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Igneous rocks in thin section Contents Image scales (frame widths) Quartz

Plagioclase Potassium feldspars

Myrmeckite Micas

Amphiboles Pyroxenes

Other primary minerals and minor minerals

Secondary (subsolidus) minerals

20x = 6 mm 40x = 3 mm

100x = 1 mm 200x = 0.5 mm

400x = 0.25 mm 1000x = 0.1 mm

Quartz

Quartz crystal in alkali granite

Plane polarized light, 20x

NEIGC86-B2-7

2

Quartz crystal in alkali granite

Cross polarized light, 20x

NEIGC86-B2-7

3

Strained quartz crystal in a metaluminous granite. Strain has caused the quartz crystal to deform into domains with slightly different extinction angles.

Cross polarized light, 20x

DIG-D

4

Fluid inclusions in quartz in alkali granite. The inclusion in the center has an irregular outer boundary, inside of which is a layer of liquid water, a layer of liquid CO2, and a central bubble of vapor (mostly CO2). At the time of trapping of this secondary inclusion, the fluid was a binary H2O-CO2 fluid.

Plane polarized light, 1000x

NEIGC86-B2-7

5

Fluid inclusions in quartz in alkali granite. Several inclusions containing (probably) water and a central vapor bubble.

Plane polarized light, 1000x

NEIGC86-B2-7

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Plagioclase

Plagioclase, unzoned, in a hornblende diorite

Plane polarized light, 40x

NEIGC84-A5-5C

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Plagioclase, unzoned, in a hornblende diorite. Note the strong, parallel sets of albite twins, and the less visible set of pericline twins inclined almost at right angles to the albite twins.

Plane polarized light, 40x

NEIGC84-A5-5C

8

Plagioclase, zoned, in a dacite porphyry. This plagioclase appears quite homogeneous in plane light.

Plane polarized light, 100x

Rhyolite-2

9

Plagioclase, zoned, in a dacite porphyry. This plagioclase has fine oscillatory zoning, in which the composition varies between more and less anorthite-rich compositions. Internal unconformities followed by euhedral overgrowths are also visible.

Cross polarized light, 100x

Rhyolite-2

10

Plagioclase, zoned, in a dacite porphyry. Notice the concentric layers (zones) of inclusions in this crystal.

Plane polarized light, 40x

Rhyolite-2

11

Plagioclase, zoned, in a dacite porphyry. Notice that the concentric inclusion zones also have different birefringence, indicating these zones have different anorthite content. The interior of this crystal has patchy zoning rather than concentric, indicating skeletal early growth. Albite, carlsbad, and pericline twins cut across the composition zones.

Cross polarized light, 40x

Rhyolite-2

12

Potassium feldspars

Orthoclase in a dacite hypobyssal intrusive.

Plane polarized light, 40x

Py-28

13

Orthoclase in a dacite hypobyssal intrusive. Notice the two carlsbad twin domains, separated by the carlsbad composition plane.

Cross polarized light, 40x

Py-28

14

Microcline from a peraluminous granite.

Plane polarized light, 20x

Kinsman

15

Microcline from a peraluminous granite. Note the "grid" or "tartan plaid" twinning pattern that results from crossing albite and pericline twin domains that form during inversion of monoclinic orthoclase to triclinic microcline during cooling. This inversion and change in crystal system is associated with increasing ordering or aluminum in crystallographic sites.

Cross polarized light, 20x.

Kinsman

16

Microcline from a peraluminous granite. Closeup of the plaid twinning. Notice how the twin domains are spindly and somewhat wispy. This is in contrast to the straight and generally continuous twin domains in plagioclase.

Cross polarized light, 100x.

Kinsman

17

Perthite from a metaluminous biotite granite. Note the faint, irregular stripes that run from the upper right to lower left.

Plane polarized light, 20x.

4.7.84H

18

Perthite from a metaluminous biotite granite. Note the lighter irregular stripes and patches of albite that separate gray stripes and patches of microcline. The color difference is mostly due to the different optical orientations of the two different minerals. The microcline and albite both exsolved (unmixed) from an originally homogeneous high temperature of intermediate composition. The sample is oriented to obscure the twinning.

Cross polarized light, 20x.

4.7.84H

19

Perthite from a metaluminous biotite granite. The somewhat less altered and narrower albite exsolution lamellae are in sharp contact with a much larger microcline domains. You can see a lamella closeup here, and then see a Becke line test from focused to a lowered stage position at the lamella-microcline contact. The Becke line goes into the higher index phase, albite.

Plane polarized light, 100x.

4.7.84H

20

Perthite from a metaluminous biotite granite. Same as the image above in cross polarized light.

Cross polarized light, 100x.

4.7.84H

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Perthite from a metaluminous biotite granite. Closeup showing the characteristic grid twinning in the microcline host (upper center and upper left) and albite twinning in the lamellae (lower center to center right).

Cross polarized light, 200x.

4.7.84H

22

Myrmeckite

Myrmekite patch that appears to be replacing microcline.

Plane polarized light, 40x.

Kinsman

23

Myrmekite patch that appears to be replacing microcline. Faint twins in the myrmekite clearly shows that the probably quartz "worms" are in a plagioclase matrix.

Cross polarized light, 40x.

Kinsman

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Micas

Muscovite, peraluminous granite. Igneous muscovite is generally colorless with good cleavage. Radiation halos can be visible around radioactive inclusions (but not visible here).

Plane polarized light, 40x.

NEIGC84-A5-6

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Muscovite, peraluminous granite.

Cross polarized light, 40x.

NEIGC84-A5-6

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Biotite, metaluminous granite, showing several grains in different orientations. Grain on the far right is oriented with the cleavages N-S, and so is almost opaque. The largest grain is inclined and is lighter in color. Grains with the cleavages E-W have the least absorption (not shown in this image).

Plane polarized light, 40x.

4.7.84G

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Biotite, metaluminous granite. This is the same area as the image above in cross polarized light. The birefringent colors of the biotite are muted because of the color of the biotite itself.

Cross polarized light, 40x.

4.7.84G

28

Biotite, metaluminous granite, showing a closeup of one of the same biotite crystals above at extinction, occupying the entire center of the image. Damage produced to this soft mineral during thin section grinding causes speckles of light on the biotite, where the crystal lattice has been deformed. This means that biotite in standard thin sections rarely goes completely extinct. This is called "incomplete extinction" or sometimes "birds eye maple extinction".

Cross polarized light, 200x.

4.7.84G

29

Biotite, peraluminous granite. This graphite- and garnet-bearing granite has high-Ti, low-Fe3+ red-brown biotite. This patch of biotite rimming garnet (mineral occupying the lower part of the image) shows the range of pleochroic colors because of different crystals being in different orientations.

Plane polarized light, 20x.

Kinsman

30

Amphiboles

Green hornblende in a diorite.

Plane polarized light, 40x.

NEIGC84-A5-5C

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Green hornblende in a diorite.

Cross polarized light, 40x.

NEIGC84-A5-5C

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Green hornblende in a diorite. The ~120 and ~60 cleavage intersections are clearly visible in this end section of a crystal.

Plane polarized light, 100x.

4.8.84Q

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Brown hornblende, hornblende gabbro. The extensive dark and light brown areas are several hornblende crystals. Note the numerous inclusions of opaques and plagioclase.

Plane polarized light, 20x.

4.8.84Q

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Brown hornblende, hornblende gabbro, in the image above. The two hornblende orientations do not happen to yield the maximum interference colors, which happen to be second order blue in this section.. Note how plagioclase inclusions have more closely spaced twins than does the hornblende.

Cross polarized light, 20x.

4.8.84Q

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Pyroxenes

Enstatite (orthopyroxene, OPX) in norite. The large OPX in the center is oriented with its c crystallographic axis oriented N-S.

Plane polarized light, 100x

NEIGC83-C1-14

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Enstatite (orthopyroxene, OPX) in norite. The large N-S oriented enstatite grain near the center of the image (see image above) is extinct, in keeping with the orthorhombic symmetry of this mineral. Birefringence ranges to upper first order.

Cross polarized light, 100x

NEIGC83-C1-14

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Enstatite (orthopyroxene, OPX) in norite. Here the section has been rotated clockwise ~45 to show the birefringence of the large grain.

Cross polarized light, 100x

N