Metamorphic Rocks, Part 3
CONTACT/REGIONAL AND
METASOMATIC ROCKS
Marble, Quartzite, and
Serpentinite
1
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
2
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
3
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
4
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
5
Pink Marble
• Nonfoliated Marble
6
Relict Sedimentary Bedding
• Relict sedimentary bedding in marble
7
Photomicrograph of Marble
• Marble, CN
• The photo shows
strongly twinned
and highly
cleaved calcite
8
Weathering in Marble
• Weathering in a marble tombstone (left)
• Lichens secrete acid, which help to dissolve marble
(right)
9
Brecciated Marble
• Angular fragments in carbonate matrix
10
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
11
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
12
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
13
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
14
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
15
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
16
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
17
Epidote and Actinolite
• France, from the Ecole de Mines
18
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
19
Photo of Apatite
• Apatite from Durango, Mexico
• Photo: Monique Claye, Ecole de Mines
20
Photomicrograph of Apatite
• Large apatite end
section (indicated by
arrows)
• Note: hexagonal shape
• Green phenocryst is
hornblende
• Width of view is
0.85mm
21
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
22
Quartzite
• Sioux Quartzite, South Dakota
• Nonfoliated
23
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
24
Use of Quartzite
• Quartzite is highly resistant to physical and
chemical weathering, so it does well in applications
like this rip-rap
25
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
26
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
27
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
28
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
29
Kyanite Photomicrographs
• (Upper - CN) Kyanite is
surrounded by quartz
• (Lower - PP)
30
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
31
Serpentinite
• Serpentinite marble
• Nonfoliated
32
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
33
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
34
Serpentinite Photo
• Serpentinite from California Mother Lode country,
in the Sierra Nevadas
• Metallic mineral appears to be pyrite
35
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
36
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
37
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
38
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
39
Talc in Serpentinite
• France
40
Brucite Photo
• Closeup of brucite
41