THE CONTRASTING RESPONSES OF MUSCOVITE AND PARAGONITE TO INCREASING PRESSURE: PETROLOGICAL IMPLICATIONS
707
The Canadian Mineralogist
Vol. 38, pp. 707-712 (2000)
THE CONTRASTING RESPONSES OF MUSCOVITE AND PARAGONITE TO INCREASING PRESSURE: PETROLOGICAL IMPLICATIONS
XXXXXXX X. XXXXXXXX§
Department of Geological Sciences, University of Maine, Orono, Maine 04469-5790, U.S.A.
XXXXXXXXX X. XXXXX
Dipartimento di Mineralogia e Petrologia, Università di Padova, X.xx Xxxxxxxxx 00, X-00000 Xxxxxx, Xxxxx
XXXXX XXXXXX AND XXXX XXXXXXXXX XXXXXXX
Dipartimento di Scienze della Terra, Xxxxxx Xxxxxxxxxx, X-00000 Xxxxxxx, Xxxxx
XXXXX X. XXXXXXX
Chemical and Analytical Sciences Division, Oak Ridge National Laboratory, P.O. Box 2008, Oak Ridge, Tennessee 37831-6110, U.S.A.
Abstract
The incorporation of Fe, Mg, and Si into muscovite in response to increase of pressure (P) has long been recognized. In the context of the appropriate mineral assemblages, the extent of this substitution has been calibrated to serve as a very useful geobarometer for high-P parageneses. In marked contrast, little or no Fm i.e., Σ(Mg + Fetotal), substitutes into paragonite regard- less of P. To date, Fm substitution into muscovite has been considered only in terms of ΔVr with little consideration of the crystallochemical aspects of this substitution. Moreover, the substitution seems to occur in response to simple exchange-reactions involving little or no dehydration. Such reactions typically have a negligible ΔVr. Drawing upon the implications of studies combining high-P refinement of the crystal structures and measurement of compressibility, we suggest that high P causes struc- tural changes in low-Fm muscovite that destabilize it. However, implementation of the Fm substitution facilitates structural adjustments, which reduce this instability. In contrast, paragonite is not only intrinsically less compressible than muscovite, but any substantial amount of Fm substitution would destabilize it.
Keywords: muscovite, paragonite, celadonite, mica, crystal structure, compressibility, high pressure.
Sommaire
On connait depuis longtemps l’incorporation du Fe, du Mg, et du Si dans la structure de la muscovite en réponse à une augmentation de la pression (P). Dans le contexte d’assemblages de minéraux appropriés, la portée de cet écart par rapport à la composition idéale sert de géobaromètre très utile pour étudier les paragenèses formées à pression élevée. Il est remarquable, par contre, que très peu du composant Fm, c'est-à-dire Σ(Mg + Fetotal), est incorporé dans la paragonite, quelle que soit la pression. Jusqu’à maintenant, on a rationalisé l’incorporation de Fm dans la muscovite uniquement en termes de ΔVr, sans trop se préoccuper des considérations cristallochimiques de cette substitution. De plus, la substitution semble intervenir en réponse à de simples réactions d’échange impliquant très peu de déshydratation ou bien sans déshydratation. Xx xxxxxx réactions typiquement on une valeur très faible de ΔVr, ou bien elle est nulle. Nous tirons profit des études récentes des structures à pression élevée et des mesures de leur compressibilité, et nous proposons qu’à pression élevée, des changements structuraux mènent à la déstabilisation de muscovite à faible teneur en Fm. L’incorporation du composant Fm facilite les ajustements structuraux, et réduit ainsi cette instabilité. En revanche, la paragonite serait non seulement intrinsèquement moins compressible que la muscovite, mais toute ajout du composant Fm mènerait à sa déstabilisation.
(Traduit par la Rédaction)
Mots-clés: muscovite, paragonite, céladonite, mica, structure cristalline, compressibilité, pression élevée.
§ E-mail address: xxxxxxxx@xxxxx.xxxxx.xxx
Introduction
Regardless of a rock’s bulk composition or mineral assemblage, an increase in metamorphic pressure leads to an increase in the Fe, Mg, and Si contents of xxxxx- xxxx (Ms), i.e., in the solid solution toward celadonite (Xxxxx 1963); at the same time, Na/(Na + K) and d(002) decrease. In contrast, the composition of paragonite (Pg) is largely unaffected by increasing P. In this paper, we discuss crystallochemical and compressibility data for muscovite and paragonite that shed light on these ob- servations. Some of the crystallochemical data and many of the petrological details are presented in Sassi (1972), Xxxxxxxx & Xxxxx (1976, 1998) and Xxxxxxxx et al. (1989, 1992, 1994a, b). Moreover, Xxxxx et al. (1994) and Xxxxxxxx & Xxxxx (1976, 1998) have reviewed how the occurrences of muscovite and paragonite (including their polytypes) and muscovite–paragonite immiscibil- ity are strongly correlated with P–T conditions of crys- tallization. For mica compressibility, we will utilize (a) the results of Xxxxx & Finger (1978), which show that layer silicates (phlogopite and chlorite) are highly com- pressible, especially along the c dimension, and (b) the recently published data of Xxxxxx & Xxxxxxx (1995, 1997), who combined compressibility data and crystal- structure refinement results for three carefully chosen, naturally occurring muscovite and paragonite speci- mens, all with compositions that are essentially in the Na–K pseudobinary system muscovite–paragonite and having Na/(Na + K) values of 7.14, 38.14, and 88.0 mole
%. The parageneses of these micas are summarized in
Xxxxxx & Xxxxxxx (1995, 1997). The sodian xxxxx- xxxx with a Na/(Na + K) value of 38.14 mole % essen- tially lies at the Na-limit for muscovite on the muscovite–paragonite solvus proposed by Xxxxxxxx et al. (1994a). The results of Xxxxxx & Xxxxxxx (1995, 1997) are summarized in Figure 1 and in the next paragraph.
Crystallochemical Aspects
Two key results of the compressibility and high- pressure crystal-structure work simultaneously per- formed by Xxxxxx & Xxxxxxx (1995, 1997) can be summarized as follows: (1) the observed compressibi- lities and crystal structures at high pressure are system- atically related to the Na/(Na + K) values of the three micas studied, and (2) the changes in structure occur- ring as a function of pressure affect the stability of the micas, especially muscovite. Also affected are the crystallochemical behavior of muscovite and paragonite in various petrological and structural regimes. In par- ticular, important effects on the intrinsic stability and crystallochemical behavior of muscovite occur because of the following changes observed in the isochemical high-P studies (see Appendix Table 1 for details).
(1) Increased α rotation (defined as rotation of the
tetrahedra within the plane of the sheet of tetrahedra: Xxxxxx 1984), plus large decreases in interlayer thick-
FIG 1. Compressibility data from Xxxxxx & Xxxxxxx (1995, 1997). (A) Variation of K, the bulk modulus, with the Na/ (Na + K) ratio of white mica in the Ms – Pg pseudobinary.
(B) Comparison of unit-cell parameters normalized to room conditions; Ms: ▼ and dashed lines, Na-rich Ms: ▲ and solid lines, Pg: ■ and dotted line. Solid symbols are chosen for V/Vo, c/co, and b/bo, and open symbols for a/ao. It can be seen that the compressibility of the a and b cell param- eters is quite similar for Ms, Na-rich Ms, and Pg, but xxxx- xxxx different for V/Vo and c/co, decreasing progressively from Ms to Pg.
xxxx lead to: (a) significantly shortened K–O bond lengths, (b) closer proximity of highly charged cations in tetrahedral sites, and (c) greater repulsion of the basal layers of oxygen across the interlayer site (especially in Na-rich muscovite).
(2) The sheets of octahedra (VI) shrink, and espe- cially they thin to near and beyond the minimum allow- able thickness, 2.04 Å according to Xxxxxx (1984). This thinning causes strong repulsion between oxygen atoms
on the top and bottom of the sheet (see the note in Appendix Table 1 concerning the thickness of the sheet of octahedra for the Na-rich muscovite at 27 kbar). As discussed below, in rocks, compositional changes of muscovite occur which counteract these destabilizing effects.
Discussion
Because the new compressibility and crystal- structure data discussed herein were obtained at room temperature, it is important to consider the potential effects of elevated temperatures (T). It is difficult to assess rigorously the effects of T on the compressibility of muscovite and paragonite, but by combining their low-T compressibility data with the low-P thermal expansion data of Catti et al. (1989), Xxxxxx & Xxxxxxx (1995) concluded that, as expected, T and P generally have opposite effects on muscovite. Unfortunately, com- bining two separate datasets precluded rigorous quanti- fication of the opposing effects. For muscovite recrystallizing in the strongly preferred orientation of slates (Xxxxxxxx et al., in prep.), T is quite low and thus can probably be completely ignored. For Fm-rich mus- covite [Fm: Σ(Mg + Fetotal)] in high-P blueschist– eclogite terranes, the effects of P are probably dominant and more than counter any effects of T. Exceptions to this assumption might include muscovite that crystal- lized in deep-seated rocks at T = 600°C (e.g., xxxxx- xxxx that formed at both high P and T, as in the case of some rocks of the Western Alps (see data tabulation used in Xxxxxxxx et al. 1994b).
The most common effects of P discussed in the literature (e.g., Xxxxxxxx & Xxxxx 1976, 1998) involve the relatively low-Al limiting assemblage muscovite + K-feldspar + plagioclase with a coexisting Fe–Mg phase, e.g., biotite. Indeed, the effect of P on the incor- poration of Mg + Fe2+ into muscovite (or conversely, the incorporation of Si) in such assemblages is the basis for the well-known “phengite geobarometer” (Xxxxxxxx 1981, 1993, Xxxxxxxx & Schreyer 1986, 1987, Velde
1965, 1967, 1968). Less attention has been given to more Al-rich assemblages, including those with an Al-silicate present. Nonetheless, as demonstrated in the compilations used by Xxxxxxxx et al. (1994a, b), the effects of P on muscovite and paragonite listed in Appendix Table 2 also occur in such assemblages. That increasing P causes an increase of the Fm content in muscovite over a wide range of aluminous bulk compo- sitions has also been confirmed by experiments per- formed at extremely high P (up to 100 kbar) by Xxxxxxx & Xxxxxxxx (1996) for a high-Al bulk composition, and by Xxxxxxx (1996) for a low-Al bulk composition (see Xxxxxxxx & Xxxxx 1998, p. 841). In summary, as discussed in detail by Xxxxxxxx & Xxxxx (1976, 1998), the bulk Al-content in mineral assemblages can xxxx- xxxx affect the extent to which Fm substitutes into mus-
covite; however, for a given Al bulk-rock content, Fm incorporation into muscovite is P-controlled.
Previous attempts (e.g., Xxxxxxxx et al. 1992) to understand atomistically the effect of high P on the com- position of muscovite centered on P-induced changes in the interlayer (XII) site. Now, in the context of the structural changes listed in Appendix Table 1, we sug- gest that the isochemical destabilizing effects described above act in concert to produce the chemical and struc- tural changes reflected in the observations considered herein (Appendix Table 2). From this broader starting point, our reasoning parallels that of the above- mentioned previous explanations, i.e., in rocks, xxxxx- xxxx undergoes a twofold compositional adjustment to mitigate the destabilizing effects caused by increasing P: (a) substitution of Fm for VIAl mitigates the over– thinning of the VI sheet and also lessens the increase in α rotation, which would otherwise lead to excessively short K–O bonds; (b) an increase in K/(K + Na) helps prop apart the 2:1 sheets, thereby keeping the basal atoms of oxygen from getting too close. These changes lead to larger cations in VI and XII sites and smaller cations in IV sites (Si replacing Al). The latter substitu- tion also helps to counter the significant increase of α that occurs when muscovite is highly compressed.
As determined at the Earth’s surface, Fm-rich muscovite has a markedly smaller α angle (6–8°) than
muscovite. This produces a larger XII site which, as dis- cussed by Xxxxxx (1984), allows the alkali ions to sink deeper into the pseudohexagonal rings of basal oxygen atoms. Hence, compared to muscovite, there is a marked shortening of the c unit-cell dimension. In contrast, the a and b cell dimensions are larger owing to the effects of Fm increase in the VI sites. As discussed by Xxxxxxxx et al. (1992), the net result is that at the Earth’s surface, Fm-rich muscovite has a somewhat larger unit-cell volume than muscovite.
The combined compositional and structural changes described above for muscovite subjected to high P lead to a much less distorted crystal structure, especially compared to that produced in the isochemical compress- ibility experiments described above. In this context, Xxxxxx & Xxxxxxx (1995, p. 176) discussed studies sug- gesting that “the stability of micas largely depends on the degree of distortion of various layers”. Assuming that the results of Xxxxx & Finger (1978) on a trioctahedral mica can be extrapolated to dioctahedral micas, they concluded that “in high-pressure environ- ments, micas with small α rotations and octahedral layers having high bulk moduli (i.e., less compressible) are more stable”, as would be produced by the substitu- tions noted above.
Finally, it is notable that increases in Fm, K, and Si
in muscovite due to increasing P cause its cell volume (as determined at conditions of the Earth’s surface) to increase, which is seemingly at odds with an increase in
P. However, this a well-documented observation
(Xxxxxxxx et al. 1992). Presumably, muscovite is involved either in dehydration or exchange reactions with chlorite, such that ΔVr is either slightly negative or, as typical for exchange reactions, near zero.
Conclusions
Analysis of combined compressibility measurements and high-P crystal-structure refinements shows that high P destabilizes low-Fm muscovite. This destabilization is mainly related to bond lengths, bond angles, and di-
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Received February 9, 2000, revised manuscript accepted May 11, 2000.