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/)