January 9, 2004
Petrology 310.
Extinction Angles
purpose of measuring extinction angles
provides quick estimates of compositions in certain mineral series
e.g., mol% An (anorthite) in plagioclase solid solution
allows distinction between orthorhombic and monoclinic members of a group
e.g., hypersthene vs. pigeonite or augite
allows distinction between amphiboles and pyroxenes
e.g., hornblende vs. augite, both common in many igneous and metamorphic rocks
definition of extinction angles
angle between a crystallographic and an optical direction
most commonly measured between a cleavage and an extinction position Fig. 7.31
measurement of this angle in thin section has minimal value
neither the cleavage nor the extinction will have a special orientation
need special orientation for extinction angle to be useful
orientation usually obtained with a principal figure (BXa, BXo, ON)
sometimes with a specific orientation relative to cleavages (normal to one or two cleavages)
how can one determine that a cleavage is vertical in thin section??
examples of cleavage(s)
micas: {001} cleavage(s) - perfect: how many??
calcite: {1011} cleavage(s)- perfect: how many??
halite: {001} cleavage(s)- perfect: how many??
hornblende: {110} cleavage(s) (perfect), pyroxene: {110} cleavages (very good): how many??
feldspars: (001) perfect, (010) very good ("second cleavage")
why are only two cleavages obtained with (001), (010) in feldspars??
types of extinction relations
parallel extinction: extinction position parallel to cleavage Fig. 7.32a
inclined extinction: extinction position inclined to cleavage Fig. 7.32b
symmetrical extinction: extinction bisects two cleavages (us. symmetrically related) Fig. 7.32c
no cleavage (or none seen in a given viewing plane) Fig. 7.32d
Pyroxenes and Amphiboles
observations of {110} cleavages for pyroxenes and amphiboles
orthorhombic monoclinic (Fig. 7.34)
orientation extinction extinction
(100) parallel parallel (Fig. 7.34c)
(110) parallel inclined
(010) parallel inclined (Fig. 7.34d)
(hk0) parallel inclined
(001) symmetrical symmetrical (Fig. 7.34b)
(h0l) symmetrical symmetrical
(hkl) inclined inclined (Fig. 7.34e)
In most optics courses one is taught that orthorhombic minerals only have parallel extinction, and that
monoclinic minerals only have inclined extinction. This is erroneous. (Why??) In oils this is usually
correct for amphiboles (why??), but not in thin section.
3D drawings of pyroxenes p. 76, Tröger
3D drawings of amphiboles p. 88-89, Tröger
Z c in monoclinic pyroxenes p. 77, Tröger
Z c in monoclinic amphiboles p. 93, Tröger
Alkali Feldspars and Plagioclases
plagioclase feldspar 3D drawings p. 268, Nesse
plagioclase feldspar optics vs. composition Figs. 233-12, 233-13, Tröger (p. 126-127)
provides quick estimates of compositions in plagioclase solid solution, %An (anorthite)
determinations use twinning or observation of special orientation of grains
common plagioclase feldspar twin laws Fig. 12.11
name type composition plane
carlsbad simple (010) Fig. 12.11c
albite repeated (010) Fig. 12.11a
carlsbad-albite both (010)
pericline repeated normal to b axis Fig. 12.11b
avoid use of twins not parallel to cleavage for plagioclase determinations
repeated twins = "polysynthetic twinning” Fig. 12.12
plagioclase determinative curves using extinction angles with grain mounts in oils
(1) grains lying on cleavages (001) and (010)
advantage: simple, rapid; doesn't require conoscopic measurement
nearly independent of structure variations for most plagioclases
disadvantage: loss of textural information, can confuse (001) and (010) cases
plagioclase determinative curves using extinction angles in thin section
(2) Michel-Lévy method Figs. 12.15, 12.16
advantage: doesn't require conoscopic observations
disadvantage: requires multiple measurements, may give multiple answers
(3) carlsbad-albite method Figs. 12.17, 12.18
uses complexly twinned plagioclases
advantage: doesn't require conoscopic observations
disadvantage: complex graph, requires multiple measurements, may give multiple answers
(4) grains normal to the a axis
grains with vertical (001), (010) cleavages
advantage: doesn't require conoscopic observations, need only one measurement
disadvantage: requires careful searching for properly oriented grain
(5) centered bisectrix method Fig. 12.37, Heinrich (1965)
grains with centered BXa or BXo figures
advantage: need only one measurement
disadvantage: requires conoscopic observations, centered bisectrix figure
3D drawings of alkali feldspars
optical features of alkali feldspars
"perthite" (exsolution of albite lamellae) Fig. 12.9
"myrmekite" (wormy intergrowths of quartz and feldspar) Fig. 12.10
"tartan" (plaid) twinning, common in microcline (pericline and albite twins) Fig. 12.20b
sanidine (untwinned, often shows orthoscopic dispersion) Fig. 12.20d