Page 92 - A Practical Introduction to Optical Mineralogy

SILICATE MINERALS FELDSPAR GROUP

(a) Albite twin
+- --+

a----

a + a'
Figure 2.15 extinction angle = - -
2
2V variation in
plagioclase (b) Combined Carlsbad-albite twin
feldspars. Mol % An
right- hand left-hand
half half
during solidification of the rock mass or of chemical weathering. Other
minerals which may form from feldspars during late stage hydrothermal
activity include the epidote mineral zoisite, or clinozoisite, which i~
produced during an alteration process called saussuritisation (see also
the epidote group minerals).
BIREFRINGENCE Low, with interference colours varying from first order greys (Ab) to
first order yellows (An).
INTERFERENCE 2V is generally large and variable in sign with the data for high and low
FIGURE series shown in Figure 2.15. The OAP orientation varies from albite to
anorthite, and Phillips and Griffen (1981) should be consulted for
details. smaller extinction larger extinction
*TWINNING Multiple twinning by the albite law is common in all plagioclase feld a + a' b + b'
angle = - - angle= - -
spars and is a characteristic feature. Albite twin lamellae, which tend to 2 2
be parallel to the prism zone, often tend to be narrow in the Na II ou • 2. 16 Measurement of extinction angles in (a) albite twin, (b) combined Carlsbad-albite
plagioclases and alternating narrow and broad in the Ca-plagioclascs. I Ill
Other twin laws which operate in plagioclases include Carlsbad (simple)
and pericline (repeated), with combinations of twins common, such a~ found in ultrabasic igneous plutonic rocks (troctolites, peridotites) and
Carlsbad-albite. Figure 2.16 gives a combined Carlsbad-albite twin , some very basic extrusive types. Figure 2.18 shows the curves needed to
showing symmetrical extinction in each half of the twin. Sedimentary obtain the composition of a combined twin in plutonic rocks, with each
. authigenic plagioclase feldspars (albites) and sodic plagioclases in low half of the Carlsbad twin being examined separately. Thus the smaller
grade metamorphic rocks may be untwinned .
symmetrical extinction angle of one half of the twin is plotted along the
*EXTINCTION Composition of plagioclases may be determined by measuring the sym ordinate and the larger symmetrical extinction angle of the oth'er half of
metrical extinction angles of albite twins measured on sections at righl the twin is plotted on to the curves. The composition is then read off
angles to the a crystallographic axis. Figure 2.17 gives full details of th along the abscissa. No other twin types are commonly used in determin-
variation in maximum extinction angle with composition, for both high ing composition.
(extrusive rocks) and low (hypabyssal and plutonic rocks) plagioclase: •toNING Common in plagioclase feldspars from extrusive rocks. Normally the
feldspars. Combined Carlsbad- albite twins may also be used for detc1 zoning shows up as a continuous change in composition from a calcium-
mination of composition, but these combined twins are usually onl rich core to a sodium-rich margin. In this case the composition should be
80 81