Oceanic Mineralogy and Petrogrphy
Minerals we encnounter on the ocean floor are considerably more mafic than those from the continents. Free quartz is a rarity. Feldspars are rich in Ca and poor in alkalis.
| Mineral | Formula | Mantle | Crust |
| olivine | (Fe,Mg)2SiO4 | Mg-rich: forsterite | |
Fe-rich: fayalite | |||
| orthopyroxene | (M)2(SiO3)2 | Mg: enstatite | (Mg,Fe): hypersthene |
| clinopyroxene | CaMg: diopside | Ca(Mg,Fe) augite | |
| plagioclase | CaAl2Si2O8 | ||
| NaAlSi3O8 | |||
| garnet | (Fe,Mg)3Al2(SiO4)3 | Fe-rich almandine; Mg-rich pyrope | |
| spinel | MgAl2O4 | (MgCr2): chromite | |
| quartz | SiO2 (extremely rare) | ||
The pyroxenes have different names in crust and mantle rocks because the mantle minerals have a much lower iron/magnesium ratio, meaning that they have a higher diopside (or enstatite) content than the crustal rocks.
Rock types
Basalt: On ridges mostly olivine tholeiites, described by major phenocryst phase, e.g., Plag-Phyric basalt, Plag-Cpx phyric basalt, etc.
Figure X: IUGS nomenclature of gabroic rocks (ODP Leg 153 Initial Results, Explanatory Notes)
Common lower crustal rock types:
norite, gabbronorite, gabbro, olivine gabbro, oxide gabbro, troctolite.
Cumulate rocks expected but never (or rarely) found on the ocean floor:
Clinopyroxenite, websterite, anorthosite, harzburgite, lherzolite, dunite.
They are found in ophiolites.
Residual rocks:
Lherzolite, harzburgite dunite.
These are melting residues, as such they are not properly igneous rocks at all!
Other Rocks:
Rodingite: Calcium carbonate- rich metamorphic/metsaomatic rock
Amphibolite: High grade altered basalt.
Greenstone: Low grade altered basalt.
Mylonite: All rock types, affected by significant tectonic strain.