Alkali olivine basalt is silica-undersaturated, characterized by the absence of orthopyroxene,
absence of quartz, presence of olivine, and typically contains some feldspathoid mineral,
alkali feldspar or phlogopite in the groundmass. Feldspar phenocrysts typically are
labradorite to andesine in composition. Augite is rich in titanium compared to augite
in tholeiitic basalt. Alkali olivine basalt is relatively rich in sodium.
The definition of tholeiite and alkali basalt here are more prescriptive than those
found in most reference authorities. This is to actually provide some descriptive
criteria to allow assignment of rocks on a hand sample basis to the tholeiite or alkali
basalt categories if detailed petrographic or chemical data are available.
http://en.wikipedia.org/wiki/Basalt; Carmichael, I.S. Turner, F.J., Verhoogen, John,
1974, Igneous petrology: New York, McGraw HIll Book Co., p.42-43.
Metamorphic rock mainly consisting of green, brown or black amphibole and plagioclase
(including albite), which combined form 75 percent or more of the rock, and both of
which are present as major constituents. The amphibole constitutes 50 percent or more
of the total mafic constituents and is present in an amount of 30 percent or more;
other common minerals include quartz, clinopyroxene, garnet, epidote-group minerals,
biotite, titanite and scapolite.
Coutinho et al. 2007, IUGS SCMR chapter 8 (http://www.bgs.ac.uk/SCMR/)
Fine-grained igneous rock with less than 20 percent quartz and less than 10 percent
feldspathoid minerals in the QAPF fraction, in which the ratio of plagioclase to total
feldspar is greater 0.65. Includes rocks defined modally in QAPF fields 9 and 10 or
chemically in TAS field O2 as andesite. Basalt and andesite, which share the same
QAPF fields, are distinguished chemically based on silica content, with basalt defined
to contain less than 52 weight percent silica. If chemical data are not available,
the color index is used to distinguish the categories, with basalt defined to contain
greater than 35 percent mafic minerals by volume or greater than 40 percent mafic
minerals by weight. Typically consists of plagioclase (frequently zoned from labradorite
to oligoclase), pyroxene, hornblende and/or biotite. Fine grained equivalent of dioritic
Note the mela-andesite and leuco-basalt categories are not recommended in this system.
If chemical analytical data are available to constrain the silica content, the basalt
or andesite category should be used.