DaG 476 Olivine-Phyric Shergottite
Paired with DaG 489, 670,
735, 876, 975 and 1037
This Martian basalt contains large olivine phenocrysts in a
groundmass of pyroxene and feldspathic glass. The olivine
phenocrysts are aligned in the direction of flow, here, generally
left/right. Minor minerals and brownish melt pockets are
present.
Field of view is 8.5 mm wide.
Many of the olivine phenocrysts are zoned. The rims are
higher in iron than the cores, possibly from having been formed in an
earlier iron-rich melt (in which case they would be xenocrysts, foreign
crystals). Field of view is 2.1 mm wide.
This
twinning in pyroxene is a result of shock. Field of view is 1.2
mm wide.
This
olivine crystal exhibits the shock feature
called undulatory or undulose extinction. A non-shocked crystal
would
darken - go to optical extinction - all at once as crossed polarizing
filters
were rotated in unison, one above and one below it. This crystal
darkens
(and
lightens) unevenly and indistinctly because impact shock pressures have
disrupted
its
internal structure. Fields of view are 2.1 mm high.
