* The plagioclase feldspar
solid-solution series displays three miscibility gaps at low temperature:
(1) peristerite in sodic compositions; (2) Bøggild in intermediate
compositions; and (3) Huttenlocher in calcic compositions. Upon slow cooling,
these “solvi” give rise to exsolution intergrowths of two
plagioclase feldspars with different compositions (and hence different
refractive indices). In most cases, the scale of the intergrowths is submicroscopic,
but such intergrowths can manifest themselves through fantastic coloration
called iridescence. The latter term is used to describe coherent scattering
of light from feldspar intergrowths of different refractive indices with
spacing on the scale of the wavelength of light. Labradorescence is the
name applied to colors caused by reflection from the two phases in Bøggild
intergrowths.
Labradorescence comes in all colors of
the rainbow, but the origin of such color variation is not entirely clear.
Some studies have demonstrated a relationship between iridescent colors
and original plagioclase composition as follows: blue = An48-52; green
to yellow = An52-55; and red = An55-59. Other studies claim that the color
relates more to the thickness of the submicroscopic lamellae, with thicker
lamellae producing colors at the red end of the spectrum, and thinner
lamellae yielding blue colors. In either case, labradorescent plagioclase
is beautiful to behold in outcrop and in hand-specimen, and has attracted
international attention since its discovery in Nain, Labrador in the 1770s.
(For a look at a specimen of iridescent
labradorite from the type locality at Nain, see Professor Dymek in room
135.)
[Note. Portions of the above text are paraphrased
from Smith, J.V. and Brown, W.L. (1988) Feldspar Minerals, 2nd Ed., Berlin-New
York-London: Springer-Verlag, 828 pp.]
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