Igneous rock in thin section

[...]... monoclinic orthoclase and sanidine, and develop like this only during inversion to triclinic microcline The triclinic twin domains can nucleate with the triclinic angles leaning one way or another, but once they start that is how the twin grows Because many twin domains nucleate, there are large numbers of thin, spindly and discontinuous domains in the final microcline product Because the grid twinning... 20x Kinsman Microcline from a peraluminous granite Note the "grid" or "tartan plaid" twinning pattern that results from crossing albite and pericline twin domains These are formed during the change from monoclinic high temperature orthoclase to triclinic low temperature microcline, associated with progressive ordering of aluminum and silicon during cooling Albite and pericline twins are impossible in. .. an inversion texture, it shows that the original feldspar was once monoclinic Microcline crystals that grow below the inversion temperature do not have grid twinning like this Cross polarized light, 20x Kinsman 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... and generally continuous twin domains in plagioclase Note the different spacing of the twin domains This shows that different volumes of the crystal nucleated different numbers of twin domains during inversion Cross polarized light, 100x Kinsman Perthite from a metaluminous biotite granite Note the faint, irregular stripes that run from the upper right to lower left In this case, the exsolved albite... The microcline and albite both exsolved (unmixed) from an originally homogeneous high temperature of intermediate composition In this photo the thin section was rotated to obscure the twinning Cross polarized light, 20x 4.7.84H 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... 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 Biotite, peraluminous granite This graphite- and garnet-bearing granite has high-Ti,... 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 Myrmeckite Myrmekite patch that appears to be replacing microcline, though its appearance is obscure in plane-polarized light Plane polarized light, 40x Kinsman Myrmekite patch that appears to be replacing microcline Twins in the myrmekite clearly... NEIGC84-A5-6 Muscovite, peraluminous granite Cross polarized light, 40x NEIGC84-A5-6 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,... 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 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 in this soft mineral during thin. .. 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 Perthite from a metaluminous biotite granite Same as the image above in cross polarized light Cross polarized light, 100x 4.7.84H Perthite from a metaluminous biotite granite Closeup showing the characteristic grid twinning in . pericline twins are impossible in monoclinic orthoclase and sanidine, and develop like this only during inversion to triclinic microcline. The triclinic twin domains can nucleate with the triclinic. leaning one way or another, but once they start that is how the twin grows. Because many twin domains nucleate, there are large numbers of thin, spindly and discontinuous domains in the final. IGNEOUS ROCK IN THIN SECTION Contents Image field widths Quartz Plagioclase Potassium feldspars Myrmeckite Micas Amphiboles Pyroxenes Other rock- forming primary minerals Accessory minerals Secondary