Petrography
What is a typical extinction angle for orthopyroxene?
0°
Biaxial minerals have
2 optic axes and 3 indices of refraction
In XPL, when looking perpendicular to the principal; section, a triclinic mineral will show extinction at _____ locations during a complete rotation of the stage
4
What is the range of wavelengths for visible light?
400 to 700 µm
In which of the following modern locations would you be most likely to find active deposition of carbonate?
Bahamas
Which of the following is not a diagnostic mineral property?
Color
In Bowen's reaction series, the terms discontinuous and continuous refer to
Continuity of composition and discontinuity of structure
What two igneous minerals are found together?
Diopside and forsterite
Clinopyroxenes have more ____ than orthopyroxenes.
Fe and Mg
What are the maximum interference colors one should expect from a mineral with a birefringence of 0.001?
First-order gray
An isotropic mineral
Has no birefringence
Naturally occurring glass
Is isotropic Can be basaltic or rhyolitic Is mostly volcanic in origin Is not a mineral (all of these)
How can you tell a monzogranite from a monzonite?
Monzonite has less quartz
What is most important in determining the equilibrium mineralogy of a metamorphic rock?
P and T and composition
How do you tell quartz from K-feldspar in thin section?
Quartz sometimes shows undulatory extinction.
ripple marks are best described as
Sedimentary structures formed in the presence of a current
A porphyritic texture implies:
Slow cooling followed by rapid cooling
Plane-polarized light, upon interring anisotropic minerals
Splits into two vibration at right angles to each other with different velocities
In which crystal systems are the optical axes and the crystallographic axes of minerals aligned?
Tetragonal, hexagonal
If, when you raise the stage, a white line moves from the outer edge of a crystal to the inner edge, then
The crystal has a higher RI than its surrounding
20 quartz grains are observed in a thin section of a sandstone. None of them appear exactly the same in XPL. What is the best explanation for this observation?
The grains have different orientations relative to the plane of the thin section.
If a mineral causes light to slow down by 20% then
The wavelength must increase by 20%
What are the maximum interference colors one should expect from a mineral with a birefringence of 0.050?
Third-order orange or red
When does even a strongly anisotropic mineral show no birefringence?
When looking perpendicular to the circular section
What is the best way to determine the thickness of a thin section?
With the interference color of a known mineral
Should one use XPL or PPL to determine if a mineral is twinned?
XPL
Fractional crystallization of a basaltic magma will produce
a liquid with more SiO2 that it started with.
A geothermometer is
a mineral assemblage that can reveal the minimum temperature attained during heating
A sandstone is best described as
a rock composed of material from 2 to 0.625 mm in diameter
Which of the following is true? a. Diopside has inclined extinction b. SiO2-saturated rocks sometimes contain leucite c. Hornblende is common in obsidian d. Andesite has 45% SiO2 e. Monzogranite has higher birefringence than biotite
a. Diopside has inclined extinction
Which of the following statements about sedimentary rocks in general is false? a. They are composed exclusively of rocks and mineral particles produced from the weathering of older rocks b. biogenic sediments are produced by the life activities of plants and animals c. chemical processes alter minerals as rocks weather d. soluble ions are produced by weathering e. diagenesis can be thought of as very low grade metamorphism
a. They are composed exclusively of rocks and mineral particles produced from the weathering of older rocks
Which of the following terms best describes trilobite?
an example of an allochem
A fine-grained igneous rock with 61% SiO2, 1% K2O, and 2% Na2O is best called a
andesite
A pleochroic mineral must always be
anisotropic
Which of these is not an example of a typical cement mineral?
apatite
Which of the following lists is written in order of chemical stability on the ocean floor from least to most?
aragonite, low-Mg calcite, high-Mg calcite
________ is the Fe-rich end member of the clinopyroxenes
augite
A fine-grained igneous rock with 10% opx, 15%, plagioclase, and 75% groundmass is best called a
basalt
A metamorphic rock with mostly olivine, epidote, augite, and albite is most likely to have _____________ as its protolith.
basalt
________ often contains pleochroic halos
biotite
Rocks with lawsonite, glacophane, and albite are best categorized in the ________ facies.
blueschist
Which of the following metamorphic facies is diagnostic of subduction zones?
blueschist
A metamorphic rock contains dolomite, garnet, phlogopite, diopside (avg. grain size 2.5 mm). What is the protolith?
calcareous sandstone
Lithification requires
cementation
Which of the following lists is given in increasing grade of metamorphism?
chlorite, biotite, staurolite
Which of the following lists is written in decreasing order of grain size?
conglomerate, sandstone, siltstone
Which of the following metamorphic rocks experienced the highest pressure?
eclogite
________ is the Fe-rich end member of the olivines
fayalite
What is the most prominent feature of rocks formed by regional metamorphism?
foliation
Which of the following terms best describes ooids?
formed in the surf zone
Which of the following terms best describes dolomite?
found more in ancient rocks than modern sediments
What is the coarse grained equivalent of basalt?
gabbro
A coarse-grained igneous rock with 20% quartz, 20% plagioclase, 20%, Kspar, 20% biotite, and 20% hornblende is best called a
granodiorite
Rocks with quartz, K-spar, sillimanite, and garnet are best categorized in the ________ facies.
granulite
Rocks with quartz, chlorite, and plagioclase are best categorized in the ________ facies.
greenschist
Rocks with quartz, muscovite, plagioclase feldspar, biotite, and chlorite are best categorized in the ________ facies.
greenschist
A mineral with a refractive index of 1.7 mounted in epoxy with a refractive index of 1.55 will show
high positive relief
To find the birefringence (δ) of a mineral, look for a mineral grain with the
highest interference color
Rocks with quartz and andalusite are best categorized in the ________ facies.
hornfels
Allochems include
intraclasts and fossils
Which of the following scenarios would result in the smallest zone of contact metamorphism?
intrusion of an rhyolitic dike 5 m wide
A Geologist suffers from a black eye after yelling "Gneiss!!!" to a classmate, who happened to be sunning on a large boulder. The geologist really meant to tell the classmate that the rock they were laying on
is a high-grade, silicate-rich, mineralogically banded metamorphic rock
The original bedding in a sedimentary rock
is only preserved in low-grade metamorphic rocks
A metamorphic rock contains Quartz, Kfsp, kyanite, biotite, plag (avg. grain size 4.3 mm). What is the most likely name for this rock?
kyanite gneiss
A metamorphic rock with mostly calcite and dolomite is most likely to have _____________ as its protolith.
limestone
Which of the following metamorphic rocks cannot form from a shale?
marble
Isograds are line of equal
metamorphic assemblage
Which of the following terms best describes fine-grained carbonate mud?
micrite
Which of the following pairs from the Folk and Dunham classifications describe essentially the same rock?
micrite and mudstone
Which of the following metamorphic rocks experienced the highest temperature?
migmatite
A highly rounded, well-sorted sediment with 40% feldspar and 60% quartz is
mineralogically immature and texturally mature
For which of the following sediments would compaction be most important in the process of lithification?
mud
In the Dunham classification of carbonates a mud-supported rock with less than 10% grains is called a
mudstone
In PPL an orthorhombic mineral will show extinction at _____ locations during a complete rotation of the stage
no
A limestone is best described as
none of the above
Mud cracks are best described as
none of the above
________ is a rock we know erupted at a very high temperature
obsidian
A coarse-grained igneous rock with 40% pyroxene, 40% olivine, and 20%, plagioclase is best called a
olivine gabbro
A mineral with a 2V angle of 0° is
orthorhombic, trigonal, or tetragonal
In the Dunham classification of carbonates a grain-supported rock with some mud is called a
packstone
10% melt of ____ will produce basalt
peridotite
Another name for dunite is
peridotite
In PPL, a mineral changes its color when rotated. This is called
pleochroism
Which of the following groups of mineral characteristics are observed best (or only) in PPL?
pleochroism, color, relief
What two minerals are never found in the same igneous rock?
quartz and forsterite
Which of the following lists contain minerals that are highly resistant to chemical weathering?
quartz, tourmaline, and zircon
A metamorphic rock contains quartz, plagioclase, chlorite (avg. grain size 0.55 mm). What is the most likely name for this rock?
quartzite
Assimilation is important in the production of
rhyolite
What is the fine-grained equivalent of granite?
rhyolite
________ is a common inclusion in quartz
rutile
A metamorphic rock contains quartz, plagioclase, chlorite (avg. grain size 0.55 mm). What is the protolith?
sandstone
A metamorphic rock with mostly dolomite, talc and tremolite is most likely to have _____________ as its protolith.
sandy limestone
A metamorphic rock with mostly wollastonite, calcite and garnet is most likely to have _____________ as its protolith.
sandy limestone
A metamorphic rock contains quartz, Kfsp, kyanite, muscovite, plag (avg. grain size 4.3 mm). What is the protolith?
shale
A metamorphic rock with mostly kyanite, muscovite, staurolite, and quartz is most likely to have _____________ as its protolith.
shale
A metamorphic rock with mostly quartz, biotite, plagioclase, and andalusite is most likely to have _____________ as its protolith.
shale
Which of the following lists is given in increasing grade of metamorphism?
shale, schist, migmatite
Which of the following pairs of minerals are not likely to be found in the same metamorphic rock?
sillimanite and chlorite
Zircon is a tetragonal mineral. Is it likely to have
straight extinction
Isotherms are bent down in
subduction zones
Metasomatism is
the change of the bulk composition of a rock during metamorphism
A shale is best described as
the most common sedimentary rock
Which of the following is not used to classify metamorphic rocks?
the texture of the parent rock
The process of lithification is
the transformation of loose sediment in to sedimentary rock
In a uniaxial mineral, when the fast ray travels parallel to the c-axis, we call that mineral
uniaxial positive
The 2Vx angle of olivine can vary from 45 to 100°. This variation is caused by
variation in composition
________ is extensively used in geochronology
zircon
________ often produces pleochroic halos
zircon
How is the φ parameter for describing grain size calculated?
φ = log2(D), where D is the grain diameter in mm
