1. Metamorphic Rocks CONTACT/REGIONAL AND METASOMATIC ROCKS
Marble, Quartzite, and Serpentinite

2. Marble, Quartzite, and Serpentinite
Marble and quartzite may be either regional or contact metamorphic
Serpentinite is formed by metasomatic alteration of mafic rocks
Marble may also involve metasomatism
Therefore these rocks do not fall into neat categories

3. Marble
Marble is usually the product of metamorphism of limestone or dolomitic limestone
Limestones often contain silicate impurities, and the impurities may be converted to minute crystals of sericite, chlorite, etc
These crystals may impart a slightly silky luster to the marble, similar to the process that occurs during the formation of phyllite

4. Metamorphic Grade of Marble
Marbles range in grade from slates to schists
Foliation may be visible in hand specimen
Foliation may be due to plastic flow during metamorphosis, or
Foliation may be relict sedimentary

5. Naming Marble
Marbles may be named for their color, for example pink or white marble
White marble is often dolomitic
Marble may also be named for accessory minerals such as brucite, grunerite, pyrrhotite, etc

6. Pink Marble Nonfoliated Marble
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Photo #449053_.jpg
Nonfoliated Marble

7. Relict Sedimentary Bedding
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Photo #449057_.jpg
Relict sedimentary bedding in marble

8. Photomicrograph of Marble
Marble, CN
The photo shows strongly twinned and highly cleaved calcite
Photo #c_116.gif

9. Weathering in Marble Weathering in a marble tombstone (left)
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Left Photo #449058_.jpg
Right Photo #449059_.jpg
Weathering in a marble tombstone (left)
Lichens secrete acid, which help to dissolve marble (right)

10. Brecciated Marble Angular fragments in carbonate matrix
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Photo #449060_.jpg
Angular fragments in carbonate matrix

11. Acid Reaction in Marble
Marble usually retains at least some carbonate component
If calcite is present, the marble will react to acid vigorously
Dolomitic marbles react very slowly to cold hydrochloric acid
Acid solutioning of marble may lead to cave formation

12. Cavities in Marble
The metamorphic process often releases large quantities of carbon dioxide
This gas escapes though the marble and may lead large fractures and cavities in the rock, in a manner similar to the formation of vesicular basalt
Marble is used as a decorative stone, and the presence of cavities is often undesirable

13. Filling Cavities
For decorative purposes, the cavities may be filled with epoxy colored to match the background color of the marble
This is often done and is generally a satisfactory solution

14. Testing for Epoxy Filling
Acid etching of limestone marble will quickly expose the epoxy as topographically high regions
The use of mineral-specific stains, for either calcite or dolomite, will leave the epoxy uncolored

15. Load-Bearing Marble
For load-bearing structures, such as marble columns, the marble should be dense, with little or no cavities
Before marble is used in critical load-bearing applications, representative sample must be tested, and these tests should include testing for epoxy filling

16. Mineralogy of Marble
Common non-carbonate minerals in marble include tremolite, actinolite, diopside, epidote, phlogopite, scapolite, and serpentine
Epidote (along with albite) occur in lower grade marbles
Hornblende, plagioclase, some mica, and, in the higher grades, diopside are common
Sphene, apatite, and scapolite are present in amphibolite facies marbles

17. High-Grade Marble Mineralogy
Under higher grade conditions, dolomite will disappear
It decomposes to yield periclase (MgO) or brucite (Mg(OH)2)
Dolomite present in high-grade metamorphics is probably due to retrogressive metamorphism

18. Epidote and Actinolite
Ecole de Mines photo
France, from the Ecole de Mines

19. Actinolite in Thin Section
(Upper - CN) Actinolite in a groundmass of Mg-rich chlorite. The photo shows the upper first-order to mid second-order interference colors of actinolite
(Lower - PP) Actinolite in a groundmass of Mg-rich chlorite
Upper Photo #:tjb4.jpg
Lower Photo #:tjb3.jpg

20. Photo of Apatite Apatite from Durango, Mexico
Ecole de Mines photo
Apatite from Durango, Mexico
Photo: Monique Claye, Ecole de Mines

21. Photomicrograph of Apatite
Large apatite end section (indicated by arrows)
Note: hexagonal shape
Green phenocryst is hornblende
Width of view is 0.85mm
Photo #mlb85m.jpg

22. Quartzite
Quartzites are often the metamorphic product of quartz sandstones
During metamorphism, the quartz grains become interlocking due to compression and recrystallization
If shearing forces are large enough, the quartz grains elongate and interlocking grain boundaries granulate
The granulation of the boundaries can only be seen in thin section

23. Quartzite Sioux Quartzite, South Dakota Nonfoliated
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Photo #449073_.jpg
Sioux Quartzite, South Dakota
Nonfoliated

24. Photomicrograph of Quartzite
Quartzite CN
Quartzite is metamorphosed quartz-rich sandstones
All of the grains are quartz; black spaces are in extinction or are holes in the thin section (plucking)
Note that all grains are xenoblastic (anhedral), typical of quartz in metamorphic rocks
Photo #quatzite.gif

25. Use of Quartzite
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Photo #449075_.jpg
Quartzite is highly resistant to physical and chemical weathering, so it does well in applications like this rip-rap

26. Sheared Quarzites
In highly sheared quartzites, the quartz grains become lenticular
All trace of the original sandstone disappears
The quartz grains may also show strain effects under the optical microscope

27. Strain in Argillaceous Sandstones
If the parent rock is an argillaceous sandstone, strain is taken up primarily in the fine-grained argillaceous phases and quartz and feldspar grains will be relatively undeformed

28. Mineralogy of Quartzites
Impure quartz sandstones are likely to produce sericite during metamorphism
Arkoses and feldspathic sandstones typically produce quartz-mica schists
They will differ from a schist produced from an argillaceous sandstone or graywacke due to their lack of chlorite or biotite

29. Kyanite Photomicrographs
(Upper - CN) Kyanite is surrounded by biotite and muscovite
The cleavage, relief, and bladed form of kyanite are clearly visible
Maximum first order red interference colors; inclined extinction that can almost be parallel.
(Lower - PP) Colorless to pale blue in plane polarized light
Tabular crystals; 2 cleavages; high relief
Upper Photo # m_44.gif
Upper Photo # m_45.gif

30. Kyanite Photomicrographs
(Upper - CN) Kyanite is surrounded by quartz
(Lower - PP)
Upper Photo # b_108.gif
Upper Photo # b_109.gif

31. Serpentinite
Product of metasomatic alteration of ultramafic igneous rocks
Serpentine minerals are usually pseudomorphous after the minerals they replace
Serpentines replacing olivine even retain the irregular curving fractures typical of olivine
The fractures may fill with very fine-grained magnetite produced during the serpentinization process

32. Serpentinite Serpentinite marble Nonfoliated
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Photo #449064_.jpg
Serpentinite marble
Nonfoliated

33. Photomicrographs of Serpentine
(Upper CN , Lower PP) The rock shown is almost 100% serpentine
The equant crystal forms seen are serpentine pseudomorphs after olivine
All members of the group have low birefringence (first order yellow maximum) and parallel extinction
The mineral habit is fibrous, and in plane polarized light grains are colorless to pale green
Grain size is typically too small to determine many optical properties
Upper Photo # b_42.gif
Lower Photo #b_43.gif

34. Serpentine in Mafic Rocks
(Upper, CN) The picture (1.5 mm field of view) shows light gray stringers of serpentine altering clinopyroxene (at extinction)
(Lower, CN) Clinopyroxene grain surrounded by gray serpentine
Upper Photo # b_41.gif
Lower Photo #b_25.gif

35. Serpentinite Photo
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Photo #449068_.jpg
Serpentinite from California Mother Lode country, in the Sierra Nevadas
Metallic mineral appears to be pyrite

36. Pseudomorphism in Serpentinite
The outlines of the crystals are also visible because of the magnetite grains which define the outline of the crystal
Pseudomorphs in serpentine are often among the finest pseudomorphs found in any rock
Serpentine replacing pyroxene may retain the cleavage, parting, or Schiller luster of the pyroxene

37. Biotite after Garnet
It consists of a more-or-less random aggregate of biotite flakes
Elsewhere in the thin section relics of garnet remain in these aggregates, which are said to pseudomorph the original mineral
This is the result of polymetamorphism - a thermal overprint on a regionally-metamorphosed rock
Photo #pseudomprph.jpg
Field of view is 2.5 mm across.
Polymetamorphic mica-schist, Eastern Alps,
Sample DW-0222.
The brown patch at the center of the field of view has the regular outline of a garnet

38. Serpentinite Mineralogy
Magnesite, in minute grains, inevitably accompanies the serpentine minerals - magnesite is a product of the metasomatic alteration
Other minerals found in serpentinites include tremolite and anthophyllite, usually as fibers or prisms on the borders of former olivine crystals

39. Serpentinite Mineralogy continued
Talc is another common alteration product
Talc may occur to the exclusion of all other secondary minerals
Resulting structures are unusual, possibly due to volume expansion during metasomatism
Slickensides are sometimes seen on serpentinites
Another fairly rare mineral is brucite

40. Talc in Serpentinite France http://cri.ensmp.fr/mineral/Z39x.jpg
Ecole de Mines photo
France

41. Brucite Photo Closeup of brucite
Ecole de Mines photo
Closeup of brucite