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A. Rectangular subhedral phenocryst of biotite. Shows typical maximum pleochroism
(moderate brown). The biotite contains three inclusions of magnetite; a tiny diamond-shaped
phenocryst of sphene can be seen along the left upper surface of the biotite; and a small
subhedral phenocryst of pyroxene can be seen in the lower right corner of the photomicrograph.
The field of view is about 1.7 x 2.5 mm. Plane-polar light. Ok Tedi R15.
B. Typical anhedral flake of magmatic biotite in a sample with phaneritic texture. Contains
inclusions of magnetite. Dark brown pleochroism. The field of view is about 0.4 x 0.6 mm.
Plane-polar light. DDH 356-166.5
C. Ragged anhedral crystal of magmatic biotite in a sample with phaneritic texture. Contains
inclusions of magnetite. Dark brown pleochroism. The field of view is about 0.6 x 0.8 mm.
Plane-polar light. DDH 356-166.5
D. Magmatic mineral association of biotite (left, brown), pyroxene (center, pale green), and
magnetite (right, black). The field of view is about 0.6 x 0.8 mm. Plane-polar light. DDH
331-218.5
E. Anhedral, partly resorbed, phenocryst in weakly-altered porphyry. This photomicrograph
shows the maximum pleochroic color (dark brown) of magmatic biotites from the Ok Tedi
Intrusive Complex. The green fibrous mineral at the center is chlorite which has replaced
biotite. Two large phenocrysts of magnetite that were partly, or totally, enclosed by the biotite
are visible in the center of the photo. Additional small granular crystals of magnetite form a
fringe around parts of the perimeter of the biotite. The field of view is about 0.8 x 1.2 mm.
Plane-polar light. JDD-94-10
F. The same phenocryst of biotite shown in E. The crystal has been rotated to a position of minimum pleochroism (pale yellow brown). Plane-polar light. JDD-94-10
G. Pseudomorph of hydrothermal biotite after a phenocryst of magmatic ferromagnesian mineral. Identification of the original mineral is no longer possible by visual means. Patchwork texture consisting of an aggregate of tiny flakes of biotite and myriad crystals of an opaque mineral. One small crystal of sphene is present just to the left of the larger opaque inclusion. The field of view is about 2 x 3 mm. Plane-polar light. DDH 345-468
H. The same pseudomorph of hydrothermal biotite shown in G. Crossed-polarizers. DDH 345-468
