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