Transcript Metamorphic Rocks, Part 3 CONTACT/REGIONAL AND …

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