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Practical Aspects of Mineral Thermobarometry

Micas: biotite and muscovite

Structure and sites

The mica structure consists of sheets of tetrahedra arranged in pairs, enclosing, sandwich-fashion, a sheet of edge-sharing octahedra. The apices of the tetrahedra connect to the octahedra of the central layer. Between each "sandwich" there are interlayer sites which can contain large cations.

The coordination of the octahedra is completed by OH anions. The general formula of mica group minerals is

X Y2-3 Z4 O10(OH)2

where X represents the interlayer site, Y the octahedral sites, and Z the tetrahedral sites. The octahedral sheet can be made up in two ways: either dominantly of divalent cations such as Mg and Fe, in which case all three sites are filled (trioctahedral mica), or else dominantly trivalent cations such as Al, in which case one of the three sites is left vacant (dioctahedral mica). If the tetrahedra are occupied solely by Si, the sandwich is charge-balanced and there is no need for interlayer cations - the resulting minerals are talc (trioctahedral) and pyrophyllite (dioctahedral). In true micas Al substitutes for Si in the tetrahedra, and charge balance is maintained by K, Na, or more rarely Ca, in the interlayer site. The important rock-forming micas are

End members and site allocations

This treatment will concentrate on the common micas biotite and muscovite. The most important substitutions and site preferences are as follows

Sometimes it is necessary to distinguish between two types of octahedral site, M1 and M2. There are two M2 sites and one M1 site per formula unit; in dioctahedral micas the M1 sites are vacant.

Biotite

The end members for which we have thermodynamic data include:

You will see from the above that, starting from phlogopite, some of the important substitutions are FeMg-1 in octahedral sites (annite), the tschermak substitution Al2Mg-1Si-1 (eastonite), and NaK-1 in the interlayer site.

In addition, Ti substitutes in octahedral sites by a complex coupled substitution whose nature is not entirely clear. Although some authors have suggested that the introduction of Ti introduces octahedral site vacancies, evidence is accumulating that an important process is the loss of protons from hydroxyl groups, to give a Ti-biotite end member KMg2Ti[AlSi3]O12.

Composition variation with grade

The major substitutions in biotite (Fe/Mg ratio, Al content) tend to reflect rock composition rather than metamorphic grade. The Ti content increases with grade if there is a saturating Ti-mineral in the rock, such as rutile, ilmenite, or sphene. Ti strongly affects the transmitted light colour (changing from green to brown to orange with increasing Ti), and biotite colour has traditionally been used as an approximate monitor of metamorphic grade.

Muscovite

The important end members include:

There are wide miscibility gaps between the K, Na and Ca micas. Muscovite can contain up to about 25% paragonite, but is generally very low in Ca. Muscovite also contains minor Ti. Fe3+ may substitute for octahedral Al, but there is no reliable recalculation scheme which could be used to estimate it.

Some muscovites show an excess of Mg and Fe above that which is attributable to celadonite or ferri-muscovite, implying that some solid solution towards trioctahedral mica is possible.

Phengite

The tschermak substitution (working in reverse as MgSiAl-2) is an important one in muscovite. It leads towards the celadonite end members, forming the range of compositions known as phengites. Phengites with high celadonite content are favoured by high P and low T, and can be an important indicator of pressure. Celadonite content in phengite is conveniently monitored by the Si content, which is 3 in pure muscovite but can be up to 3.9 in the most Si-rich phengites known. The equilibrium Si contents in some critical assemblages have been measured experimentally.

The crystallography and crystal chemistry of phengites has been investigated by Massonne and Schreyer (1986), and their observations provide an interesting example of the interrelationships between composition, crystal structure, and physicochemical properties.

The individual tetrahedral sheets of the mica structure ideally have hexagonal symmetry, but in practice, because of the size difference between tetrahedra and octahedra, the fit between a tetrahedral and an octahedral sheet is accompanied by distortions. The oxygen spacing of the octahedral layer is generally somewhat smaller, and this is accommodated in the tetrahedral layer by rotation of the tetrahedra in the plane of the sheet, and by flexing of the tetrahedra out of the plane of the sheet. In pure muscovite the tetrahedral rotation angle is about 9°.

3T phengites

The substitution MgSiAl-2 changes the relative sizes of the octahedral and tetrahedral sites, enlarging the octahedra and reducing the tetrahedra. This improves the match between the dimensions of the tetrahedral and octahedral sheets, and the tetrahedral rotation angle thus decreases with increasing celadonite content, reaching zero at about 3.5 atoms Si p.f.u.. Phengites of this composition seem to favour the three-layer stacking sequence with trigonal symmetry (the 3T polytype), rather than the more usual 2-layer monoclinic form. They can be distinguished in thin section because they are uniaxial.

Massonne and Schreyer could distinguish a change in the trend of physical properties with Si content at about 3.5 Si, implying a change in the substitution mechanism beyond this point. This has implications for activity-composition models in phengites: we should be cautious about applying the same model across the full compositional range.

Na, and the paragonite content of phengites

Another area where caution is even more necessary is the behaviour of Na. Phengites commonly show a strong inverse correlation between Na and Si. Again, this probably relates to the distortion of the tetrahedral layers. The Na cation is small, and the substitution is probably favoured by the larger tetrahedral rotation in pure muscovite. Paragonite itself (the phase) shows negligible MgSiAl-2 substitution, even at high pressure.

Now, paragonite turns out to be important for thermobarometry in blueschists and eclogites, because of its well-constrained high-pressure breakdown reactions such as

Paragonite = Jadeite + Kyanite + water

The Na-K substitution in white mica is strongly non-ideal, with a solvus and mixing parameters quite well determined at low pressures, in micas with negligible celadonite content. However, use of this model will overestimate the limit of Na substitution in high-P phengite, so that the activity of paragonite, even in a sodium-saturated phengite, will probably be seriously underestimated. So, unfortunately, it's not really possible to use the Na content of phengites for geobarometry.

Still to come:

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This page last modified 12 October 2004