Page 32 - Petrophysics 2E

MINERAL CONSTITUENTS OF ROCKS-A REVIEW 7

complex and their chemical formulas differ in various publications; in
such cases the most common formula reported in the list of references
was selected.

IGNEOUS ROCKS

Igneous rocks (about 20% of all rocks) are the product of the
cooling of molten magma intruding from below the mantle of the crust.
Igneous (plutonic) rocks are divided into three easily recognizable rocks,
which are subdivided by the rate of cooling (Figure 1.1). The granites
are intrusive rocks that cooled slowly (at high temperature) below
the surface, whereas gabbro is a rock resulting from more rapid (low
temperature) cooling in the subsurface. Diorite is a rock that cooled
below the surface at a temperature intermediate between granite and
gabbro. The minerals differentiate during the slow cooling, forming large
recognizable, silica-rich crystals with a rough (phaneritic) texture.
The second classification is extrusive (volcanic) rock that has
undergone rapid cooling on or near the surface, forming silica-poor
basaltic rocks. Rhyolite, or felsite, is light colored and estimated to be
produced on the surface at a lower temperature than the darker andesite
that formed at a temperature intermediate between that of rhyolite and
the dark-colored basalt. As a result of rapid cooling on the surface, these
rocks have a fine (alphanitic) texture with grains that are too small to be
seen by the unaided eye [5].
Minerals precipitating from melted magma, or melt, do not crystallize
simultaneously. Generally, a single mineral precipitates first and, as the
melt cools slowly, this is joined by a second, third, and so forth; thus the
earlier-formed minerals react with the everchanging melt composition.
If the reactions are permitted to go to completion, the process is called
equilibrium crystallization. If the crystals are completely or partially
prevented from reacting with the melt (by settling to the bottom of
the melt or by being removed), fractional crystallization takes place
and the final melt composition will be different from that predicted by
equilibrium crystallization. The mechanism by which crystallization takes
place in a slowly cooling basaltic melt was summarized by Bowen [6] as
two series of simultaneous reactions; after all of the ferro-magnesium
minerals are formed, a third series of minerals begins to crystallize from
the melt. From laboratory experiments Bowen discovered that the first
two series of reactions have two branches:

(a) The plagioclase grade into each other as they crystallize; the crystals
react continually with the melt and change composition from an
initial calcium plagioclase crystal to sodium plagioclase.