Ch 24 Mineral Assemblages

The Phase Rule in Metamorphic Systems
• Phase rule, as applied to systems at equilibrium: F=C-f+2 the phase rule (6-1) f is the number of phases in the system C is the number of components: the minimum

The Phase Rule in Metamorphic Systems
Cool to the temperature of the reaction curve, periclase reacts with water to form brucite: MgO + H2O Mg(OH)2
The rock probably represents an equilibrium mineral assemblage from within a metamorphic zone

The Phase Rule in Metamorphic Systems
b)

f<C
Common with mineral systems that exhibit solid solution
Chapter 24. Stable Mineral Assemblages in Metamorphic Rocks
• Equilibrium Mineral Assemblages
• At equilibrium, the mineralogy (and the composition of each mineral) is determined by T, P, and X • “Mineral paragenesis” refers to such an equilibrium mineral assemblage • Relict minerals or later alteration products are thereby excluded from consideration unless specifically stated
The Phase Rule in Metamorphic Systems
2) Equilibrium has not been attained

The phase rule applies only to systems at equilibrium, and there could be any number of minerals coexisting if equilibrium is not attained
Liquid
Plagioclase
plus
Liquid
Plagioclase
The Phase Rule in Metamorphic Systems
c)

f>C
A more interesting situation, and at least one of three situations must be responsible: 1) F < 2 The sample is collected from a location right on a univariant reaction curve (isograd) or invariant point
The Phase Rule in Metamorphic Systems
• A typical sample from a metamorphic terrane Likely select a sample from within a zone, and not from right on an isograd • Alternatively, pick a random point anywhere on a phase diagram Likely point will be within a divariant field and not on a univariant curve or invariant point • The most common situation is divariant (F = 2), meaning that P and T are independently variable without affecting the mineral assemblage
The Phase Rule in Metamorphic Systems
3) We didn’t choose the # of components correctly
b) Components that substitute for other components



Adding a component such as NaAlSi3O8 (albite) to the 1-C anorthite system would dissolve in the anorthite structure, resulting in a single solid-solution mineral (plagioclase) below the solidus Fe and Mn commonly substitute for Mg Al may substitute for Si Na may substitute for K
The System MgO-H2O
Figure 24-1. P-T phase diagram illustrating the reaction brucite = periclase + water, calculated using the program TWQ of Berman (1988, 1990, 1991). From Winter (2001). An Introduction to Igneous and Metamorphic Petrology. Prentice Hall.
The Phase Rule in Metamorphic Systems
Reaction: periclase coexists with brucite: f=C+1 F = 1 (#2 reason to violate the mineralogical phase rule) To leave the curve, all the periclase must be consumed by the reaction, and brucite is the solitary remaining phase f = 1 and C = 1 again
The Phase Rule in Metamorphic Systems
Suppose we have determined ee scenarios: a)

f=C
The standard divariant situation in metamorphic rocks
The Phase Rule in Metamorphic Systems
If F 2 is the most common situation, then the phase rule may be adjusted accordingly: F=C-f+2 2 f C (24-1)
• Goldschmidt’s mineralogical phase rule, or simply the mineralogical phase rule
Figure 21-9. The P-T phase diagram for the system Al2SiO5 calculated using the program TWQ (Berman, 1988, 1990, 1991). Winter (2001) An Introduction to Igneous and Metamorphic Petrology. Prentice Hall.
The Phase Rule in Metamorphic Systems
3) We didn’t choose the # of components correctly
• Some guidelines for an appropriate choice of C

Begin with a 1-component system, such as CaAl2Si2O8 (anorthite), there are 3 common types of major/minor components that we can add a) Components that generate a new phase Adding a component such as CaMgSi2O6 (diopside), results in an additional phase: in the binary Di-An system diopside coexists with anorthite below the solidus
The Phase Rule in Metamorphic Systems
Consider the following three scenarios: C=1 f = 1 common f = 2 rare f = 3 only at the specific P-T conditions of the invariant point (~ 0.37 GPa and 500oC)
The Phase Rule in Metamorphic Systems
3) We didn’t choose the # of components correctly