed systematic hydrothermal according to Moore and Ito (1979)]. Fransolet (1975, experiments between 400 and 800°C, at 1 kbar and 1977) noted sharp contacts between alluaudite and under oxygen fugacities controlled by the Ni–NiO, ferrisicklerite from the Buranga pegmatite, Rwanda, Fe2O3–Fe3O4,Cu2O–CuO, and Fe–Fe3O4 buffers. The without any replacement texture. This observation aim of this paper is to report the results of these indicates that the metasomatic replacement process experiments, which will provide a tool for constraining proposed by Moore (1971) cannot be generalized. the temperature and oxygen fugacity conditions that He´reng (1989) re-examined the samples from the prevailed in granitic pegmatites during the crystalliza- Buranga pegmatite, and observed three different primary tion of alluaudites. parageneses: alluaudite + triphylite, alluaudite + fillowite, and alluaudite + arrojadite. The large size of the alluaudite grains from the Buranga pegmatite, Previous studies which can reach 1 cm in length, is also a good argu- ment for distinguishing primary from secondary fine- Crystal structure, chemical composition, grained alluaudites. More recently, Fransolet et al. and nomenclature of natural alluaudites (1994, 1997, 1998, 2004) observed several parageneses involving primary alluaudites: alluaudite + arrojadite By using a single crystal from the Buranga pegmatite, in Hagendorf-Su¨ d, Germany; alluaudite + fillowite in Rwanda, Moore (1971) determined the crystal Rusororo, Rwanda, and Kabira, Uganda; alluaudite + structure of alluaudite in the monoclinic C2/c space ferrisicklerite + heterosite in Kibingo and Wasurenge, group, and proposed the general structural formula Rwanda. X(2)X(1)M(1)M(2)2(PO4)3, with Z = 4. The structure As observed for the phosphates of the triphylite– consists of kinked chains of edge-sharing octahedra lithiophilite series, which progressively transform to stacked parallel to {101}. These chains are formed by a ferrisicklerite–sicklerite and to heterosite–purpurite succession of M(2) octahedral pairs linked by highly due to oxidation and Li-leaching, the primary alluau- distorted M(1) octahedra. Equivalent chains are con- dites, which are weakly oxidized, progressively trans- nected in the b direction by the P(1) and P(2) phos- form into oxidized secondary alluaudites. In order to phate tetrahedra to form sheets oriented perpendicular maintain charge balance, Na is leached out of the to [010]. These interconnected sheets produce channels alluaudite structure, according to the substitution parallel to the c axis, channels which contain the dis- mechanism Naþ þ Fe2þ ! Ã þ Fe3þ; as observed torted cubic X(1) site and the four-coordinated X(2) by Mason (1941) and Fransolet et al. (1985, 1986, site (Fig. 1). 2004). This oxidation mechanism, coupled with Na According to Moore (1971), the cations are distrib- leaching, explains the transformation of hagendorfite, uted among the different crystallographic sites as a 2+ 3+ 3+ Na2MnFe Fe (PO4)3, into alluaudite, hNaMnFe 2 function of their ionic radii. Accordingly, the large 2+ 3+ (PO4)3, and of ferrohagendorfite, Na2Fe 2Fe (PO4)3, X(2) site contains Na, K and vacancies; X(1) contains 2+ 3+ 2+ into ferroalluaudite, hNaFe Fe 2(PO4)3. Na, Mn, and Ca; M(1) contains Mn and Fe ; and the In the genetic processes affecting Fe–Mn phosphates small M(2) site contains Fe3+,Fe2+, Mn, Mg, and Li. in granitic pegmatites, minerals of the alluaudite group Because Mn dominates on the M(1) site, and Fe2+ and occupy a crucial position. Due to their flexible crystal Fe3+ dominate on the M(2) site, Moore (1971) 2+ 3+ 2+ 3+ structure, which is able to accommodate Fe and Fe proposed the ideal formula Na2MnFe Fe (PO4)3, in variable amounts, alluaudites are very stable and from which the majority of natural alluaudites can be crystallize from the first stages of pegmatite evolution derived. to the latest oxidation processes. Since the petrogenetic Moore and Ito (1979) investigated the crystal importance of accessory phosphates has been demon- chemistry of several natural alluaudite samples, strated in the ultrahigh-pressure rocks of the Dora- and also proposed a systematic nomenclature for the Maira massif (Brunet et al. 1998), it now clearly alluaudite group, which is based on the cation distri- appears that experimental studies on these rare bution among the different crystallographic sites. The minerals are necessary to better understand the genesis mineral is given a generic name that depends on the 123 Contrib Mineral Petrol (2006) 152:399–419 401 Fig. 1 A projection of the alluaudite structure. The PO4 tetrahedra are densely shaded, the M(1) octahedra are shaded, and the M(2) octahedra are unshaded. The circles indicate Na at the A(1) (= X(1)) and A(2)¢ (= X(2)) crystallographic sites predominant M(2) content (Mn = varulite, Fe2+ = et al. (2000a) proposed the new general formula 3+ hagendorfite, Fe = alluaudite), and characterized by [A(2)A(2)¢][A(1)A(1)¢A(1)†2]M(1)M(2)2[PO4]3 for a prefix reflecting the M(1) content (Fe2+ = ferro-, and alluaudite-type compounds. In this formula, A(1) and Mg = mag-). For example, the compositions NaA2¢ A(2)¢ correspond to X(1) and X(2), respectively A1 2+ M1 2+ 3+ M2 A2¢ A1 M1 Na (Fe ) (Fe Fe ) (PO4)3,Na Na (Mn) (Fig. 1). Except in some Cu- and H-bearing alluaudite- 2+ 3+ M2 A2¢ A1 M1 3+ M2 (Fe Fe ) (PO4)3,andNa Na (Mn) (MnFe ) type compounds, the A(1)¢, A(1)†, and A(2) sites are (PO4)3 correspond to the minerals ferrohagendorfite, empty (Hatert et al. 2000a). hagendorfite and varulite, respectively, whereas In general, the cation distribution in synthetic the more oxidized compositions hA2¢NaA1(Fe2+)M1 alluaudite-type phosphates is controlled by the ionic 3+ 3+ M2 A2¢ A1 M1 3+ 3+ M2 (Fe Fe ) (PO4)3 and h Na (Mn) (Fe Fe ) radii of the cations, as suggested by Moore (1971) and (PO4)3 correspond to ferroalluaudite and alluaudite, Moore and Ito (1979).
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