intercrystalline cation partitioning between minerals of the triplite-zwieselite-magniotriplite and...

Contrib Mineral Petrol (1994) 118:239-248 �9 Springer-Verlag 1994

Paul Keller. Francois Fontan Andr6-Mathieu Fransolet

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

Received: 29 October 1993 / Accepted: 8 July 1994

Abstraet 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 un- derstood nor are thermodynamic data known. Conse- quently, phosphate associations and assemblages from 8 granitic pegmatites - Clementine II, Rubicon II and III, and Tsaobismund (Namibia); Hagendorf-Stid 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 XVe = 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

P. Keller ([~) Institut ftir Mineralogie und Kristallchemie, Universitfit Stuttgart, Pfaffenwaldring 55, D-70550 Stuttgart, Germany

F. Fontan Laboratoire de Min6ralogie, URA 067, Universit6 Paul-Sabatier de Toulouse, All6e Jules-Guesde 39, F-31400 Toulouse, France

A.-M. Fransolet Institut de Min6ralogie, Universit6 de Li&ge, Bfitiment B 18, Sart Tilman, B-4000 Liege, Belgium

Editorial responsibility: W. Schreyer

when both Fe-Mn phosphates are intergrown mutually or with other Fe-Mn-Mg mineral solid-solutions. 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)wr = [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.

Introduction

Minerals of the solid-solution series between triphylite, LiFePO4, and lithiophilite, LiMnPO4, as well as between triplite, Mn2PO4 F, zwieselite, FezPO4F , and magniotriplite, (Mg, Fe, Mn, Ca)2PO4F, are constituents of many granitic pegmatites, occasionally in large amounts up to several thousand tons in weight. Their intergrowth textures and possible genetic relationships were investigated most recently using associations from 8 pegmatites (Keller et al. 1994). The present paper deals with the chemical composition of associated phosphates from the same 8 pegmatites, in order to determine further evidence for their genetic relationships, and for a discussion of pegmatite evolution in general.

Firstly, detailed chemical investigations should per- mit the Fe/(Fe + Mn) ratio of the triplite series to be used, as successfully as the Fe/(Fe+ Mn) ratio of the triphylite series, for the interpretation of pegmatite evolution (Fransolet et al. 1986; Keller 1988; Keller and Von Knorring 1989; Keller 1991; Keller etal. 1994). Then, pegmatite differentiation can be discussed on the

240

basis of the Fe/(Fe + Mn) ratio of the primary Fe-Mn phosphates.

Secondly, the search for compositional zoning could yield further evidence of disequilibrium conditions during the evolution of granitic pegmatites, in addition to the occurrence of dendritic phosphates (Keller 1988, 1991) and other pegmatite characteristics (London 1992). Despite the long-accepted idea that the Fe-Mn phosphate minerals in a pegmatite are characterized by a distinct chemistry, constant throughout the pegmatite body, minerals of the triphylite series with different chemical compositions and chemically heterogeneous triplite-zwieselite have been described from single pegmatites by several authors (Volborth 1954; Fransolet 1975; Fontan 1981; Lahti 1981; Fransolet et al. 1986; Keller and Von Knorring 1989; Keller 1988, 1991). Compositional zoning, however, has never been report- ed.

Furthermore, the main cation partitioning between the two solid-solution series could possibly be used in the future in order to evaluate conditions of formation, particularly temperature and pressure, for the interesting late stages of pegmatite evolution, provided that the thermodynamic data of both phosphates can be determined. The petrographic results and preliminary data on partitioning of main elements presented in this investigation may encourage petrologists to study this interesting system for pegmatite genesis by experimental methods.

Selected materials and their intergrowth textures

Specimens were selected with respect to their intergrowth texture and their Fe/(Fe + Mn) ratio. Whereas the intergrowth textures usually indicate genetic relationships between the phosphates (Keller et al. 1994), the Fe/(Fe + Mn) ratio depends, according to a hypothesis introduced by Ginsburg (1960), on the degree of fractionation of the initial pegmatitic melt as well as the evolutionary stage of pegmatite formation (e.g. Franso- let et al. 1986; Keller 1991).

Most phosphate specimens are of large size, as representative of pegmatites. Some fine-grained triphylite s.l.1 forming inclusions and reaction rims, usually over- looked in the past, was also considered.

As frequently observed among the investigated samples, triphylite s.1. ist topotactically transformed into an oxidized product belonging to the ferrisicklerite-sickler- ite, " 3+ 2+ LlxFe(l_xlMnx PO4 with 0_<x_<l, or to the heterosite- purpurite, (re,Mn)PQ, series. As documented in a large number of cases (Mason 1941; Fontan et al. 1976; Fransolet et al. 1985, 1986; Keller and Von Knor-

1 In this paper s.1. (sensu lato) is meant to include all members of the two solid-solution series discussed, that is lithiophilite with triphylite, zwieselite and magniotriplite with triplite. In fact, in the diagrams to follow, the products of secondary alteration of triphylite s.l., ferrisicklerite, heterosite and purpurite are included as well.

ring 1989; Keller 1991), the Fe/(Fe+Mn) ratio of triphylite s.1. fortunately remains constant during this alteration process. Consequently, in case where the primary triphylite s.l. was totally transformed, the chemistry of the oxidized products was determined, and it was assumed that the cation content was equivalent to that of the pre-existing triphylite.

The pegmatites considered in this study, and a short description of their genetic relationships, indicated by observed texture types described elsewhere (Keller et al., 1994), are given below. The selected samples are briefly characterized in Table 1.

Clementine II, Okatjimukuju farm, Karibib, Namibia (Keller and Von Knorring 1989). Cogenetic phosphates display a texture indicated as type 1.1. Textures of type 3.1. and type 3.3. represent different crystallization sequences with earlier triplite and later triphylite. Hagendorf-Siid, Waidhaus, Germany (e.g. Schmid 1955; Strunz et al. 1975). The intergrowth textures result from replacement of triplite s.l. by triphylite. Whereas a typical corona texture (type 3.2.) may be formed during an initial stage with weak interactions, much stronger replacement have caused a distinctly different texture (type 2.2.). Rabenstein (Hiihnerkobel pegmatite), Zwiesel, Germany (Schmid 1955). Texture type 1.2. represents cogenetic phosphates. A corona texture (type 3.2.) was formed during replacement of zwieselite by triphylite, whereas crystallization sequences with earlier zwieselite and later triphylite is indicated by texture type 3.3. Pegmatite No. II at Rubicon, Karibib, Namibia (Keller et al. 1994). The cogenetic phosphates display texture type 2.1. Pegmatite No. III at Rubicon, Karibib, Namibia (Keller et al. 1994). A thick rim of late alteration products obscures the intergrowth texture. Tsaobismund, Namibia (Karnin 1980; Fransolet et al. 1986). The cogenetic phosphates display texture type 2.1. A texture in ap- pearance like type 3.1. seems to be formed by an agglomeration process, in contrast to other pegmatites. Valmy, Massif des Alb~res, France (Fontan and Fransolet 1986). Lamellar intergrown masses of triphylite, sarcopside and graftonite were replaced by magniotriplite. During this process, a corona was formed, composed of secondary triphylite, sarcopside, and apatite. Viitaniemi, Er/ij~irvi, Finland (Volborth 1954, 1956; Lahti 1981). Cogenetic phosphate minerals display texture like type 2.2. The phosphates of type 3.1. belong to a crystallization sequence with early triplite and late lithiophilite.

Analytical methods

About 500 point analyses were performed either by the fully auto- matic Camebax SX 50 electron microprobe (Laboratoire de Min6ralogie, Toulouse) or by a Cameca MS 46 (Institut ffir Miner- alogie und Kristallchemie, Stuttgart) using an operating voltage of 15 kV and a beam current of 20 nA for all elements except F. The operating conditions for F are 4 kV and 50 nA. The standards used with the Camebax SX 50 are: CaF 2 (for F), apatite (for P), albite (Na), andradite (for Fe, Ca, and Si), graftonite (Mn), titanite (Ti), olivine (Mg) and ZnS (Zn). Those used with the Cameca MS 46 are: triplite (P, Fe and Mn), apatite (Ca), MgO and pure Mg (Mg), AI203 (A1), albite (Na), metallic Zn (Zn), and metallic Ti (Ti).

Usually, the chemical composition was determined at the centre of grains as well as at several points toward their rim. If possible, data were collected along common boundaries of both phosphates. Although these contacts are obscured by alteration reactions in several samples, it was possible in most cases to document zoning and/or heterogeneous variations of composition by such a procedure. Only the mole fractions and the Fe/(Fe + Mn) ratios for

241

Table 1 List of selected samples with coexsisting members of the triphylite and triplite series, as well as alteration products of the former

Phosphate mineral pairs Physical relationship Intergrowth texture a

Clementine II, Okatjimukuju, Karibib, Namibia CleL1 Ferrisicklerite CleT1 Triplite/zwies. CleL2 Ferrisicklerite CleT2 Triplite CleL3 Ferrisicklerite CleT3 Triplite CleL4 Ferrisicklerite CleT3 Triplite

CleL5 Ferrisicklerite CleT4 Triplite CleL6 Ferrisicklerite CleT5 Triplite Hagendorf-S/id, Waidhaus, Germany HagL1 Triphylite HagT1 Zwieselite HagL2 Triphylite HagT2 Triplite/zwies. HagL3 Triphylite HagT3 Triplite/zwies, HagL4 Triphylite HagT4 Triplite HagL5 Triphylite HagT5 Triplite/zwies. HagL6 Triphylite HagT6 Triplite HagL7 Triphylite HagT7 Triplite/zwies. HagL8 Triphylite HagT8 Triplite HagL9 Triphylite HagT9 Triplite Rabenstein, (Hiihnerkobel), Zwiesel, Germany RabL1 Heterosite RabT1 Zwieselite RabL2 Heterosite RabT2 Zwieselite RabL3 Heterosite RabT3 Zwieselite RabL4 Heterosite RabT4 Zwieselite RabL5 Heterosite RabT5 Zwieselite RabL6 Heterosite RabT6 Zwieselite Rubicon II, Okongava-Ost, Karibib, Namibia Ru2L1 Ferrisicklerite Ru2T1 Triplite Ru2L2 Ferrisicklerite Ru2T2 Triplite Ru2L3 Ferrisicklerite Ru2T3 Triplite Ru2L4 Heteros./purpurite Ru2T4 Triplite Ru2L5 Ferrisicklerite Ru2T4 Triplite Rubicon III, Okongava-Ost, Karibib, Namibia

Central portions 1.1. + 3.1. Common edges 3.1. Common edges, fine grained ferrisicklerite 1.1. Common edges, fine grained ferrisicklerite, intergrown with magnetite 1.1. Central portions 3.3. Common edges 3.3.

Central portions, coarse grained triphylite Common edges, fine grained triphylite Central portions Close contacts Central portions Close contacts Central portions, coarse grained triphylite Common edges, fine grained triphylite Central portions

2.2. 2.2. 2.2. 2.2. 2.2. 2.2. 3.2. 3.2. 2.2.

Central portions 1.2. Common 6dges 1.2. Central portions 3.2. Common'edges 3.2. Central portions 3.3. Common edges 3.3.

Central portions, coarse grained ferrisicklerite 2.1. Common edges in vicinity of garnet 2.1. Common edges, fine grained ferrisicklerite 2.1. Close contacts with garnet 2.1. Common edges, ferrisicklerite intergrown with magnetite 2.1.

Central portions Common edges Central portions, coarse grained triphylite Common edges, minute grains of triphylite

Ru3L1 Lithiophilite Ru3T1 Triplite Central portions Tsaobismund, Namibia TsaL1 Triphylite TsaT1 Triplite/zwies. TsaL2 Triphylite TsaT2 Triplite TsaL3 Triphylite TsaT3 Triplite TsaL4 Triphylite TsaT4 Triplite/zwies. Valmy, Massif des Alb~res, France ValL1 Triphylite ValT1 Magniotriplite Central portion ValL1 Triphylite ValT2 Magniotriplite Central portion Viitaniemi, Er/ijfirvi, Finland ViiL1 Lithiophilite ViiT1 Triplite Central portions ViiL2 Lithiophilite ViiT2 Triplite Common edges ViiL3 Lithiophilite ViiT3 Triplite Central portions ViiL4 Lithioph./triphyl. ViiT4 Triplite Common edges

2.2.

2.1, 2.1. 3.1. 3.1.

3.2. 3.2.

2.2. 2.2. 3.1. 3.1.

~Keller et at. 1994

representative core and rim compositions are given in Table 2. To receive a copy of the full data set, please contact the first author.

Crystal structures and solid-solution series

Triphylite-lithiophilite

The minerals of the triphylite-lithiophilite series, Li(Fe, Mn)PO4, are isotructural with olivine (Finger and Rapp

1970). The M(1) site of the structure is occupied by Li +, while the Fe 2+ and Mn z+ ions fill the M(2) site in a complete solid-solution series. It is not known whether Mg 2+ replaces the main elements in the M(1) or in the M(2) site. In the more likely case of substitution in M(2), a single-site crystalline solution is developed. The unit- cell dimensions change linearly as a function of the Fe/ (Fe + Mn) ratio (Fransolet et al. 1984). Consequently, an ideal solution behaviour could be expected. However, the difference between the volumes of the two end-mem-

242

Table 2 Representative mole fractions and Fe/(Fe + Mn) ratios of the correlated phosphate-mineral pairs given in Table 1. Symbols as in Table 1

CleL1 CleT1 CleL2 CleT2 CleL3 CIeT3 CleL4 CleT3 CleL5 CleT4 CleL6 CleT5

Mole fractions XF~ 0.705 0.425 0.736 0.406 0.756 0.386 0.747 0.386 0.620 0.341 0.641 0.278 X~n 0.265 0.471 0.238 0.482 0.223 0.509 0,233 0.509 0,345 0.542 0.325 0,601 XMg 0.024 0.083 0.023 0,090 0,016 0,080 0.017 0.080 0.031 0.093 0.032 0.097 Xc~ 0.006 0.021 0.003 0.022 0.005 0.025 0.003 0.025 0.004 0.024 0.002 0.024 Fe/(Fe+Mn) 0 .726 0.474 0.756 0.457 0,772 0.432 0.762 0.432 0,642 0.386 0.664 0.316

HagL1 HagT1 HagL2 HgT2 HagL3 HagT3 HagL4 HagT4 HagL5 HagT5 HagL6 HagT6

Mole fractions

Xve 0.817 0.565 0,842 0.495 0,739 0.460 0,768 0.392 0.733 0.442 0.768 0.367 X~n 0.182 0.410 0.157 0.487 0,261 0.509 0.231 0.579 0.261 0,512 0.228 0.597 XM~ 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.001 0.006 0.018 0.004 0.019 Xc, 0.001 0.025 0.001 0.018 0,0 0.031 0.001 0.028 0.0 0.028 0.0 0.017 Fe/(Fe+Mn) 0 .818 0.579 0.844 0.504 0,739 0.475 0.769 0.404 0.731 0.463 0.771 0.381

HagL7 HagT7 HagL8 HagT8 HagL9 HagT9 RabL1 RabT1 RabL2 RabT2 RabL3 RabT3

Mole fractions

Xw 0.748 0.461 0.768 0.426 0,714 0.414 0.790 0.521 0.837 0.449 0.762 0,496 XMn 0.249 0.505 0.232 0.544 0,286 0.561 0.183 0.346 0.142 0.381 0.200 0.366 XMg 0.001 0.001 0.0 0.004 0.0 0.0 0.024 0.117 0.018 0.157 0.033 0.122 Xca 0.002 0.033 0.0 0.026 0.001 0.025 0.003 0.016 0.003 0.013 0.005 0.016 Fe/(Fe+Mn) 0 .750 0.477 0.768 0,439 0.714 0.425 0.812 0.601 0.855 0.540 0,792 0.575

RabL4 RabT4 RabL5 RabT5 RabL6 RabT6 Ru2L1Ru2T1Ru2L2 Ru2T2 Ru2L3 Ru2T3

Mole fractions

XFe 0.812 0.463 0.738 0.480 0.767 0.446 0.566 0.333 0.575 0.234 0.691 0.267 XMn 0.165 0.391 0.221 0.364 0.197 0.380 0.427 0.620 0.417 0.717 0.303 0.687 X~g 0.021 0.134 0.037 0.141 0.033 0.157 0.005 0.011 0.006 0.014 0.002 0.012 XCa 0.003 0.012 0.004 0.015 0,003 0.016 0.002 0.036 0.002 0.035 0,003 0.034 Fe/(Fe+Mn) 0.831 0.542 0.770 0,569 0.796 0.540 0.570 0.349 0.580 0,246 0.695 0.280

Ru2L4 Ru2T4 Ru2L5 Ru2T4 Ru3L1 Ru3T1 TsaL1 TsaT1 TsaL2 TsaT2 TsaL3 TsaT3

Mole fractions

XFo 0.504 0.199 0.635 0.199 0.289 0.135 0.667 0.386 0.691 0.317 0.701 0.417 XMn 0.492 0.767 0.357 0.767 0.707 0.817 0.262 0.359 0.242 0.399 0.229 0.346 XMg 0.004 0.011 0.003 0.011 0.002 0.004 0.070 0.240 0.067 0.269 0.070 0.223 Xca 0.0 0.023 0.005 0.023 0.002 0.044 0.001 0.014 0,001 0.015 0.0 0.014 Fe/(Fe+Mn) 0 .506 0.206 0.640 0.206 0.290 0,141 0.718 0.519 0,740 0.443 0.754 0.547

TsaL4 TsaT4 ValL1 ValT1 VaiL1 ValT2 ViiL1 ViiT1 ViiL2 ViiT2 ViiL3 ViiT3 ViiL4 ViiT4

Mole fractions

XF~ 0.721 0.366 0.766 0.483 0.766 0,454 0.419 0.195 0.470 0.152 0.459 0.225 0.488 0.187 XMn 0.214 0.367 0.111 0.143 0.111 0,132 0,579 0.753 0.528 0.795 0.540 0.730 0.511 0.769 XMg 0.065 0.256 0.122 0.372 0.122 0,411 0.002 0.004 0.001 0.005 0.001 0.003 0.001 0.003 Xca 0.001 0.012 0.001 0.002 0.001 0,003 0.001 0.048 0.001 0.048 0.0 0.042 0.001 0.041 Fe/(Fe+Mn) 0 .771 0.499 0,873 0.772 0.873 0,776 0.420 0.206 0.471 0.161 0.459 0.236 0.489 0.196

bers is ra ther small. Thus, the volume change of mixing da ta belong to Fe-rich triphylite. Mole fractions for the is less than one might guessed, e.g. by compar i son to two minera l series were de te rmined within the following other Fe -Mn solid solutions, l imits: 0.285 < XFe --< 0.843 and 0.098 _< XM, --< 0.712. Rel-

A l though the invest igated samples display a suffi- at ively low mole fractions of Mg have been repor ted for ciently wide compos i t iona l range, the major i ty of the t r iphyli te s.l. (Fontan et al. 1976; Fon t an 1978, 1981;

Fransolet etal. 1985, 1986) with the highest value XMg = 0.151, observed in triphylite from Valmy (this paper). Only traces of calcium (Xca -< 0.014) were detected.

Triplite-zwieselite and magniotriplite

Triplite, Mn2PO4F , zwieselite, Fe2PO4 F, and magniotriplite, (Mg, Fe, Mn)2PO4F, are isotructural and belong to a unique structure type very different from that of olivine (Waldrop 1969, Tadini 1981). The distribution of divalent cations over the M(1) and M(2) sites is not random. A weak preference of Mg for the slightly smaller M(2) site is observed by X-ray crystal structure determination. No definite conclusion regarding the site preference of Fe can be deduced from M6ssbauer spec- troscopy (Kostiner 1972). Thus, it is uncertain whether or not a behaviour far from a simple crystalline solution should be envisaged. Unfortunately, it is also unknown whether the unit-cell volume changes linearly as a function of the chemical composition or whether there is an excess volume due to mixing.

In nature the pure Fe and Mg end-members have not been observed. According to previous publications (Ot- to 1935; Fontan 1981), the mole fractions vary within the following ranges: 0.11<XMn<0.99, O . O < X w _<0.654 and 0.0 < XMg _< 0.66. The samples considered here cover these compositional ranges relatively well: 0.116 _< XMn -< 0.824, 0.127_<Xve-<0.525, 0.0-<XMg _<0.418 and 0.0_<Xca__<0.057.

Compositional zoning

243

Correlations between the origin of intergrowth textures and the degree of compositional zoning are likely, but not without exceptions. For instance, triplite/ zwieselite of texture type 1.1. from Clementine II (Cle T1/T3) displays only a weak compositional zoning within a very small rim (about 20 gm wide). However, the Rubicon II triplite of texture type 2.1. - also cogenetic with triphylite - displays strong compositional, zoning (Ru2T1/T3). Such pronounced zoning has usually been detected for textures resulting from a strong replacement process, e.g. triptite s.I. from Hagendorf- Sfid (texture type 2.2, HagT1/T2, Hag T3/T4). No compositional zoning was detected in Valmy.

Compositional zoning is developed not only along interfaces between the two phosphate minerals but also between the phosphates and other minerals forming solid-solution series with respect to Fe, Mn, and/or Mg, such as garnet, gahnite, magnetite, and mica. In Rubi- con II, for example, the Fe/(Fe + Mn) ratio of triplite at contacts with ferrisicklerite decreases and the zoning is more intense, when garnet occurs in the vicinity of both phosphates (Ru2T1/T3 and Ru2T2). This ratio reaches an extreme value along the intimate contact between triplite and garnet (Ru2T4). The chemical composition of ferrisicklerite or heterosite at common edges with triplite also depends on the occurrence of garnet. In contrast with triplite; however, the compositional zoning is strong and the Fe/(Fe + Mn) ratio is high in absence of garnet (Ru2L1/L3), whereas both decrease in the vicinity of garnet (Ru2L2, L4). Remarkably strong zoning with much lower Fe/(Fe + Mn) ratios has also been detected near inclusions of gahnite at Tsaobismund.

This is the first report of compositional zoning of triplite s.1. and triphylite s.l. with respect to their main elements Fe, Mn, Mg, and Ca. The differences in composition between centres and rims of crystals and the width of the rims around grains - usually about 20 to 300 pm wide - are not constant but may depend on the mineral, either triplite s.l. or triphylite s.l., and the genetic origin of the texture type.

The compositional zoning is usually more pronounced in triplite s.l. than in triphylite s.l. The only exception is the material from Rabenstein, where the zoning of zwieselite within a rim of about 100 gm width is weak to medium (Table 2, compositions RabT1/T2, RabT3/T4) whereas that of heterosite is relatively strong (Table 2, RabL1/L2, RabL3/L4).

The grain sizes of triphylite s.l. are sometimes very small. For instance, at Rubicon II the size of grains (0.1- 0.2 mm wide) is approximately the same as the rim of coarse grains (about 1 mm in diameter), which displays a composition different from the core. For that reason, the chemical composition changes with the grain size. Such a grain-size effect as an indirect result of compositional zoning was observed in Clementine II (texture type 1.1., CleL1/L3 and L1/L4), Hagendorf-Sfid (texture type 3.2., HagL7/L8), and Tsaobismund (texture type 3.1., Tsa L3/L4).

Intercrystalline partitioning of main cations

Data selection

Although hardly any conditions of formation were known in this first approach to main-element partitioning between Fe-Mn phosphates, an attempt was made to plot Roozeboom diagrams, and to calculate empiri- cal partition coefficients on molar basis (KD). For this purpose, correlated pairs of data as given in Table 1 and 2 were selected from the data base. When the compositional zoning is developed, the data were taken from the phosphate central portions, as well as from common interfaces between the two phosphates, or between the phosphates and garnet or magnetite. In the few cases where only a grain-size effect was observed, chemical data of the coarser and the smaller grains were considered as core and rim composition, respectively.

The mole fractions of main cations

The evaluated mole fractions are Xve = Fe/ (Fe + Mn + Mg + Ca), X~n = Mn/(Fe + Mn + Mg + Ca) and XMg = Mg/(Fe + Mn + Mg + Ca) (Figs. 1, 2). Thus,

244

1 XF e

XMn 0.8

i i i i i I I I . i __~/~ o .,=a- Mn, centre: kD = 0.443, R = 0.974 . ~ ' ~ . ; ~ ' ~ / / ' ~ \ --:--;u?: Mn, contact: .i! ..-" / . I x kD =0.288, R = 0.902 " x , f 41 ~ Z] S" / / /_."

\ �9 , , . ." , , - / / /

0.6 - ~ # i / / ~ '" -

L '~ t " /

. , . / - 0.4 ~" ~ t ; * *

.9 �9 i i . . . " ~ 0.2 . /1 / , , , ~ . . . . . " ~ F e , co n tact :

!; + / ! / ~ ...'-" ~ o = , 8 8 7 . . = 0.908

E entre: k 9 = 3.2126 ~ 0.9910 o .... ," , , I I I I I I

0 0.2 0.4 0.6 0.8

mole fract ions of t r iphy l i te s.I. XFe, XMn

centre contact centre contact

A Clementlne II �9 x Rubicon III o Hagendorf-SOd I ~r Tsaoblsrnund -/r v Rabenstein �9 + Valmy 0 Rubicon II �9 [] Vl i tanleml �9

Fig. 1 Partitioning of Fe 2+ and Mn 2+ between coexisting minerals of the triplite and triphylite series. Both mole fractions XM~ = Mn/(Fe + Mn + Mg + Ca) and X w = Fe/(Fe + Mn + Mg +Ca) are plotted simultaneously in order to make deviations from ideality visible. Preferred Mn fractionation plots in the upper left, preferred Fe fractionation in the lower right portion of the diagram. The four curves given represent data fittings using ideal solution models as discussed in the text. Note that the term s.l. for the two mineral series includes the topotactic alteration products of triphylite (e.g. ferrisicklerite and heterosite) as well

0.5 XMg

I q I I I I I j , .

J Mg, contact ; s s -

0.4 ,0= 0,19,8, , = 0.98a..~. .~ . / . . ~ - ~ J _

I f

"~ 0 . 3

;g

0.2 /

7 v , ' / ' ~ ' \ o '.~ v 1 7 " ~ M g , centre= V ~ "kD ~ 0.2356, R =0 .995

,~_~e ~ o.1 / ~

o . , - ', 1 I I 1 1 I 0 0.04 0.08 0.12 0.16

mole fractions of triphylite s.I, XM9

Fig. 2 Partitioning of Mg between coexisting minerals of the triplite and triphylite series (and its topotactic alteration products). The two curves given represent data fittings using ideal solution models as discussed in the text

only the very small concentrations of Ti, Zn, Na (and other elements with mole fractions less than 0.01 from both solid-solution series) have not been considered here, although they may have also affected the partitioning of main elements between the two phosphates, as F - and (OH)- of triplite s.l. may have done. The partition coefficients are defined as follows:

Tr Li Tr KDM = X~.(1--XM)/(1--XM).XM, with M = Fe, Mn, Mg or Ca; Li = triphylite s.l. and Tr -- triplite s.l.

Several interesting conclusions can be drawn from plots of the mole fractions (Roozeboom diagrams) of triplite s.l. versus those of triphylite s.l. (Figs. 1, 2). The crystal structure of triplite has preference for Mn, Mg, and Ca, whereas the triphylite structure prefers Fe. The mole fractions which were determined either at the central portions or at the rim of the phosphates are evident- ly separated from each other. Furthermore, it is surprisingly possible to correlate each data set with a distinct distribution curve fitted by constant KD values (Figs. 1, 2, 6). The data for coarser and smaller grains, both without compositional zoning, also fit well with the distribution curves for core and rim composition, respectively. The development of two different distribution curves for the core and the rim of minerals indicates that the partition coefficients are sensitive to varying conditions of formation. With respect to this observation, the detected scattering of data in the Roozeboom diagrams is surprisingly small. The plots of mole fractions from both core and coarse grain compositions (Figs. 1, 2) are not only rather smooth but are also close to ideal distribution curves, plotted according to the equation X~ Li = XM/[K,~ + (1 -- KD~j~XLil~j. The weighted correlation coefficients are RFe=0.990, RMn=0.974 and RMg = 0.995. The set of data from common boundaries and from the smaller grains have lower correlation coefficients, RFe = 0.968, RMn = 0.902 and X~g = 0.965, mainly due to analytical uncertainties. Nevertheless, it is easily visible that the distribution curves of Xw and XMn are not symmetrical with respect to the intersecting di- agonal of Fig. 1. Deviations from ideality can also be demonstrated by Fig. 3, where the ideal distribution curve and the best fitted curve, both calculated for X~, are compared. Thus, non-ideal solution has to be considered, at least for one of the two solid-solution series.

While the plot of XFe deviates smoothly and system- atically with increasing values (Figs. 1, 3), the scattering of XMn seems to be random and relatively strong (Fig. 1). However, if l n K , of triplite s.l. is plotted versus the dominant of both mole fractions, either XMg or XCa, it becomes evident that the observed scattering of partition coefficients KD~, and KI~Fo is mostly dependent upon XMg and XCa (Figs. 4, 5). A plot of lnKD against (XMg +Xca), e.g. like that given by Ganguly & Saxena (1987) for garnet and coexisting biotite/clinopyroxene, does not fit as well. This possibly indicates different effects of either XMg or Xca on the variation of KD. The high scattering of Xca is likely due to the low concentrations (Fig. 5). The plot of lnKD versus XMg and Xca for triphylite s.l. do not fit as well as those of triplite s.l.,

0.7 ] I i i ] ~ I / / I i

XFe nonldeal solution . . i

0.6 - / /

ideal solution

d 0.5 ~,/,V/

~ 0.4

.2 ~ 0 . 3 -

E 0 . 2 -

O l , r I I I I 0.3 0.4 0.5 0.6 0.7 0.8 0.9

mole f r a c t i o n s of triphylite s.I. XFe

Fig. 3 Plot of mole fractions XF~ = Fe/(Fe + Mn + Mg + Ca) from the centre of triplite s.l. against triphylite s.1. The solid line is calculated using the ideal solution model of Fig. 1. The element distribution, however, fits better with the asymmetric broken line (provisionally determined by Y = A + BX + CX2), indicating nonideal solution

InkDM n

-0.2

-0.4

-0.6

7

-0 .8

o InkDMn =2'17xMg-l"06, R = 0.983

-1 .2 I J I I J J I I 0.1 0.2 0.3 0.4

t r ip l i te s.I. wi th X M g > X C a XMg

Fig. 4 Plot of lnKD versus XMg of triplite s.l. with XMg > Xca. Considered are data~ 'om centres and contacts

almost certainly because the much smaller quantities of Mg and Ca are more strongly affected by analytical errors.

The Fe/(Fe + Mn) ratio

To consider the influence of magnesium and calcium, no exact calculations are possible because of missing thermodynamic data. For that reason, the mole fractions Fe/(Fe + Mn) of triplite s.1. - which are also the ratios

245 -0.5

I I I I I

InkDM n

- 0 . 6 - X -

- 0 . 7 -

-0.8-

-0.9

-1

- 1 . 1 O f O O

/ 7 -1 .2-

InkDMn=20.16 XCa-1.66, R=0.861

-1.3 I I I ] P 0.025 0.03 0,035 0.04 0.045 0.05

triplite s.L with XCa>XM9 Xca

Fig. 5 Plot of lnK D versus Xca of triplite s.l. with Xca >XMg. Considered are da t a~om centres and contacts

0.8 4 I I [ I I $

Fe/(Fe + Mn), centre= / 0.7 - kD= 2.737, R = 0.988 " ~ / ~

J / "~ 0.6 ~.~ v o l SI = oZ o ,, = 0.5 * / . / , z /o 7~;-" o 0.4 O / , s , ,, " I�9

+ 0.3 p s

u. 0.2 J = .*," "-~. / .=;" "~Fe/~Fe + ~ , ~ontact=

J . _ . ~ - " kD--4.237, R=0.957

Ol I " " I I I I I 0.3 0.4 0.5 0.6 0.7 0.8 0.9

Fe/(Fe+Mn)rat io of t r iphy l i t e s.l.

Fig. 6 Plot of the Fe/(Fe + Mn) ratio of triplite s.1. against triphylite s.l. The data from both centres and contacts correlate well with the solid and dashed lines, respectively. Both curves represent data fittings using ideal solution models as discussed in the text

used in the hypothesis of Ginsburg (1960) - are provisionally plotted against those of triphylite s.1. (Fig. 6). The scattering observed could be mainly due to different conditions of formation, if analytical errors are excluded. However, it is more difficult to determine compara- ble data from the rim than from the central portions, because the rims are usually very small. Additionally, the common boundaries between the phosphates are obscured by alteration products. Thus, the most extreme chemistry could easily be missed.

The Fe/(Fe + Mn) ratios of triphylite s . l . and triplite s . l . can be theoretically correlated using the distribution curve (Fig. 6). The correlation between the Fe/(Fe + Mn)

246

ratio of the two solid-solution series, as presently determined, is sufficiently significant. The preliminary correlation equation with R=0.988 is:

Fe/(Fe + Mn)Tr = [Fe/(Ve + Mn)L~]/ {2.737 -- (1.737)[Fe/(Fe + Mn)Li] }.

Discussion

The observed compositional zoning and the resulting distribution curves for the core and rim chemical compositions are evidence that the partition coefficients are dependent upon the conditions of formation. It seems likely that temperature is more important than pressure. Because the pressure seems to be almost constant during the crystallization of Fe-Mn phosphates in a given pegmatite body. The influence of pressure on element partitioning may also be insignificant due to a small volume change of mixing, at least for the triphylite series. The scattering of partition coefficients as well as a possible camouflaged equalization effect can originate from several sources if analytical errors are excluded.

Firstly, the conditions of formation were different in the various pegmatites considered. Besides the relatively unknown variations of temperature (and pressure) for the various localities, variable chemical compositions of the system are evident from the observed chemistry of the phosphate minerals. Thus, not only binary but at least ternary solution including Mg and Ca must be taken into account. It has already been proved that the scattering of partition coefficients is essentially caused by the mole fractions XMg or Xca (Figs. 4, 5).

Secondly, the assumption that equilibrium conditions have been reached is not only affected by misinterpretation of the observed intergrowth textures, but also by kinetic aspects of the cooling history.

According to the intergrowth textures, the compositional zoning may have various reasons, when subse- quent diffusion is excluded. In a given assemblage the zoning can be a product of contemporaneous crystallization. Then core and rim compositions represent the initial and final equilibrium conditions, respectively. In the case of replacement processes, the rim composition can represent equilibrium conditions for the late stage of crystallization. However, it seems resonable that the closure temperature for diffusion is below either the temperature of solidification or of replacement for the following reasons.

Assuming that initially zoned phosphates have crys- tallized, as in many minerals capable of solid solution, their growth zoning must have been obliterated by diffusion while above closure temperature. For example, crystals of triphylite s.l. are always chemically homoge- neous, independant of their sizes, when they occur free of other intergrown minerals forming Fe-Mn-Mg solid solution. Consequently, element partitioning resulting from diffusion could have not only affected, but most likely even created compositional zoning, when the

phosphates are intergrown with each other, or with other minerals forming Fe-Mn-Mg solid solutions. Under such conditions, equilibrium may be reached only at the grain contacts, just before the system is closed. Then the rim compositions will represent the partitioning rele- vant for conditions near the closure temperature. The core compositions possibly change in accordance with the parameters of the homogenization stage. Neverthe- less, it may approximately represent the bulk chemistry reached during crystallization, because the zoned rims are very thin. However, according to Lasaga (1983) and Chakraborty and Ganguly (1991), it must be stressed that not only the rim but also the core composition may be affected by diffusion. Such interferences must be taken into account particularly when finegrained aggre- gates are considered.

Following the ideas discussed above, the Fe/ (Fe + Mn) ratios calculated from core compositions are the most appropriate values of the bulk composition, (i) when the Fe-Mn phosphates were not intergrown with other (Fe,Mn)-bearing minerals, and (ii) if they are intergrown, provided the effects described by Saxena (1983) and Chakraborty and Ganguly (1991) due to diffusion on the core composition do not exist. Therefore, the Fe/(Fe + Mn) ratios of triphylite s.t. and triplite s.l. can be theoretically correlated by the distribution curves of the Fe/(Fe + Mn) ratio (Fig. 6).

Using the observed partition coefficients, calculated with the Fe/(Fe + Mn) ratios of core compositions, no obvious relationships with the texture types can be stat- ed. However, there is a tendency that the partition coefficients correlate with the most likely T-conditions of formation. For example, the lowest values are detected for specimens from Valmy. At this locality magniotriplite has replaced a triphylite-sarcopside-graftonite intergrowth, which is typical for relatively high-temperature processes in the evolution of pegmatites. The highest values belong to specimens from Hagendorf- Slid, where the intergrowth textures indicate late-stage replacement products. On the other hand, the phosphates from Clementine II, with textures indicating different conditions of formation, have similar KD values. Another puzzling example is a fractionation sequence in Rabenstein, displaying different textures and successive- ly lower Fe/(Fe + Mn) ratios, but decreasing distribution coefficients.

Conclusions

The selected associations and assemblages of minerals belonging to the triphylite-lithiophitite and triplite- zwieselite-magniotriplite solid-solution series mainly originate from well-zoned, granitic rareelement pegmatites. The intergrowth textures of the phosphates document different genetic relationships between the minerals (Keller et al. 1994). In most of the investigated pegmatites triplite s.l. is either earlier than triphylite s.l. or cogenetic. The only exception with earlier triphylite is encountered at Valmy.

In this initial approach on main cation partitioning between the phosphates, the attempt to plot Rooze- boom diagrams was successful. Mole fractions such as XF~ = Fe/(Fe + Mn + Mg + Ca) fit relatively well on smooth curves (Figs. 1, 2), However, deviations from ideality can easily be seen in Figs. 1 and 3. It is evident from plots of lnKD versus the mole fractions XF~ and X~n that the deviations of mole fractions from ideal distribution curves is mainly dependent upon XMg or Xc~. The scattering which is observed for mole fractions Fe/(Fe + Mn) could mainly be caused by changing conditions of formation, provided that misinterpretation of equilibrium condition with respect to the kinetic aspects of the cooling history are excluded. In any case, surprisingly small differences between the partition coefficients of core values were detected, despite dramatically different texture types (see Keller et al. 1994).

This is the first report on compositional zoning of Fe-Mn phosphate minerals. The mole fractions which were determined either at the core or at the common edges of the two phosphates correlate with two distribution curves fitted by different KD values. Thus, the chemical composition is clearly dependent upon the conditions of formation. As already discussed, the role of temperature seems to be more effective than that of pressure. Field observations (Volborth 1954; Keller and Von Knorring 1989; Keller 1988, 1991) corroborate the results of chemical investigations: the first crystallization at high temperature produces Fe-rich solid solution members of both the triphylite and the triplite series, with more Mn at lower temperatures.

The origin of compositional zoning can be discussed only provisionally, with respect to the very preliminary nature of this investigation into main element partitioning of pegmatitic Fe-Mn phosphates on one hand, and considering results from well-studied minerals (e.g. garnets and feldspars, Loomis 1983) on the other. However, it seems to be likely that the rim compositions are mainly the result of element partitioning caused by diffusion, and that of the cores represent approximately the bulk chemical compositions reached during primary crystallization.

Under such conditions, and also if the Fe-Mn phosphates are not intergrown mutually or with other Fe- Mn bearing minerals, a quantification of main element fractionation between triplite s.l. and triphylite s.l. is particularly easy for the Fe/(Fe + Mn) ratio. Equations with sufficiently high correlation coefficients were determined. Consequently, the Fe/(Fe + Mn) ratio of triplite- zwieselite solid solution members can be used in the future for the interpretation of pegmatite evolution, in the same manner as this ratio for the triphylite series has been used. Additionally, pegmatites can be mutually compared by the successful hypothesis of Ginsburg (1960).

Finally, the intercrystalline element partitioning between triphylite s.l. and triplite s.l. could be an important tool in determining PTcondi t ions for the very interesting late stages of pegmatite evolution. However,

247

experimental and thermodynamic data are needed for this endeavour.

Acknowledgements The critical reading, helpful comments and language corrections by Prof. Dr. W. Schreyer (Bochum), Dr. A. Pring (Adelaide), Prof. Dr. D. London (Norman), Prof, Dr. P. Cer@ (Winnipeg), Prof. F. Cech (Praha), and Prof. J.P. Fortun6 (Toulouse) have considerably improved the presentation of the manuscript.

P.K. is also indebted to PD Dr. R. Wurster (Hohenheim) for assistance with electron microprobe analyses, to R. Mfiller (Stutt- gart) for providing specimens from Viitaniemi, and to the Geolog- ical Survey (Windhoek) and the Deutsche Forschungsgemein- schaft (Bonn) for financial support.

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