Classification of Igneous rocks (composition) Flashcards
How do we classify Igneous rocks?
Based on Composition
Based of Fabric and Texture
Base on Field relation
Most Straight forward approach to determining rock mineralogy that involves visually identifying the minerals and determining their percentage by volume.
Modal Composition or Mode
an indirect scheme using data derived from chemical analysis of a rock sample. this is commonly used in aphanitic or glassy volcanic rocks.
Normative Mineralogy
Why is Normative Mineralogy used in Aphanitc and glassy volcanic rocks?
because the modal mineral composition of these rocks can not be determined.
Rock group with <45% weighted percent silica (SiO2)
Ultrabasic
Rock group with 45-52% weighted percent silica (SiO2)
basic
Rock group with 52-66% weighted percent silica (SiO2)
Intermediate
Rock group with >66% weighted percent silica (SiO2)
Acidic (silicic)
Generally enriched in the elements iron and magnesium and are referred to as Ferromagnesian or Mafic minerals
Dark-colored Minerals
Depleted in Ferromagnesian elements and are generally enriched in elements such as silicon, oxygen, potassium, and sodium
Light-colored Felsic Minerals
Rocks that are Dark or greenish in color that’s rich in olivine and may or may not contain pyroxene or amphibole
Ultramafic
Dark-colored rocks containing pyroxene and amphibole. it may or may not contain some olivine and biotite
Mafic
Grayish to salt and pepper colored rocks rich in plagioclase and amphibole. It may or may not contain amounts of Biotite and Quartz.
Intermediate
Light-colored or red rock rich in potassium feldspar and quartz. It may or may not contain some amount of Biotite or Muscovite
Felsic
> 90% Dark-colored crystalline mineral
Ultramafic
70-90% Dark-colored crystalline mineral
Mafic
40-70% Dark-colored crystalline mineral
Intermediate
<40% Dark-colored crystalline mineral
Felsic
Quartz ± Feldspar and/or magnesium orthopyroxene
SiO2 oversaturated
Feldspar and/or Magnesium orthopyroxene only
SiO2 Saturated
Forsterite olivine, nepheline, leucite, and other feldspathoids ± feldspar and/or orthopyroxene minerals. excluding quartz
SiO2 undersaturated
Forsterite olivine, nepheline, leucite, and other feldspathoids ± feldspar and/or orthopyroxene minerals. excluding quartz
SiO2 undersaturated
<30% Dark Minerals according to Shand Classification
Leucocratic
30-60% Dark Minerals according to Shand Classification
Mesocratic
60-90% Dark Minerals according to Shand Classification
Melanocratic
> 90% Dark Minerals according to Shand Classification
Hypermelanic
<10% Dark Minerals according to Ellis Classification
Holofelsic
10-40% Dark Minerals according to Ellis Classification
Felsic
40-70% Dark Minerals according to Ellis Classification
Mafelsic
70-90% Dark Minerals according to Ellis Classification
Mafic
> 90% Dark Minerals according to Ellis Classification
Ultramafic
Igneous rock based on AlO concentration:
Al2O3 > CaO + Na2O +K2O
Peraluminous
Igneous rock based on AlO concentration:
Al2O3 = Na2O +K2O
Subaluminous
Igneous rock based on AlO concentration:
Na2O +K2O < Al2O3 < CaO + Na2O +K2O
Metaluminous
Igneous rock based on AlO concentration:
Al2O3 < Na2O +K2O
Peralkaline
A very dark-colored rock, depleted in SiO 2 and commonly enriched in the minerals pyroxene, olivine, amphibole, and plagioclase. Ultramafic plutonic rocks occur in Earth’s Mantle
Peridotite
A fine-grained dark-colored (mafic), SiO 2- poor (Basic) rock rich in plagioclase, pyroxene, and olivine
Basalt
A coarse-grained dark-colored, silica-poor rock rich in plagioclase, pyroxene, and olivine
Gabbro
Fine-grained gray-colored to salt and pepper-colored rocks rich in hornblende, pyroxene, and plagioclase. Andesite and diorite contain more than half to almost two-thirds SiO 2.
Andesite
A coarse-grained gray-colored to salt and pepper-colored rocks rich in hornblende, pyroxene, and plagioclase. Andesite and diorite contain more than half to almost two-thirds SiO 2.
Diorite
A coarse-grained light-colored (felsic) rock, containing approximately two-thirds SiO2, rich in plagioclase, alkali feldspar, and quartz, and also contains small amounts of hornblende and biotite.
granodiorite
A fine-grained light-colored (felsic) rock, containing approximately two-thirds SiO2, rich in plagioclase, alkali feldspar, and quartz, and also contains small amounts of hornblende and biotite.
Dacite
A fine-grain light-colored (felsic) rock containing more than two-thirds SiO 2 (silicic or acidic) and rich in quartz, alkali feldspar with small percentages of plagioclase and biotite
Rhyolite
A coarse-grain light-colored (felsic) rock containing more than two-thirds SiO 2 (silicic or acidic) and rich in quartz, alkali feldspar with small percentages of plagioclase and biotite
Granite
light-colored non-crystalline vesicular rocks characterized by the absence of crystals.
pumice
dark-colored non-crystalline vesicular rocks characterized by the absence of crystals.
Scoria
non-crystalline rocks include those with glassy textures
Obsidian
Non-crystalline rocks that are enriched in rock
pyroclastic fragments that are volcanic ash to gravel size.
Tuff
magma emplaced onto the surface of the Earth as coherent lava flows or as fragmental deposits. These rocks are typically aphanitic and glass
Extrusive (volcanic)
igneous rocks formed at depth; typically phaneritic
Intrusive (plutonic)
formed at intermediate depths not clearly distinct from those of volcanic and plutonic rocks. They can have fabric similar to that of plutonic and volcanic
rocks.
hypabyssal rocks
