1. Introduction to the Metamorphism of Carbonate Rocks
IN THIS LECTURE
Calcite marbles
Decarbonation
Dolomitic marbles
Calc-silicate rocks
Fluid composition in marbles

2. Marbles
The term marble is used for metamorphosed calcareous rocks in which carbonate minerals dominate.
This represents essentially two end-member compositions
Very pure calcite limestones
Impure calcite or dolomitic limestones
Metamorphism of these two end-member compositions produces two different rock types
Pure calcite marbles which are petrologically not very interesting
Dolomitic marbles which are petrologically interesting

3. Calcite Colour colourless Pleochroism non pelochroic Form
variety of habits, but usually coinsist of scalenohedron and rhombohedron combinations. In most rocks calcite forms anhedral grains or grain aggregates
Relief
moderate negative to high positive, marked change with stage rotation nw = ne = 1.486
Cleavage
perfect rhombohedral cleavage, angle between cleavages 74°57‘
Birefringence
0.172, extreme, creamy high order colours
Twinning
lamellar twins parallel to one edge of the cleavage rhomb or along the long diagonal of the rhomb

4. Calcite Optic Character uniaxial Extinction
extinction is inclined or symmetrical to cleavage traces
Composition
dominantly CaCO3, but substitution of Mg, Fe, Mn, or Zn and minor Sr and Ba
Alteration
altered to dolomite during diagenesis, calcite is soluble in natural waters and may be removed by solution
Occurrence
common and widespread as a major mineral in limestones, and an accessory in igneous, metamorphic and sedimentary rocks
Distinguishing Features
cleavage, variable relief, extreme interference colours

5. Calcite
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6. Calcite vs Dolomite
Distinguishing calcite, dolomite and other rhombohedral carbonates from each other can be very difficult without obtaining chemical analysis or using chemical stains.
Often can use associated mineralogy to help decide
We’ll come back to this

7. Calcite Marbles
In general, metamorphism of a pure calcite limestone simply produces a pure calcite marble.
Petrologically not very interesting since calcite is stable to very high pressures and temperatures.
Relatively pure limestones that contain a small amount of quartz are more interesting as they show one of the simplest examples of the most common reaction type in carbonate rocks, decarbonation reactions.
CaCO3 + SiO2 -> CaSiO3 + CO2
Calcite + quartz -> wollastonite + fluid
However, at pressures of more than a couple of kilobars the temperature required to form wollastonite is beyond the range of normal regional metamorphism

8. Wollastonite Formula CaSiO3 Pyroxenoid group.
Usually pure, but Mn and Fe2+ can substitute for Ca
Crystal System
Triclinic -> Biaxial
Crystal Habit
Columnar and fibrous elongate grains, often with twinning 
Cleavage
Perfect cleavage on {100}, good cleavages on {001} and {-102} Splitery cleavage fragments. Angles of cleavage: 84.5 degrees, and 70 degrees
Color/Pleochroism
Colorless, white, greyish, often with yellowish or brownish tint. Vitreous. No pleochroism

9. Wollastonite
Refractive Indices a: b: g: d:
Increase with Fe and Mn content. Wollastonite resembles tremolite and pectolite, but both have a higher birefringence.
Extinction
Parallel  Elongate crystals display parallel extinction.
Distinguishing Features
Colorless to grey in thin sectionwith moderate to moderatly high relief. First order interference color yellow-orange. One perfect cleavage and two good cleavages producing splintery cleavage fragments. H = G = Streak is colorless or white. 
Occurrence
Occurs commonly as a product of contact and/or regional metamorphism in limestone and dolomite. Associated minerals include calcite, and grossular in hornfels, tremolite, epidote group members, diopside, and other Ca-Mg silicates.

10. Wollastonite

11. P-T Stability of Calcite + Quartz

12. The Role of Fluid Composition
How then do we explain the presence of wollastonite in marbles that have not been to such high temperatures?
Reduce the pressure of the CO2 phase.
At temperatures of the greenschist facies and above, H2O and CO2 supercritical fluids are completely miscible
Hence the partial pressure of CO2 in a mixed H2O-CO2 fluid may be much less than the total fluid pressure.

13. P-T Stability of Calcite + Quartz

14. Phase Rule Constraints
The observed effect of adding H2O to the calcite + quartz + wollastonite + CO2 equilibria accords with the phase rule.
Recalling that F = C – P + 2
In the H2O-absent system there are four phases and three components (CaO, SiO2 and CO2) giving one degree of freedom.
This means that the full assemblage can only exist along a univariant curve.
If H2O is added to the system then the number of components is increased by one but the number of phases stays the same since H2O is miscible with CO2.
Hence there are two degrees of freedom
Therefore fluid composition is a variable in addition to T and P and by specifying one of these three variables the equilibrium conditions can be represented by a univariant curve on a plot with the other two variables as axes.

15. Effect of Fluid Composition

16. T-XCO2 diagrams
These types of plots are known as isobaric T-XCO2 diagrams
On these types of plots divariant equilibria plot as a line known as an isobaric univariant curve.
Therefore if the P is specified there is still one degree of freedom within the system.

17. Dolomitic Marbles
Limestones that contain dolomite provide much more useful indicators of metamorphic grade because of a range of Ca-Mg silicates can form in the more usual P-T conditions of metamorphism, such as talc, tremolite and diopside.
With prograde metamorphism there is a zonal sequence of mineral-appearance isograds similar to what we saw with pelites.
This zonal sequence in regionally metamorphosed dolomitic limestones appears to be
Talc (not always present)
Tremolite
Diopside or forsterite
Diopside + forsterite
This zonal scheme was first identified by Eskola, one of the fathers of metamorphic petrology in 1922.

18. Dolomitic Marbles and the Phase Rule
The zonal scheme identified by Eskola, although applying generally, is actually much more complex in natural systems.
Why is this?
Again look at the phase rule.
Dolomitic marbles can be described by five components
CaO, MgO, SiO2, H2O and CO2
No assemblages have more than five phases, normally four minerals and a mixed fluid phase.
Therefore according to the phase rule, there should be two degrees of freedom in most systems and thus most mineral assemblages will occur over a wide range of pressures and temperatures depending on what the composition of the fluid phase is.

19. Calc-Silicates
Calc-silicates are rocks rich in Ca-Mg silicate minerals but with only minor amounts of carbonate present.
Like dolomitic marbles, calc-silicates are useful indicators of metamorphic grade.
They can be correlated with the pelite zones in the following manner
Pelite zone Calc-silicate zone
Garnet Zoisite-calcite-biotite
Zoisite-hornblende
Staurolite Anorthite-hornblende
Kyanite
Sillimanite Anorthite-pyroxene

20. Calc-Silicates
Calc-silicates contain significant amounts of other chemical components especially Al, K and Fe.
Therefore their mineralogy is more complex than that of dolomite marbles and additional phases include
Zoisite
Garnet
Hornblende
Ca-pyroxene like diopside
Calcic-plagioclase
K-feldspar
Phlogopite and vesuvianite
In general zoisite and grossular garnet are only stable if the fluid phase is rich in water, while calcic-plagioclase is favoured by CO2 dominated fluids.

21. Diopside Formula CaMgSi2O6 Crystal System Monoclinic -> Biaxial
Crystal Habit Short, stubby, prismatic crystals with square, rectangular, or eight sided cross section Granular, lamellar, or columnar masses Anhedral grains
Cleavage Fair to good cleavage on (110), Partings on (100) and (001) Imperfect cleavage intersecting at 87º and 93º ie typical pyroxene cleavage
Color/Pleochroism No pleochroism Colorless to pale green in thin section

22. Diopside
Refractive Indices a = b = g = d =
Extinction inclined in (010) sections
Distinguishing Features light green color, cleavage
Occurrence Commonly found in metamorphosed carbonate rocks like skarns and marbles. Found with: tremolite, actinolite, grossular garnet, epidote, wollastonite, forsterite, calcite and dolomite

23. Diopside
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24. Epidote Formula Ca2(Al,Fe)Al2O(SiO4)(Si2O7)(OH)
Complete solid solution from clinozoisite (Al: Fe 3+ = 3:0) to epidote (Al:Fe 3+ = 2:1)
Crystal System
Monoclinic -> Biaxial (ep –ve, czo ve)
Crystal Habit
coarse to fine granular ; also fibrous
Cleavage
{001} perfect, {100} imperfect perfect cleavage in one direction
Color/Pleochroism
clinozoisite: pale green to gray epidote: pistachio-green to yellowish-green to black

25. Epidote
Clinozoisite epidote a = b =  g = 
Max Birefringence
Refractive indices and birefringence increase with iron content
Extinction
Parallel to length of elongate crystals and to the trace of cleavage.
Distinguishing Features
Epidote is characterized by its green color and one perfect cleavage. H= 6-7. G = 3.25 to Streak is white to gray. Clinozoisite and epidote are distinguised from eachother by optic sign, birefringence, and color
Occurrence
Occurs in areas of regional metamorphism; forms during retrograde metamorphism and forms as a reaction product of plagioclase, pyroxene, and amphibole. Common in metamorphosed limestones with calcium rich garnets, diopside, vesuvianite, and calcite.

26. Epidote
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27. Actinolite-Tremolite
Formula
Ca2(Mg,Fe2+)5Si8O22(OH)2
Crystal System
Monoclinic -> Biaxial
Crystal Habit
occurs as columnar, bladed or acicular grains, elongated parallel to c axis, may be fibrous, basal sections are diamond shaped, with typical amphibole cleavage
Cleavage
two amphibole cleavages on {110}, intersect at 56 and 124°
Colour/Pleochroism
colourless to pale green to dark green, darker colours and stronger pleochroism associated with high Fe contents

28. Actinolite-Tremolite
Refractive Indices
a = b = g =
Birefringence
maximum interference colours are upper 1st to mid 2nd order
Extinction
Inclined extinction greater for tremolite than actinolite
Distinguishing Features
Can exhibit simple and lamellar twins
Alters to talc, chlorite and carbonates
Resembles hornblende but often has lower extinction angle
Occurrence
common occurrence is in contact and regional metamorphosed limestone and dolomite. Also found in metamoprhosed mafic and ultramafic rocks. It is the common fine-grained alteration product of pyroxenes.

29. Actinolite-Tremolite
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