The main minerals, which reflect the most essential associations with preiswerkite are phlogopite, aspidolite, pargasite, sadanagaite, corundum, spinel, anorthite, dolomite and calcite. As it was noted by several authors [
21,
26], preiswerkite formation is favored by the presence of an aqueous fluid, Si-poor and an Al-Na-rich system. For this reason, preiswerkite-bearing marble petrogenesis is considered further in terms of a metasomatic process, even if it was realized in a local scale.
The common feature of all metasomatic parageneses is a change from less hydrous to more hydrous minerals (amphiboles to micas to chlorite). The reason for the production of such a time sequence of mineral distribution could only be a systematic change in composition and properties of the mineral-forming fluids as they permeate the protolith and react with it.
In the model system K-Na-Mg-Al-Si-CO
2-H
2O, the most common original rocks (evaporites, clays, etc.) and the secondary minerals developed after them can be described satisfactorily. However, the metamorphism of siliceous dolomites requires an analysis of phase equilibria in the presence of a mixed H
2O-CO
2 fluid [
40]. Let us consider the connection of chemical potentials of СО
2 and Н
2О in a binary fluid (СО
2 + Н
2О) at
and
. Under these conditions, the molar fraction of CO
2 in the fluid
is defined by the expression:
where from
The values of the chemical potential of H
2O in the fluid (
) are determined, as is known, by the expression:
where
is the standard chemical potential of water at a given temperature and pressure;
is fugacity, fugacity coefficient and partial pressure of H
2O in the fluid, respectively;
is the molar fraction of CO
2 in the fluid.
To ascertain the main parameters of the mineral-forming environment, which govern the stability of minerals in the marbles, let us consider the topology of the diagrams plotted in coordinates of the chemical potentials [
41]. The analysis of the model system is based on the fundamental principles of Korzhinsky [
41,
42]: local equilibria exists among minerals, all components have differential mobility and the upcoming fluids react with the protolith. Such an approach has been used successfully to model the chemical transformation of metasomatic rocks [
42].
Depending on the mineral and chemical composition of the protolith, two specific versions of preiswerkite-bearing mineral associations can be formed: (1) with minerals of mica group; (2) with minerals of amphibole group.
5.2.1. Mineral Equilibria with (Micas)
In plotting the qualitative
and
diagrams of the Luc Yen mica transformations, it was assumed that the mineral composition of the preiswerkite-bearing marbles is governed by four minerals: phlogopite, aspidolite, preiswerkite and corundum. The chemical compositions of the micas are determined by the proportions of Mg, Al and Si. However, in all micas the ratio Mg/Si is equal to one because of substitution Mg[Si]←Al[Al] (see
Figure 1). Thus, we obtain two virtually inert components (Mg and Al) (
Figure 5).
Thus, the number of minerals (phases) in the model system is taken as four and the number of inert components as two, leaving three completely mobile components (K, Na and H
2O). We take the temperature and pressure as constant external equilibrium factors. The equations of the chemical reactions for the lines of monovariant equilibrium were calculated for the theoretical (normative) compositions of the minerals (
Table 3), the chemistry of which is very close to that of their natural analogs. The equations of the chemical reactions are given in
Table 4.
Table 3.
Theoretical compositions of minerals (solid-solution components) adopted for calculating chemical reaction equations.
Table 3.
Theoretical compositions of minerals (solid-solution components) adopted for calculating chemical reaction equations.
Mineral | Symbol | Crystallochemical Formula |
---|
Phlogopite | Phl | KMg3(AlSi3O10)(OH)2 |
Aspidolite | Asp | NaMg3(AlSi3O10)(OH)2 |
Preiswerkite | Pwk | Na(Mg2Al)(Al2Si2O10)(OH)2 |
Corundum | Crn | Al2O3 |
Table 4.
Equations of chemical reactions occurring on lines of monovariant equilibrium.
Table 4.
Equations of chemical reactions occurring on lines of monovariant equilibrium.
N | Lines * | Chemical Reactions |
---|
1 | [Pwk] | Phl + Na+ = Asp + K+ |
2 | [Crd] | Phl + Na+ = Asp + K+ |
4 | [Phl] | 4Asp + 7Crn + 2Na+ +3H2O = 6Prw + 2H+ |
3 | [Asp] | 4Phl + 7Crn + 6Na+ +3H2O = 6Prw + 2H+ + 4K+ |
Mineral equilibria in such a system can be depicted graphically in three-dimensional space. Therefore it is more feasible first to analyze in detail the behavior of the lines of monovariant equilibrium of a reaction on the diagram in coordinates of two completely mobile components taking the temperature, pressure and chemical potential of H2O as constant external equilibrium factors and then on the basis of this analysis, to consider the diagram in coordinates.
In the qualitative
diagram, we consider the transformations of phlogopite into aspidolite (singular Reactions 1,2,
Figure 5), phlogopite+corundum into preiswerkite (Reaction 4) and aspidolite+corundum into preiswerkite (Reaction 3). It should be noted that with low values of
preiswerkite breaks down into the association aspidolite + corundum according to Reaction 3,
Figure 5. As the chemical potential of potassium increases, preiswerkite becomes unstable and is replaced by phlogopite + corundum (Reaction 4), and as
increases, the field of preiswerkite-bearing parageneses is somewhat broadening (Reactions 2, 4,
Figure 5).
Figure 5.
Qualitative
diagram of mica parageneses in the system K-Na-Mg-Al-Si-H
2O. I, II, III, IV—stable divariant fields; the numbers in circles indicate the reaction numbers in the
Table 4.
Figure 5.
Qualitative
diagram of mica parageneses in the system K-Na-Mg-Al-Si-H
2O. I, II, III, IV—stable divariant fields; the numbers in circles indicate the reaction numbers in the
Table 4.
In plotting the qualitative
, we take the temperature, pressure and chemical potential of potassium as constant external equilibrium factors. Mineral equilibria in such a system can be depicted graphically in two-dimensional space (
Figure 6).
The formation of preiswerkite in this system is controlled by two reactions (3 and 4,
Figure 6), by which it is completely destroyed when
decreases. Under these conditions, the stability of aspidolite (Reactions 1 and 2,
Figure 6) is determined only by the chemical potential of sodium in the system and is independent of the
of the mineral-forming medium. It should be noted that preiswerkite is stable in association with phlogopite (field III) at low activities of Na
+, and with aspidolite (field IV) at high activities of Na
+.
5.2.2. Mineral Equilibria with Amphibole (Sadanagaite)
In plotting the qualitative diagram () of Na-Mg-Al-Si-CO2-H2O system for the transformations of amphibole into preiswerkite, it assumed that sadanagaite governs the main features of the amphibole composition in the Si-poor and Al-rich pods in marbles of the Luc Yen deposit. Sadanagaite was found here in the associations with preiswerkite, anorthite, dolomite, spinel and corundum. The proportions of the Mg, Al and Si oxides determine the mineral composition of the rocks. Titanium oxide, which is present in the rocks in the form of ilmenite, titanite and rutile, can be considered a separate component. Lumping FeO and MgO together, we obtain three virtually inert components: (Mg, Fe)O, Al2O3, and SiO2. The main components producing the transformation of the original rocks are sodium and H2O.
Thus, the number of minerals (phases) in the model system is taken as six and the number of inert components as three, leaving two completely mobile components. We take the temperature and pressure as constant external equilibrium factors. Mineral equilibria in such a system can be depicted graphically in two-dimensional space. Therefore, we analyze in detail the behavior of the lines of monovariant equilibrium of a reaction on the diagram in coordinates of two completely mobile components ().
The equations of the chemical reactions for the lines of monovariant equilibrium were calculated for the theoretical (normative) compositions of the minerals (
Table 5), the chemistry of which is close to that of their natural analogs. The equations of the chemical reactions are given in
Table 6.
Thus, let us consider the diagram of the multisystem in
coordinates, because water in the mineral-forming medium essentially affects the stability of hydroxyl-bearing minerals (sadanagaite and preiswerkite) and also the equilibrium corundum + spinel (Reactions 1–3,
Table 6). It should be noted that Reactions 8–15 (
Table 6) are omitted because nonvariant points (Sdg), (Crn) and (Spl) are metastable and dolomite is in excess.
Figure 7 gives this diagram in its entirety.
Monovariant singular equilibria of the Reactions 1–3 (
Table 6) are controlled only by the chemical potential of СО
2 in the fluid and do not depend on the activity of sodium in the medium. Thus, as it was shown above, in case of increasing
(
and
are decreasing), an association of corundum+dolomite is replaced by spinel according to Reactions 1–3. This monovariant line divides the diagram into two parts (
Figure 7). In the left part (fields I-III), only corundum is stable, and in the right part (fields IV-VI), spinel is stable with an excess of dolomite; with an increase of aluminum content, corundum + spinel association is possible
It follows from the diagram that when the values of
are increasing, in a solution with low activity of H
2O (fields I-III) corundum (in excess of dolomite) associated with anorthite (field I), then with sadanagaite (field II) and at maximum sodium activities (field III) preiswerkite+corundum+dolomite association is formed (Reaction 4,
Table 6).
At higher values of (fields IV-VI) with an excess in dolomite, spinel appears instead of corundum and with increasing of , the association spinel+anorthite (field I) is replaced by an association spinel+sadanagaite (field II) and then by an association preiswerkite+corundum+dolomite (field III).
The stability of the main minerals in marbles (in the presence of dolomite ± calcite in all parageneses) as a function of the chemical potentials of sodium, potassium and Н2О in the mineral-forming medium is clearly illustrated by two-dimensional diagrams.
Unfortunately, the obtained diagrams do not allow quantitative analysis of the specific metamorphic conditions that facilitate Na-trioctahedral micas transformations due to the lack of thermodynamic data for these minerals.