Page 159 - Laboratory Manual in Physical Geology

Hand sample
x2

Plagioclase feldspar crystals

Pyroxene
crystals

Photomicrograph ( 26.6)
Original sample width is 1.23 mm

FIGURE 5.9 Gabbro (mafic, intrusive, phaneritic). Gabbro—a mafic, phaneritic igneous rock made up chiefly of ferromagnesian and

plagioclase mineral crystals. The ferromagnesian mineral crystals usually are pyroxene (augite). Quartz is absent.
Hand sample
Photomicrograph ( 26.6) (actual size)
Original sample width is 1.23 mm
Ferromagnesian
mineral
crystals
Plagioclase
feldspar
crystals

Glass

FIGURE 5.10 Basalt (mafic, extrusive, aphanitic). Basalt is a mafic, aphanitic igneous rock that is the extrusive equivalent of gabbro, so it is
dark gray to black. Microscopic examination of basalts reveals that they are made up chiefly of plagioclase, ferromagnesian mineral crystals, and glass.
The ferromagnesian mineral crystals generally are pyroxene, but they also may include olivine or magnetite. Basalt forms the floors of all modern
oceans (beneath the mud and sand) and is the most abundant aphanitic igneous rock on Earth.

Plagioclase

Hornblende
Hand sample (actual size)

Hand
sample
x2

FIGURE 5.11 Diorite (intermediate, intrusive, phaneritic).
Diorite—an intrusive, phaneritic igneous rock that has an FIGURE 5.12 Andesite (intermediate, extrusive, aphanitic).
intermediate MCI and is made up chiefly of plagioclase feldspar Andesite—an intermediate, aphanitic igneous rock that is the
and ferromagnesian mineral crystals. The ferromagnesian mineral extrusive equivalent of diorite. It is usually medium gray. This sample
crystals are usually amphibole (hornblende). Quartz is only rarely has a porphyritic-aphanitic texture, because it contains phenocrysts
present and only in small amounts (<5%). of black amphibole (hornblende) set in the aphanitic groundmass.

140 ■ L ABOR ATORY 5