|
 Beautifully
crystallized well-shaped crystals typically are exceptionally rare and occur
only when crystal growth is unimpeded by growth of other crystals. This is
most likely to occur when mineral nucleation rates are low and the few crystals
that do nucleate grow in gas- or liquid-filled cavities, fractures or
spaces. Most igneous minerals crystallize from liquid magma, but
crystallization of other later formed minerals and continued chemical reaction
with the surrounding melt typically interfere with the shape and stability of
early well-formed crystals.
Cavities or spaces are
produced by expanding of gases or volatile-rich solutions as a result of
decompression (decreasing pressure) in igneous rocks emplaced or erupted near or
at the Earth's surface. These holes are called miarolitic cavities in
plutonic rocks. Similar crystal-lined cavities or spaces also occur in
lavas and pyroclastic volcanic rocks. The cavities and fractures that
occur in sedimentary rocks are typically caused by localized solution of the
soluble carbonate or evaporite rock. If the cavities are beneath the water
table or partially filled with water, precipitation of well-formed 
minerals may occur. Botyoidal mineral aggregates are typical of
precipitation of minerals from evaporating aqueous solutions above the water
table. The botyoidal mass of the copper mineral azurite (shown on the
left) probably formed in an air-filled cavity. Solution and secondary
recrystallization are important processes in the supergene
enrichment of copper and other economic ores. Solution of unstable
minerals creates void space that may be partially filled in by well-formed or
botryoidal aggregates of secondary minerals precipitated by low temperature
aqueous solutions, depending upon whether the cavity is water- or
air-filled. This is most likely to occur in arid or semi-arid
climates.
Solution of metamorphosed
carbonates can also occur, but the creation of open space in metamorphic rocks
is more typically the result of small volumetric changes resulting through the
replacement of the original mineral assemblage by a lower volume, denser mineral
assemblage under conditions of increasing pressure or fracturing. Although
metamorphic minerals exclusively crystallize in the solid state, a small amount
of water increases reactivity, diffusion rates, and crystal growth rates.
There are some metamorphic minerals such as garnet, staurolite, and andalusite
that often crystallize in extremely well-formed crystals.
Replacement deposits of
minerals may occur as a result of contact metamorphism or the action of
hydrothermal 
solutions infiltrating unstable rocks. The beautiful crystals of benitoite,
neptunite, and natrolite (upper left photograph) occur in veins in a brecciated
(fractured) serpentinite near the San Benito River in California.
Benitoite (BaTiSi3O9) and neptunite (KNa2Li(Fe,Mn)2TiO2(Si4O11)2)
only occur in a few other localities in the world, presumably because of their
unusual compositions. At low temperature this can occur in
sedimentary rocks. Mississippi Valley Type replacement mineralization
occurred when metal-bearing brines from sedimentary basins react with carbonate
melts. The sphalerite, chalcopyrite, and quartz in the specimen to the
right may have formed in this way. They may also have precipitated from a
higher temperature hydrothermal solution.
 Well-formed
crystals can occur in hydrothermal solution veins if vugs or open fluid-filled
spaces exist after crystallization ceases. The well-shaped smoky quartz
crystal in the right top corner of this page precipitated from silica-bearing
hydrothermal solutions. The same is also likely to be the case for the
well-formed transparent, colorless quartz crystals shown to the left. The
orange coloring on parts of this specimen is due to iron-oxide staining on the
surface of the quartz.
| 
