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Metamorphic Rocks, Part 3

CONTACT/REGIONAL AND METASOMATIC ROCKS

Marble, Quartzite, and Serpentinite

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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

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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

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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

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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

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Pink Marble

• Nonfoliated Marble

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Relict Sedimentary Bedding

• Relict sedimentary bedding in marble

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Photomicrograph of Marble

• Marble, CN• The photo shows

strongly twinned and highly cleaved calcite

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Weathering in Marble

• Weathering in a marble tombstone (left)• Lichens secrete acid, which help to dissolve marble

(right)

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Brecciated Marble

• Angular fragments in carbonate matrix

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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

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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

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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

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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

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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

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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

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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

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Epidote and Actinolite

• France, from the Ecole de Mines

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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

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Photo of Apatite

• Apatite from Durango, Mexico• Photo: Monique Claye, Ecole de Mines

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Photomicrograph of Apatite

• Large apatite end section (indicated by arrows)

• Note: hexagonal shape• Green phenocryst is

hornblende• Width of view is

0.85mm

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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

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Quartzite

• Sioux Quartzite, South Dakota • Nonfoliated

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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

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Use of Quartzite

• Quartzite is highly resistant to physical and chemical weathering, so it does well in applications like this rip-rap

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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

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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

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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

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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

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Kyanite Photomicrographs

• (Upper - CN) Kyanite is surrounded by quartz

• (Lower - PP)

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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

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Serpentinite

• Serpentinite marble• Nonfoliated

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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

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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

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Serpentinite Photo

• Serpentinite from California Mother Lode country, in the Sierra Nevadas

• Metallic mineral appears to be pyrite

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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

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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

The brown patch at the center of the field of view has the regular outline of a garnet

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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

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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

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Talc in Serpentinite

• France

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Brucite Photo

• Closeup of brucite