Page 35 - Petrophysics 2E

MINERAL CONSTITUENTS OF ROCKS-A REVIEW 9

(b) The other series of crystallization that is taking place simultaneously
forms minerals that are compositionally distinct. The reaction series
(olivine-pyroxene-amphibole-biotite) is discontinuous; thus the
reaction between crystals and the melt occurs only during specific
periods of the cooling sequence.
(c) After all of the ferro-magnesium minerals and plagioclase are formed,
the third series of minerals begins to crystallize as the melt continues
to cool slowly. First potassium feldspar precipitates, followed by
muscovite and finally quartz [7-93.

The Bowen series of specific crystallization occurs only for some
basaltic magmas (a variety of different reaction series occurs within
different melts), but the processes discussed by Bowen are significant
because they explain the occurrence of rocks with compositions
different from that of the original melted magma.

METAMORPHIC ROCKS

The metamorphic rocks (about 14% of all rocks) originate from
mechanical, thermal, and chemical changes of igneous rocks [ 101.
Mechanical changes on or near the surface are due to the expansion
of water in cracks and pores, tree roots, and burrowing animals. If the
igneous rocks undergo deep burial due to subsidence and sedimentation,
the pressure exerted by the overlying rocks, shear stress from tectonic
events, and the increased temperature result in mechanical fracturing.
When unequal shear stress is applied to the rocks as a result of continental
motion of other force-fields, cleavage of the rocks (fracturing) occurs;
alternatively, slippage of a regional mass of rocks and sediments (faulting)
occurs. The pressure produced by overlying rocks is approximately
1.0 psi per foot of depth (21 kPa per meter of depth). The changes
induced by overburden pressure occur at great depth in conjunction
with other agents of metamorphism.
Chemical metamorphosis of igneous intrusive rocks, aided by high
pressure, temperature, and the presence of water, results in chemical
rearrangement of the elements into new minerals. This produces foliated
rocks with regularly oriented bands of mineral grains because the new
crystals tend to grow laterally in the directions of least stress. This
chemical metamorphism of granite yields gneiss: a foliated granite with
large recognizable crystals of banded feldspars. Gabbro changes to
amphibolite, whose main constituent is the complex mineral known as
hornblende.
The chemical metamorphosis of the extrusive rocks, rhyolite, basalt,
etc., produces changes to easily recognizable rocks. Rhyolite, light