Citrine
is a quartz with a yellow included color.
Crystals are usually stubby. Can also be found
as crystals in a geode. Most citrine are heat treated amethyst
or sometimes smokey quartz
that is heat treated. The difference is in the color. Heat treated
amethyst is more reddish yellow. Natural citrine is more subdued.
Naturally occuring citrine is relativaly scarce. We at the mineral
gallery has some intersting natural clusters from Namibia.
It closely assembles topaz and is sometimes
wrongly labelled as madeira topaz. Citrine is widely used as a gem,
and is the most valuable Quartz gem.
Quartz crystallizes in the trigonal
trapezohedral class of the rhombohedral subsystem of hexagonal
symmetry. The quartz symmetry class lacks a center of symmetry or
planes of symmetry. The (c) crystallographic axis is perpendicular to
three polar axes (a) separated by 120 degrees in a plane. Because the
polar axes differ on each end, the application of mechanical stress
to such an axis produces electrical charges of opposite sign at each
end (piezoelectricity); conversely, applied electrical fields produce
mechanical stresses. The piezoelectric property makes quartz valuable
in pressure gauges, electronic frequency-control devices, radios, and
other applications. Most quartz crystals are twinned, although this
may not be visible to the eye. The polar axes and enantiomorphism
permit complex twinning.
When heated above 573¡ C (1,063¡ F)
at 1 bar (14.50 lb/in6) of pressure, quartz assumes higher symmetry
as the threefold c axis becomes sixfold. This hexagonal form is known
as high quartz or Beta quartz. The transformation temperature is pressure-dependent,
increasing by approximately 25 C degrees (45 F degrees) per kilobar.
When high quartz is cooled below the inversion temperature, inversion
to low quartz occurs rapidly. The first mild heating of quartz
commonly will be accompanied by the emission of light
(thermoluminescence), and irreversible color changes may occur in
colored varieties. For example, amethyst may be transformed to
citrine at 250¡ C (482¡ F) or higher. Vigorous rubbing of
one quartz crystal by another may also produce visible light (triboluminescence).
