Metamorphic Rocks, Part 3 CONTACT/REGIONAL AND METASOMATIC ROCKS

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Metamorphic Rocks, Part 3CONTACT/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