Page 22 - A Practical Introduction to Optical Mineralogy

THE MICROSCOPIC STUDY OF MINERALS SYSTEMATIC DESCRIPTION OF MINERALS
removing the upper analyser from the optical train) and the mineral
the field of view. Sometimes a series of coloured ovals will appear,
arranged about a point on the isogyre, especially if the mineral section is grain rotated until a cleavage trace or crystal trace edge or twin plane is
very thick or if the mineral birefringence is very high. The stage is then parallel to the crosswires in the field of view. The position of the
rotated until the isogyre is in the 45° position (relative to the crosswires) microscope stage is again noted and the difference between this reading
and concave towards the NE segment of the field of view. In this position and the former one gives the extinction angle of the mineral grain.
Several grains are tested since the crystallographic orientation may vary
the isogyre curvature can indicate the size of the optic axial angle (2V) of
and the maximum extinction angle obtained is noted for that mineral.
a mineral. The more curved the isogyre the smaller the 2V. The curva-
ture will vary from almost a 90° angle, indicating a very low 2V (less than The results of measurements from several grains should not be aver-
aged.
10°) to 180o when the isogyre is straight (with a 2V of 80° to 90°). When
the 2V is very small (less than 10°) both isogyres will be seen in the field Extinction angles are usually given in mineral descriptions as the
of view, and the interference figure resembles a uniaxial cross, which angle between the slow (y) or fast (a) ray and the cleavage or face
breaks up (i.e. the isogyres move apart) on rotation. The first order red edge (written as y or a·cl), and this technique is explained in detail in
accessory plate (length slow) is inserted and the colour noted on the Chapter 4.
concave side of the isogyre: In many biaxial minerals the maximum extinction angle is obtained
from a mineral grain which shows maximum birefringence such as, for
example, the clinopyroxenes diopside, augite and aegirine, and the
blue means that the mineral is positive ( +ve)
yellow means that the mineral is negative ( -ve) monoclinic amphiboles tremolite and the common hornblendes. How-
ever, in some minerals the maximum extinction angle is not found in a
If the accessory plate is length fast (as mentioned in the preceding section showing maximum birefringence. This is so for the clinopyrox-
section) the colours above will be reversed, that is a yellow colour will be ene pigeonite, the monoclinic amphiboles crossite, katophorite and
positive and blue negative (see Fig. 4.20). arfvedsonite, and a few other· minerals of which kyanite is the most
important (see also Ch. 4, Section 4.10).
Extinction angle Throughout the mineral descriptions given in Chapter 2, large varia-
Anisotropic minerals go into extinction four times during a complete tions in the maximum extinction angle are shown for particular minerals.
360° rotation of a mineral section. If the analyser is removed from the For example the maximum extinction angles for the amphiboles
optical train while the mineral grain is in extinction, the orientation of tremolite-actinolite are given as between 18° and 11° (y·cleavage).
some physical property of the mineral, such as a cleavage or trace of a Tremolite, the Mg-rich member, has a maximum extinction angle be-
crystal face edge, can be related to the microscope crosswires. tween 21° and 17°, whereas ferroactinolite has a maximum extinction
All uniaxial minerals possess straight or parallel extinction since a angle from 17° to 11°. This variation in the extinction angle is caused
prism face or edge, or a prismatic cleavage, or a basal cleavage, is mainly by variations in the Mg: Fe ratio. Variation in extinction angles
parallel to one of the crosswires when the mineral is in extinction. are common in many minerals or mineral pairs which show similar
Biaxial minerals possess either straight or oblique extinction. chemical changes.
Orthorhombic minerals (olivine, sillimanite, andalusite, orthopyrox-
enes) show straight extinction against either a prismatic cleavage or a Twinning
prism face edge. All other biaxial minerals possess oblique extinction, This property is present when areas with differing extinction orienta-
although in some minerals the angular displacement may be extremely tions within the same mineral grain have planar contacts. Often only a
small: for example, an elongate section of biotite showing a basal cleav- single twin plane is seen, but in some minerals (particularly plagioclase
age goes into extinction when these cleavages are almost parallel to one feldspars) multiple or lamellar twinning occurs with parallel twin planes.
of the microscope crosswires. The angle through which the mineral has
then to be rotated to bring the cleavages parallel to the crosswire will Zoning
vary from nearly oo to 9° depending on the biotite composition, and this Compositional variation (zoning) within a single mineral may be ex-
angle is called the extinction angle. pressed in terms of changes of 'natural' colour from one zone to an
The maximum extinction angle of many biaxial minerals is an import- adjoining one; or by changes in birefringence; or by changes in extinc-
ant optical property and has to be precisely determined. This is done as tion orientation. These changes may be abrupt or gradational, and
follows. A mineral grain is rotated into extinction, and the angular commonly occur as a sequence from the core of a mineral grain (the
position of the microscope stage is noted. The polars are uncrossed (by early-formed part) to its edge (the last-formed part).
10 11