Dolerite is the medium-grained member of the basic igneous rocks.
That is to say, it has a grain size of 1-5mm and typically an SiO2
content in the range 45-57%. It is thus compositionally equivalent to
the basalt (a fine-grained basic rock) and gabbro (a coarse-grained
basic rock). The family definition is based on normative mineral
calculations. In current international recommendations the medium-
grained rocks shoud be named after the coarse-grained (plutonic)
rock with a micro- prefix. However, because of their widespread
use, the terms dolerite and diabase (the American equivalent) are
still acceptable rather than the term microgabbro. Because grain
size is related to rate of cooling, the medium-grained basic igneous
rocks will occur in small to medium sized intrusions, such as dykes
and sills.
Dolerites are therefore neither acid nor usually strictly volcanic!
Cheers
Tim
--
Dr Tim Young
Email: [log in to unmask]
Web: www.geoarch.co.uk
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