Page 81 - A Practical Introduction to Optical Mineralogy

SILICATE MINERALS

Figure 2.7 Or II 1111• 2.8 An
Feldspar Ill •h
composition trlprlnture
diagram.
"'""''·

high labradorite
TWO FELDSPARS

high oligoclase /
/
/
/
.....
..... ..... K-high albite
(- monalbite)
Ab An
example in quickly cooled extrusive igneous rocks, the potassium fcld
spar that forms has a tabular crystal habit and is called sanidine, with
optical properties peculiar to that mineral. However, when plutonu
igneous rocks crystallise, their cooling rate is much slower, which cou
An
trois the kinetics of order-disorder in the feldspar structure, lead in • to primitive
the formation of orthoclase, with a prismatic habit and slightly differ ·nt anorthite
optical properties from those of sanidine.
Much work has been carried out on the feldspar minerals in rcc ·ut
years, and Smith (1974) summarised the differences between the diff ·r
ent feldspar types. Figures 2.8, 2.9 and 2.10 illustrate the compositiou
and nomenclature of the feldspars at various temperatures. The t 1 111
' temperatures' refers to the temperatures at which effective structtuul
re-equilibration ceases, and the mineral structure will not be subje t 1!1
any change thereafter. It is a time-temperature-kinetic relationshrp
The feldspars are divided into three groups - high (representing •
trusive rocks), intermediate (representing hypabyssal rocks and smull
intrusions) and low (deep seated plutonic rocks and metamorphn
rocks).
Figure 2.8 shows feldspars quenched from high temperature. /\11
arbitrary boundary from albite (Ab) with anorthite (An) equal Itt low sanidine
orthoclase (Or) defines the alkali feldspar and plagioclase feldspnr perthites (orthoclase)
fields. The plagioclase feldspars are divided into six divisions at 1 0, JO, low albite·~~~~::,.__::~:::,_::::,__;:,._...o.___,__...o.___,__...o.___,____,____,_...o._.::-...., ..... ..o~.
Ab Or
50, 70 and 90 mol per cent An, and the feldspars in these divisions ll ll'
named on the diagram as (high) albite0-10% An, oligoclase 10-30% /\11,
andesine, labradorite, bytownite and anorthite 90-100% An. In th •
alkali feldspars, the boundary at 40% Or between high albite and hi •h
68