Fe-Mg Order-Disorder in Tremolite–Actinolite–Ferro-Actinolite At
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American Mineralogist, Volume 83, pages 458±475, 1998 Fe-Mg order-disorder in tremolite±actinolite±ferro-actinolite at ambient and high temperature BERNARD W. EVANS1 AND HEXIONG YANG2 1Department of Geological Sciences, Box 351310, University of Washington, Seattle, Washington 98195-1310, U.S.A. 2Geophysical Laboratory, 5251 Broad Branch Road, NW, Washington, D.C. 20015-1305, U.S.A. ABSTRACT The crystal structures and M-site populations of a series of unheated and heat-treated, near-binary members of the actinolite series were determined from single-crystal X-ray diffraction data. Small, systematic differences in crystal structure (mean and individual bond lengths, octahedral distortions, and Mg-Fe site preferences) between the actinolite and cummingtonite series are documented and rationalized in terms of the bond-valence model. Mg-Fe21 site preferences among the M1, M2, and M3 sites in the actinolite series are small. Between M1 and M3, site preference (Fe21 slightly in favor of M1) is indepen- dent of temperature. Mg is favored in M2 over M1 in unheated samples (less strongly than in cummingtonite-grunerite), but Fe21 and Mg are virtually disordered in samples re- equilibrated at 700 8C. Site preference between M2 and M3 and possibly also between M1 and M2 appears to undergo a reversal as a result of heat treatment, but consideration of all sources of error renders this conclusion a tentative one, although the observation could be explained in terms of a bond valence de®ciency. The ratio of Fe21/Mg atoms on M4 cannot be accurately determined, but re®nements indicate that, like cummingtonite-gru- nerite, it is larger than on all other M sites. Expansion of the cell dimensions with an increase in Fe/Mg ratio follows a pattern similar to that of thermal expansion, and a and b are in¯uenced by cummingtonite-grunerite solid solution. The data are consistent with a cell volume of 942.8 AÊ 3 (or molar volume of 2.8388 J/bar) for end-member ferro- actinolite, a ®rst approximation that assumes a reciprocal, quadrilateral DV of zero. INTRODUCTION mation for the series, which will assist calculation of the Successful thermodynamic modeling of the solution thermodynamic solution properties of the entire amphi- properties of multisite minerals such as the amphiboles bole quadrilateral. The only possibility for order-disorder 2 requires a description of the temperature, pressure, and in strictly quadrilateral amphiboles, of course, is Fe 1 and compositional dependence of atom ordering on sites. This Mg on the M sites. information is needed to make an informed decision on The actinolite series, ideally MCa2(Mg,Fe)5Si8O22(OH)2, the degree of complexity of the solution model that is has a crystal structure characterized by double silicate ultimately adopted. Ordering of Mg and Fe21 on the M chains running parallel to c. The chains are composed of cation sites of amphiboles is known to proceed down to two crystallographically distinct tetrahedra, T1 and T2, temperatures as low as ;300 8C, and amphiboles in nu- and are linked together by strips of cations occupying merous terrestrial parageneses are participants in equilib- three nonequivalent octahedra, M1, M2, and M3, and an ria corresponding to temperatures at which site fraction- eightfold-coordinated polyhedron, M4. Published X-ray ation is pronounced. Con®gurational entropies may be structure re®nements of members of the actinolite series calculated directly from such data, and the overall ener- include very few that are signi®cantly Fe-bearing and getics of solution (enthalpy and Gibbs energy) show a none whose state of order has been re-equilibrated at high temperature and composition dependency that re¯ects the temperature. Re®nement of a natural manganoan ferro- ordering states (e.g., Ghiorso et al. 1995). actinolite (Mitchell et al. 1970, 1971) showed nearly ran- An earlier study (Hirschmann et al. 1994) presented dom mixing of Fe21 and Mg on the octahedral sites M1, the results of a single-crystal X-ray diffraction study M2, and M3: Fe21/(Fe211Mg) 5 0.61(M1), 0.57(M2), (XRD) of heat-treated and quenched natural amphiboles and 0.58(M3); in this re®nement all Mn, and no Fe, was on the cummingtonite±grunerite binary join. This com- assigned to M4, and Fe31 was assigned to M2. A low-Fe panion study of the upper join in the Ca-Mg-Fe amphi- actinolite was re®ned by Litvin et al. (1972) and Litvin bole quadrilateral, tremolite±actinolite±ferro-actinolite (1973). If in this sample Fe31 is assigned to M2 (Haw- (the actinolite series, for short), was conducted to com- thorne 1983, p. 382) rather than equally to M1, M2, and plete a database of ordering and crystallographic infor- M3, then there is a preference of Mg for M2 and Fe21 0003±004X/98/0506±0458$05.00 458 EVANS AND YANG: Fe-Mg ORDER IN ACTINOLITES 459 TABLE 1. Sample characteristics Fe/(Fe 1 Mg) Specimen no. Rock type (%) Location References NMNH 93728 act-talc rock 11±12 Greiner, Zillertal, Austria (1) 67-3172 cpx hornfels 18±19 Carr Fork Mine, Bingham, Utah (2) 67-3701 cpx hornfels 23 Carr Fork Mine, Bingham, Utah (2) NMNH 156831 uralite from cpx 27±38 Green Monster Mine, Prince of Wales Is., Alaska (3) 11B iron formation 53±54 Luce Lake, Labrador, Canada (4), (5), (6) 12BA iron formation 53 Bloom Lake, Quebec, Canada (5), (6), (7) AMNH 44973 quartzite 57±59 Cumberland, Rhode Island (6), (8), (9) ID4-4C iron formation 62 Isua Belt, East Greenland (10) NMNH 168215 alteration of hbl 81±85 Goose Creek Quarry, Leesburg, Virginia Notes: (1) Skogby and Annersten (1985; cf. sample 679), (2) Atkinson and Einaudi (1978), (3) Kennedy (1953), (4) Klein (1966), (5) Wilkins (1970), (6) Burns and Greaves (1971), (7) Mueller (1960), (8) Mustard (1992), (9) Mitchell et al. (1970, 1971), (10) Dymek and Klein (1988). for M1 and M3; however, the cummingtonite component Fe21/(Fe211Mg) values from 10 to 85%. Natural rather (21%) reported in this actinolite is suspiciously large. than synthetic actinolites were chosen because their rel- These actinolite site-occupancies are similar to those in ative freedom from crystal defects (e.g., Ahn et al. 1991; the cummingtonite series, except that the preference of Maresch et al. 1994), their larger crystal size, and their Mg for M2 appears to be less strong in the actinolites. ease of characterization offset slight departures from bi- Measurements using infrared and MoÈssbauer absorption nary compositions. Given the decreased resolution of spectroscopy, notwithstanding ongoing reassessments MoÈssbauer spectra of actinolite with increasing Fe con- (Wilkins 1970; Burns and Greaves 1971; Goldman and tent (Goldman 1979; Skogby and Annersten 1985) and Rossman 1977; Goldman 1979), have suggested stronger the site resolution of XRD, we chose the latter as the site preferences for Fe and Mg atoms between the M1 1 preferred technique to determine the M-site occupancies M3 and M2 sites than the X-ray re®nements. In all these of actinolite as a function of temperature. We discovered studies, no Mg was placed on M4. that the M1, M2, and M3 site preferences in the actinolite Synthetic and natural tremolites and actinolites (up to series as a whole are not pronounced, and therefore con- 13% ferro-actinolite end-member) were studied by MoÈss- centrated primarily on a comparison between unheated bauer spectroscopy at 77 K following heat treatment at samples and samples re-equilibrated hydrothermally and 600 to 800 8C by Skogby and Annersten (1985), Skogby under reducing conditions at high temperature, mostly (1987), and Skogby and Ferrow (1989). Fe21 was found 700 8C. to strongly favor M4 over M2 and slightly favor M1 1 M3 over M2. A decrease in equilibration temperature was EXPERIMENTAL PROCEDURES 21 accompanied by the transfer of some Fe from M2 to Specimen preparation M4, while the contents of M1 1 M3 were unchanged. Many more X-ray structure re®nements have been Electron microprobe analysis (see below) was used to done on aluminous calcic amphiboles or hornblende sen- select samples of naturally occurring members of the ac- su lato. Relative to Mg, the Fe21 preference on the M- tinolite series (Table 1) from an extensive collection as- sites is M3.M1.M2 (Robinson et al. 1973; Litvin 1973; sembled from many sources (see Acknowledgments) by Hawthorne and Grundy 1973, 1977, 1978; Bocchio et al. rejecting those with more than minimal amounts of non- 1978; Hawthorne et al. 1980; Ungaretti et al. 1983; Phil- quadrilateral components. To obtain a wide spread in bulk lips et al. 1988; Makino and Tomita 1989; Oberti et al. XFe contents, this meant including some samples with 1993a, 1995a, 1995b). The preference of Fe21 for M3 variable Fe/Mg ratios from grain to grain (e.g., sample over M1 is consistent although not strong, differing sig- NMNH 156831). Fe-rich actinolites with very small Al, ni®cantly from quadrilateral amphiboles. The reported ra- Na, and Fe31 contents are quite rare. The best source tio of Fe21/Mg of M2 is variable; its accuracy suffers rocks were found to be metamorphosed banded iron for- from the dif®culty of assessing the content of other cat- mations and skarns where calcic pyroxene had been re- ions occupying that site, such as Al, Fe31, Ti, and Cr. It placed by uralitic actinolite. Almost all samples show mi- has been claimed that high-temperature (volcanic) horn- nor correlated variations in Na, Al, Si, Mg, and Fe, blendes have Fe21 and Mg randomly distributed on M1, representing incipient coupled substitutions of the types M2, and M3, in contrast to low-temperature (metamor- that lead to hornblende. The ®nal list (Table 1) comprises phic and skarn) samples (Makino and Tomita 1989).