lab pt 2 Flashcards
Carbonate Thin Section Textures
Sparite refers to crystalized calcite. It is usually .02-1 mm milky crystals that lack distinct internal structures. It is sub-euhedral and “jigsawed” together
Mud: This is microcrystaline calcite that is likely to precipitate due to algaes in calm enviroments.
Ooids
These are concentric, small, carbonate grains with D< .25 mm. Modern ooids have a radial structure. They are mainly distinguished by size and shape.
An oolite is a rock made of ooids
Peloids
These are grains of micro or cryptocrystalline calcite without internal structures
They tend to be silt to fine sand sized (.03-1mm) and are composed of pellets.
Quartz in thin section
General: No Twinning, low relief
PPL: Clear to slightly murky, conchoidal fracture
XPL: 1st order grey, undulous extinction
Volcanic Monocrystalline Quartz
This has all the properties of regular quartz but it will not have any inclusions
Polycrystalline quartz
This appears as one crystal in PPL and many in XPL. It is usually eroded quartzite or sandstone grains.
Chert
Appears as one crystal in PPL and in XPL it is a bunch of white and black dots
Chalcedony
This is similar to overgrowths in PPL but is very fibrous in XPL/PPL
Quartz Overgrowths
These are tough to ID because they are optically continuous in XPL but under close inspection there is a faint line of the original quartz grain
Chert Cement
This will look like chert but between the grains.
Feldspars
They generally twin, have 1st order grey birefringence, 2 directions of cleavage, moderate-high relief.
These can alter to clays that gives them a cloudy/muddy appearence.
K-Spar
Orthoclase has simple twinning, perthitic textures of exsolved albite (tartan twinnnig), and is generally cloudier than plagioclase
Microline has tartan twinning
Sanidine has simple contact (Carlsbad) twins
Plagioclase
Plagioclase has polysynthetic twinning. This looks like thin pinstripes that go extinct 180 degrees off from one another.
Schist
This will have bright and dark sheets of mica
Rhyolite and Basalt
This will be amorphous but includes euhedral plagioclase and sanidine
Recycled quartzite
Micas
Biotite: in PPL it is brown and pleochroic, it has low order interference colors also look for birds eye patterns
Muscovite: PPL it is clear to green and has BRIGHT interference
Micrite
This is microcrystalline calcite that is brown/green in PPL and looks like mud
Calcite
This is similar to quartz but will have rhombohedral cleavage, simple twins, lamallar twins, sub-hedral crystals, and bright blue interference.
Aragonite
Overall it is very fibrous.
Secondary aragonite forms botroidal fans and primary aragonite forms seems or groundmass of acicular crystals.
Dolomite
Variable relief in PPL with extremely high order interference in XPL. When a limestone is dolomitized it will have euhedral crystals.
It is also less likely to twin than calcite.
Geopetal Structure
This refers to when shells are infilled with sparite and remnant sand grystals are at the base of the shell. It is important because it indicates stratigraphic up
Pisoids
These are very similar to ooids but 2pisolites. They are larger, can be slightly oblong and are distinct from oncoids by not having a nucleating fossil and distinct from peloids by having concentric banding.
Oncoids
These are large oblong shaped grains that have a nucleas > 2 mm. They look like roundish stromatilites and can even form around shells. They are best distinguished by finding a nucleating particle unlike ooids and pisoids that have a tiny round nucleas.
Grapestones
This is when there is a oolite or pisolite intraclasts make another rock. They are distinguished in xpl by going extinct simultaneously even though composed of many small round grains.
Forominiferans
These look like coiled chambers
Corals
These will have slightly “tree-like” interiors. Look for radial symmetry to distinguish from other fossils.
Bryozone
These are colonial organisms that form a fuzzy network that can be either elongated or radiating
Echinoderums
These are sea urchin spines that go extinct as a singular grain and are “C” shaped. It also refers to any skeletal fragments that go extinct as a singular grain in xpl.
Brachiopod fossils
These are like clams, the individual valves/shells may be seperated. Look for small tubular or collumnar edges to ID.
Gastropod fossils
These are snail shells. In comparison to formineferans the are conical and foraminiferans are very circular.
Plotting on a Ternary Diagram
- ) Choose the axis you want to plot on (prefer upper two)
- ) Choose the second axis you want to plot on (prefer base)
- ) find the intercept on the line that is parallel to the second axis.
- ) double check by going reverse
Recycled Orogenic Sediments
These have a large abour of total Q and lithics but low feldspar. On the Qm diagram it is in the right edge, on the Qt diagram it is in the UR
Arc Sediments
These fall in the bottom center of both diagrams. They have a large amount of lithics/spars and little quartz.
Craton sediments
These are in the UL of the diagrams and have alot of quartz with very little lith and at most minor spar.
Basement uplift sediments
These have a large amount of feldspar. They plot in the LL.
