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.