Page 83 - A Practical Introduction to Optical Mineralogy

SILICATE MINERALS FELDSPAR GROUP

An intergrowths (roughly from An 40 to An 60 ); and Huttenlocher inter-
Figure 2.10 Feldspars
formed after slow growths, from An 70 to An 85 •
(prolonged) cooling. The B~ggild intergrowths can contain up to 6 % Or and are dis-
tinguished by labradorite iridescence (caused by the structure).
Although the individual feldspar types will be described in detail, the
general optical properties for feldspars are given below.
coLou R Colourless with occasional white or pale brown patches where alteration
to clay minerals has occurred.
HAB IT Euhedral crystals - tabular, or prismatic with large basal faces - may
occur aS" phenocrysts in some extrusive rocks, but most feldspars are
either subhedral (prismatic) or anhedral in most rocks.
, , EA VAGE All feldspars possess two cleavages { 001} and { 010}, intersecting nearly
at right angles on a (100) section. Several partings may occur.
KELIEF Low from just below 1.54 (K-feldspar) to above 1.54 (most plagio-
clases). Figure 2.14 gives details.
I ll· RAT ION All feldspars may alter to clay minerals. The individual descriptions give
details.
low Maximum interference colours are first order white in Ca-poor plagio-
albite Or clase, and first order yellow in Ca-rich plagioclase.
!'i ll MII· REN CE Variable in sign and size. Usually large, so that a single optic axis figure is
I·I(;U RES
often required for examination.
I IINCIION Figure 2.12 gives details for alkali feldspar. Plagioclase feldspars show
Na-sanidine represents a structural change from triclinic to monoclinic. repeated twinning, and the symmetrical extinction angles measured on
The boundary at 70% Or separating Na-sanidine from K-sanidine is an the twin plane are used to obtain plagioclase composition. Figures 2.16
arbitrary one. and 2.17 show these extinction angles, but the relevant section in the
In Figure 2.9 the feldspars formed after short cooling histories (fast plagioclase feldspar descriptions must be consulted for details of this
rate of cooling) show unmixing with perthite development. Homo- technique.
geneous feldspars (similar to those found in Fig. 2.8) occupy small areas !W INN ING K-rich alkali feldspars exhibit simple twinning, but the plagioclase feld-
in Figure 2.9, with K-feldspar (or orthoclase) restricted to a small field . spars show polysynthetic twinning or repeated twinning or complex
Most alkali feldspars are perthites, consisting dominantly of an akali multiple twins.
feldspar host with an exsolved plagioclase feldspar phase resulting from / O N ING Common in most plagioclase feldspars, particularly in phenocrysts in
segregation from an original higher temperature homogeneous feldspar. extrusive rocks.
When plagioclase feldspar is dominant the unmixed feldspar is called an 1'1 KIII ITES Feldspars frequently show effects of unmixing or exsolution resulting in
antiperthite. Some perthites consist of roughly equal amounts of inter- intergrowths- plagioclase feldspars within alkali feldspar host and vice
grown alkali feldspar and plagioclase, and these are called mesoper- versa.
thites (Fig. 2.9). Submicroscopic cryptoperthites may occur, for
example in orthoclase, which tend to give the host mineral optical
properties between it (the host) and the exsolved phase.
Figure 2.10 shows the feldspar fields after prolonged cooling histories
such as will occur in large plutons or in metamorphic rocks. K-feld par
is restricted to low microcline and homogeneous plagioclases ar •
restricted to nearly pure albite, anorthite An 85 _ , 00 and the compositional
range from An 15 to An 70 , with all these having no more than aboul
2 mol % Or in the structure. The non-homogeneous types consist of
complex series of intergrowths of which three are important: peristeritcs
(containing equal amounts of alkali feldspar and plagioclase) ; B~ggild
70
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