Spectro 186-312B, Francis 2013
LAB 9 - OPTICAL DETERMINATION OF PLAGIOCLASE COMPOSITIONS.
This week’s lab focuses on the plagioclase feldspars. The accurate determination of the composition of
plagioclase feldspar is an essential part of any petrographic study because the An content of plagioclase
decreases systematically with crystal fractionation during igneous differentiation, and thus many
igneous classification schemes are in part based on the An content of plagioclase. A review of the
crystal and optical properties of plagioclase is presented in the following pages, followed by a
description of several Methods for the estimation of plagioclase compositions in thin sections.
Tasks:
1) Determine the plagioclase composition in one of sections 8.3.n that you examined last week in
section 1 of Lab 8.
2) Determine the composition of the plagioclase phenocrysts in one of sections Al-nnn.
3) Determine the composition of the plagioclase microlites in one of sections PY-nn
4) The plagioclase crystals in sections SF-20 & SF-31 are strongly zoned. Estimate the maximum
variation in An content between the cores and rims of these plagioclase crystals in one of these
sections. These best way to do is to measure two sets of extinction angles, one for the center of
an appropriate plagioclase grain, and the other for the rim.
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Spectro 186-312B, Francis 2013
Feldspars
The determination methods that follow usually involve the measurement of the angle between a
vibration direction (commonly Nsmall (N')) and the trace of some crystallographic plane (such as a
cleavage or twin composition plane) for a grain of known orientation. This requires a detailed
knowledge of the crystallographic features of the plagioclase feldspars that include:
Cleavage:
Plagioclase has 2 directions of cleavage that are almost at right angles to each other. One (001)
is perfect, while the other (010) is distinct. Both may be observed in any one grain. If only one
cleavage trace is observed, it is more likely to be the better developed (001), however, grains
whose orientation results in sections (plane of the microscope stage) parallel to (001), only the
(010) cleavage will be seen.
Twinning:
The plagioclases are twinned according to at least 7 twin laws. The three most common (80-90
% of all feldspar twins) types of twinning are:


Albite twins:
a)
Polysynthetic twinning with twin plane parallel to (010) and perpendicular
to the (001) cleavage. Contact plane (CP) is the twin plane.
b)
Twin lamellae (CP) parallel to the typical direction of elongation ('a' crystallographic
axis) of lath-shaped crystals.
c)
Twin lamellae are length fast except for An contents greater than An 75.
d)
45% of twins for igneous feldpars, 65% of twins for metamorphic
feldspars.
Pericline twins:
a)
Polysynthetic twinning with twin axis parallel to the 'b' crystallographic axis [010], but
whose CP varies in orientation about b depending on feldspar composition.
b)
Twin lamellae may be length slow or length fast.
e)
Twin lamellae less continuous than those of albite twinning, commonly pinch and
swell.
d)
10% of twins in igneous felds, 25% of twins in metamorphic feldspars.
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Spectro 186-312B, Francis 2013

Carlsbad twins:
a)
Simple contact twin with twin axis parallel to the ‘c’
crystallographic axis [001], CP parallels (010) and the
Albite twin lamellae.
b)
CP parallel to the typical direction of elongation of lath
shaped crystals.
Twin lamellae are length fast except for plagioclases
with An greater than 75.
c)
25% of twins in igneous felds, 4% of twins in
metamorphic feldspar.
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Spectro 186-312B, Francis 2013
c
axis
Plagioclase:
b
axis
a
axis
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Spectro 186-312B, Francis 2013
Methods for the Determination of Plagioclase Composition
I. Thin Section
For best results, it is always advisable to use several of the methods given below and compare results.
For homogeneous feldspars they should agree within ~ 5%.
In many of the following methods, there is an ambiguity in the sign of the extinction angle. The
simplest method to remove this ambiguity is to compare the refractive index of the feldspar with that of
coexisting quartz (Ne = 1.55, Nw = 1.54) or the glue of the thin section (N = 1.54). Feldspars with
Nsmall less 1.54 have An contents less than An 20 and vice versa. Comparisons with the refractive
index of the glue of the thin section (n = 1.54) are best done along the edge of the petrographic slide,
with the feldspar grain at the extinction position in which Nsmall parallels the microscope’s polarizer. In
addition, in the case of plutonic rocks, feldspars with An between 20 and 30 are optically negative (-),
while those with An less than 15 are optically positive (+). Mineral association is also a good guide;
rocks with feldspars greater than An 20 have higher mafic to felsic mineral ratios compared to those
with feldspars with An less than 20. The later are typically characterised by the obvious presence of
one or more of quartz, orthoclase, or nepheline.
An additional complication arises because the optical properties of plagioclase depend on the degree of
Si - Al ordering as well as composition. Unless otherwise stated, most of the determinative curves
apply to low temperature plagioclases and are strictly applicable only to plutonic rocks. If you are
dealing with plagioclases from volcanic rocks, use only curves which may be applied to high
temperature plagioclases.
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Spectro 186-312B, Francis 2013
METHOD 1 : Michel-Levy Method – maximum extinction angle in the zone perpendicular to (010).
a) Recognition:
i) Albite twins present and have equal illumination when
parallel to the crosshairs.
ii) extinction angles on either side of a twin plane are
approximately equal.
ii) twin compositional planes are sharp and do not move
sideways if the focus is varied, i.e., they are vertical.
iii) twins show equal illumination eight times during a
complete rotation of the stage.
b) Procedure:
i) find the largest angle between N' and (010) by trial of a
number of grains. Tolerance of a few degrees is permissible in the
orientation of the grain. If not perfectly oriented, average the
readings for the individual lamellae.
Look up the composition on the curve in the following figure. Test
the mean refractive index of the plagioclase against quartz or glue and
remove the ambiguity in the determination. For crystals less calcic
than about An75, the angle to N' is less than 45, ie. the lamellae
are length fast. For plagioclase more An-rich than An75, the angle
to N' is greater than 45 degrees and the lamellae are length slow.
c) Range:
Useful for plagioclase of every composition.
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Spectro 186-312B, Francis 2013
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Spectro 186-312B, Francis 2013
METHOD 2: Carlsbad-Albite Method - section perpendicular to (010) with both Carlsbad and
Albite twins.
a) Recognition:
b) Procedure:
Same as in Method 1. Note - determine (010) is vertical by observing
Albite twin composition planes. Carlsbad composition planes are often
irregular.
i) measure two sets of symmetrical (±2o)
extinction angles of albite twins, one set on each side of the
Carlsbad contact plane. Use the average of the two smaller angles
and the average of the two larger angles.
ii) read the smaller of the two angles on the dashed curves of
the chart; read the larger angle on the solid curves.
intersection gives the composition of the plagioclase.
c) Range:
Useful for all plagioclase compositions.
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Their
Spectro 186-312B, Francis 2013
Carlsbad-Albite Method
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Spectro 186-312B, Francis 2013
METHOD 3: Microlite Method - maximum extinction angle in the zone parallel to a.
a) Recognition:
b) Procedure:
Microlites in volcanic rocks which are elongate parallel to a.
i) no orientation is required.
ii) measure the angle on each grain between its elongation and
N'(almost always less than 45 degrees). Find the largest of
these 'smaller' angles and look up the composition using the
microlite curve in the figure below.
c) Range:
Good for all plagioclase in volcanic rocks although precision is
poor for maximum extinction angles in the range 0 to 12 degrees.
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Spectro 186-312B, Francis 2013
Sources of Error in Plagioclase Determination in Thin Section:
a)
Effect of orthoclase in solid solution with sodic plagioclase. Considered to be negligible insofar
as it affects the optics of plagioclase determination.
b)
Inaccuracies in the extinction curves. Portions of some curves are poor, but by using several
methods of determination no great error will remain undetected. Curves for low temperature
plagioclase apply to all plutonic and most dike rocks. Curves for high temperature plagioclase
should be used only for volcanic rocks.
c)
Inaccuracy of measurement of extinction angles. Average error need not be more than 1 degree,
which gives an error in composition of plagioclase of 1-2% An.
d)
Inaccuracy of orientation of sections used for determination. This error is several times greater
than the previous one. Depending on the method used, errors of 5% An are to be expected. This
compares favourably with the errors to be expected using the immersion methods, where 5%
discrepancy in An content is not excessive.
e)
Intermediate states of Al-Si order in plagioclases of sub-volcanic and volcanic rocks.
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