Although
this is a paleontological rather than a mineralogical society we do often
encounter crystals (calcite, dolomite, quartz and pyrite) in veins, vugs, and
occasionally in the fossils themselves. But where do they come from? What is
their origin?
Most
rocks are composed of minerals which are in crystal form. The main difference
is whether the crystals are measured in angstroms, microns, millimetres or metres.
The size of the crystals is determined by the conditions needed for crystal
growth when the rock was formed.
Many
of the crystalized minerals we collect are formed by hydrothermal solutions
precipitating out minerals in cracks and other voids in any type of rock
existing rock. The host rock can be any of the major types; igneous,
metamorphic or sedimentary. Given the right conditions, large and pure
crystals of minerals can form in any parent rock. Crystals are an intrinsic
property of all minerals. The only meaningful question would be whether the
crystal growth occurred in an igneous, metamorphic, or sedimentary environment.
Extrusive
igneous rocks (e.g. lava) usually cool much too quickly for significant
crystals to form. Intrusive igneous rocks (e.g. granite) can cool slowly
enough for substantial crystals to form. Igneous rocks can also host crystals
of many minerals (including some quartz, such as that in geodes from Brazil or
Mexico), and also crystals of gem minerals like topaz, beryl, tourmaline, and
smoky quartz, which form in pegmatites.
Metamorphic
rocks can sometimes allow significant crystals to form, albeit small, because
the higher pressure makes metamorphic rocks denser, with fewer open cavities;
examples are garnet, kyanite, and staurolite.
Sedimentary
rocks typically don't allow for much crystal growth during their creation, but
once formed probably have a greater number of crystals that other types.
Sedimentary rocks, being nearer to the surface (therefore under less pressure)
and often being more porous and also more water-soluble, are the most likely
kind to have open spaces and cavities which provide places where crystals can
grow.
The
crystals found at Mont Saint Hillaire in Quebec were formed when that part of
the continent went over a hot spot in the earth’s crust and a magmatic
intrusion forced its way up through the carbonate rocks. The interaction of
the hot acidic magma with the alkali carbonate rock produced a plethora of
different crystals.
Another type of
sedimentary rock, Evaporite deposits, constitute much of the Silurian rock of
North America, formed when a sea is evaporating. This often forms large
crystals due to the high concentrations of dissolved saltwater constituents
left behind. Therefore, many of the most common well-crystallized minerals
(much of the calcite, gypsum, dolomite, quartz, pyrite, and halite) have formed
in sedimentary rocks.
Many
minerals can occur in all types of host rock; lets take quartz for example, we
find both clear and smoky quartz in early Ordovician limestone in some of the
quarries south-east of Ottawa, whereas Brazilian amethyst occurs in geodes in
volcanic rock. These geodes tend to be long and narrow branched pipes- it has
recently been discovered that they are the hollow remains of trees that were
buried in lava from an erupting volcano. Colorado smoky quartz occurs in
pegmatites which are massive igneous magma intrusions into the earth’s crust ,
and the quartz crystals associated with pyrite, galena, and other ore minerals
from many mines worldwide are mostly of the "hydrothermal vein"
origin which don't fit any of the three rock types.
Robert Sensenstein (With help from Pete Modreski
and Art Bergreen from the Rocks and fossils newsgroup)
