Descriptions and slides are from Scholle, P.A., 1979, AAPG Memoir 28.
Slide #1
Upper Cambrian Gatesburg Formation, Pennsylvania
Quartz is the dominant framework grain of most sandstones and is
recognized primarily by its low birefringence, lack of cleavage,
low positive relief, and uniaxial interference figure. A large number
quartz types have been defined based mainly on the number of subcrystals
in the grain and their extinction behavior when the stage is rotated
under crossed polarizers. Some of these quartz types may be related
to specific source rocks.
X.N.
0.15 mm
Slide #2
Tertiary Yellowstone Tuff, Wyoming
Detail of volcanic quartz crystal. This grain has straight extinction, a euhedral
outline, and a large "negative crystal" or vacuole. The vacuole has the same
crystallographic orientation as the complete quartz grain, hence the term
"negative crystal." This feature is common but not ubiquitous in quartz of volcanic
origin.
X.N.
0.24 mm
Slide #4.
Upper SilUrian Clinton Formation, Virginia
A single-crystal quartz grain with strongly undulose extinction. Grain
extinguishes completely with more than 5 degrees of stage rotation. Such grains
may be more abundant in strained rocks (expecially metamorphics), but evidence
is incomplete.
X.N.
0.06
Slide #6
,
Cretaceous Kogosukruk Tongue of Prince Creek Formation, Alaska
Detrital chert grains are seen as speckled grains composed of microcrystallized crystalline quartz. These are derived from a sedimentary
source.
Chert is either classed a quartz type or as a sedimentary
rock fragment
depending on sandstone classification is used
X.N.
0.15
Slide #10
Ordovician Newtown gneiss, Connecticut
Complex twinning in a plagioclase feldspar grain. Exact types of twinning are
best determined on universal stage, but albite, carlsbad, and pericline twins are
probably present here. Twin types can sometimes indicate source area. Pink tint
in grains resulted from staining for plagioclase
X.N.
0.38 mm
Slide #12
Ordovician Newtown gneiss, Connecticut
A complex plagioclase grain. and outer rim are clearly of different compositions.
Twinning throughout grain, but core considerable alteration (mainly vacuolization
and sericitization while rim is largely unaltered. Illustrates that alteration can take
place in source rocks and is highly composition dependent.
X.N-
0.10 mm
Slide #15
Pennsylvlnian Sangre de Cristo Formation, New Mexico
Untwinned orthoclase. Many feldspars, of all types, are untwinned
and when
unstained, can be distinguished only by such features as alteration (seen here),
cleavage, fracture, relief, or optic figures. The alteration seen here is coarser,
more extensive, and more regularly oriented than in quartz grains.
X.N.
0.08 mm
Slide #18
Triassic Chinle Sandstone,Arizona
Sedimentary rock fragments (SRF's), shale clasts. Shale fragments are
difficult to distinguish from rank metamorphic rock fragments, especially slates.
SRF's are generally softer and thus more likely to be deformed or embayed by
adjacent harder grains, as seen in this example. Individual clay crystal orientation
is poor, also favoring ah SRF interpretation.
X.N
0.10 mm
Slide #19
Triassic Chinle Sandstone, Arizona
Same view as previous photo but crossed polarizers. Note poor
orientation and high birefringence probably an illitic clay shale clast.
X.N.
0.10 mm
Slide #22
Triassic Dockum Formation, Texas
Abundant carbonate rock fragments. These grains are less varied in
composition and texture than in the previous example. The uniformity of
composition and lack fossil fragments make it likely that this rock is a reworked
caliche (calcium carbonate rich soil crust).
X.N.
0.38
Slide #28
Tertiary Horse Springs Formation, Nevada.
Abundant volcanic glass shards composed of opalline silica. Fragments of shardfilled sediment can occasionally be found and also are excellent indicators of a
source area (acidic volcanism
0.10
Slide #29
Cretaceous Navesink Formation, New Jersey
Glauconite, seen here as rounded grains, is generally green in both plain and
polarized light. Rounding does not necessarily indicate abrasion, and these
grains were probably produced as fecal pellets in situ. Glauconite commonly as
pellets or within skeletal grains and, when not detrital, is a good indicator of a
marine depositional setting.
X.N.
0.38 mm
Slide #30
Upper Cretaceous Mesaverde Group, Colorado
Abundant organic matter in seams. Note variation in color deep reddish-brown to
black -- this is a function not only of thermal maturation but also of the type and
thickness of organic detritus. The material seen here is derived terrestrial, woody
sources
0.10 mm
Slide #33
Cretaceous Monte Antola Formation, Italy
Detrital micas (muscovite). The grains with the bright (2nd order) blue
birefringence are flakes. They are nearly colorless in plain light. The slight
speckled texture (reminiscent of birch bark) is characteristic of micas. Muscovite,
because of its greater chemical stability, is more common than biotite in most
rocks.
X.N.
0.10 mm