Page 91 - A Practical Introduction to Optical Mineralogy

SILICATE MINERALS FELDSPAR GROUP

Plagioclase feldspars All other plagioclases occur between these, with nomenclature:
End members high/low albite (Ab) NaAISi 3 0 8 all triclinic
high/low anorthite (An) CaA1 2 Si 2 0 8 0.820::!: : 1.295::!: : 0.720::!: An mol % 0 10 30 50 70 90 100
a = 93°30'::!:, {3 = 116°::!:, y = 88°-91° Ab oligoclase andesine labradorite bytownite An
Low albite (An 0 ) Labradorite (An 50 )
There is continuous variation in the principal Rls of plagioclase feld-
c (3 c spars from Ab to An, similar to the alkali feldspars, and this is given in
I
I I I Figure 2.14.
I I I
I I I 2V is variable depending on the structural state and the composition,
and Figure 2.15 shows the curves for variation of 2V for high (solid line)
and low (dashed line) plagioclase feldspar.
D = 2.62-2.76 H = 6-61/2
coLOuR Colourless, occasionally near opaque because of clay development.
HABIT Subhedral prismatic in plutonic and hypabyssal rocks, to euhedral pris-
matic or tabular in extrusive rocks.
LEAVAGE Similar to alkali feldspars. Two perfect cleavages {001} and {010}
meeting at nearly right angles on the (100) plane. Several partings may
occur.
RELIEF Low, but apart from albite all plagioclase feldspars have Rls greater
than 1.54.
"' 1 F. RATION Already partly dealt with under alkali feldspars, but it is worth repeating
here that plagioclase feldspar alters either to montmorillonite with
limited water available, or kaolin if excess water is available. This
alteration may be the result either of late stage hydrothermal activity

c
.!2
ti
~
~
0
><
<U
Anorthite (An,,.,) "0
c

Ab An
n. 1.527 1.577
n,, 1.5 I 1.585 Mol 'X, An
ll y 1.5 9 I. 90
I<IMur" . 14 I ~ I Vll l lllllon In pill lo •I11s f' ldspu rs untl gluss mo I from th s
'
0.01 2 0.01 . ll ld l ll