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Figure 27
A. Euhedral crystal of plagioclase feldspar in potassically altered porphyry. A thin rind of
potassium feldspar has replaced the outer rim of the crystal. The field of view is about 2.3 x 3.4
mm. Crossed-polar light. DDH 318-246.7
B. Three crystals showing varying degrees of replacement. The one on the right displays only a
thin rind which does not extend around the entire perimeter of the crystal. Approximately half
of the central crystal has been replaced - a central core with ragged outer boundary is surrounded
by an outer pseudomorph of potassium feldspar after plagioclase feldspar. Potassium feldspar
has nearly entirely replaced the crystal of plagioclase in the lower left part of the photograph -
only tiny remnants of plagioclase remain (weak sieve texture). The field of view is about
3.4 x 2.3 mm. Crossed-polar light. DDH 318-246.7
C. Rectangular phenocryst of plagioclase feldspar that has been partly (about two-thirds)
replaced by pseudomorphic potassium feldspar. The field of view is about 0.8 x 1.2 mm.
Crossed-polar light. DDH 303-243
D. Phenocryst of partly replaced plagioclase feldspar that displays albite twins in its core. The
right side of the crystal has either: (1) never been replaced, or (2) been broken subsequent to
replacement. The field of view is about 0.7 x 1.0 mm. Crossed-polar light. DDH 318-246.7
E. Mottled replacement texture in plagioclase feldspar. Plagioclase (white) is replaced by
potassium feldspar (gray). At some locations (upper left) the potassium feldspar is partly
localized by visible fractures; at another location (lower right) potassium feldspar is seeming
unrelated to a fracture. The field of view is about 0.8 x 1.2 mm. Crossed-polar light. DDH
318-282.25
F. Advanced replacement of plagioclase by potassium feldspar (black). Only tiny remnants of
plagioclase (white; best seen in upper left side of phenocryst) remain. These show similar
extinction properties when the stage is rotated. DDH 316-314.7. The field of view is about
2.5 x 1.7 mm. Crossed-polar light.
G. Advanced replacement of plagioclase by potassium feldspar (dark gray). Tiny remnants as in B are scattered throughout the thin-section. These go to extinction approximately simultaneously as the stage is rotated. The field of view is about 2.3 x 3.4 mm. Crossed-polar light. DDH 302-101.3
