Page 95 - A Practical Introduction to Optical Mineralogy

FELDSPAR GROUP
SILICATE MINERALS

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Mol % An 1 albite 1 oligoclase 1 andesine 1 labradorite I bytownite I anorthite
Figure 2.17 Maximum extinction angles for albite twins in high and low plagio Figure 2.18 Graph for extinction angles of Carlsbad-albite twins. The extinc-
clase feldspars. tion angle for an albite twin is measured in each half of a Carlsbad twm. The
smaller angle is plotted along the ordinate and the larger angle into the nest of
curves (see Fig. 2.16 for details of measurement of albite twin extinction angles).
given as, for example, An 70 (core) to An, 2 (margin). If the zoning i~
Thus, for example, a Carlsbad-albite twin with angles of extinction. of lOo. (the
reversed (sodium-rich core to calcium-rich margin) the precise compo
smaller) and 30° (the larger) has a composition of An 60 • The negative ordm~te
sition variation should again be given, or if the zoning is oscillator ,
values (below the horizontal line representing 0°) are needed for feldspars whtch
where separated zones of equal extinction occur, an indication of tlw
have refractive indices of less than 1.54.
zonal variation should be given.
occuRRENCE Plagioclase feldspars are almost always present in igneous rocks (with seen optically. Anorthosites contain plagioclase feldspars as the chief
the exception of some ultramafic and ultra-alkaline types) and oft 'II constituent, comprising well over 80 % of the volume of the rock. The
comprise more than half the rock's total volume. Plagioclase varies 111 plagioclase composition varies from bytownite to andesine, although
composition with the type ofrock it is found in ; thus bytownite occur~ 111 with any particular anorthosite intrusion the compositional range is
ultrabasic rocks and labradorite in basic rocks, andesine is typical nl quite small. _
intermediate rocks, and oligoclase is common in acid rocks. Pure albite is the typical feldspar of spilites, often with relict cores of a
In basic lavas, calcium-rich plagioclase feldspars occur both 1 more anorthitic plagioclase. This may indicate a late stage magmatic or
phenocrysts and as constituents of the groundmass. In basic pluto1111 metasomatic process by which the original feldspar in the basalt
intrusions, layering and differentiation can occur, with feldspar-1 k h becomes increasingly sodium rich, a process called albitisation. How-
layers common. In these intrusions plagioclase may show a compn ever, in some spilites the albite may be a primary crystallising mineral.
sitional range from An., to An, 0 and is frequently zoned. The ' low' 1 n metamorphic rocks, the composition of the plagioclase reflects the
plagioclase found in plutonic rocks is often antiperthite, particular! 111 m tam rphic grade of the rock , the plagioclase becoming more calcium-
acid types, with exsolved alkali feldspar (K-feldspar). In other plutOII h ri has the grade increases. Albite is the typical plagioclase of low grade
rocks, especially those with a long cooling history, pc rist 'lil t• r tiona I rocks, with oligoclase occurring at garnet grade. In granulites
(Fig. 2.10) may occur in which Schiller effect can be een. T h 11111 1 ll llll ·harnockitcs, andesine or rarely labradorite is the common plagio-
basic intrusions may show either B0ggild inte rgrowths, ca us ·d hv ·lus ·. Pia io lase feldspars do not occur in eclogites, the various feldspar
unmixing of two plagioclase component , or Huttenl o h r int r I'OW I h , 'tllllpon nts ( o, A I t .) ntering either the clinopyroxenes or garnet
phos • pr s nt. Pur · uno thit mn o ur in therm ally metamorphosed
which occur in bytownites in which two basi pia io Ius ·o mpon ' Ill
unmix; however, both these intergrowth typ s or mr und 1r 1111 ·I ~ l' II 'll ll'll ll ~ I I ·k ·
'