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.

DaG 476 - Zoned Olivine Megacryst

This twinning in pyroxene is a result of shock.  Field of view is 1.2 mm wide.

DaG 476 Shock twinning in pyroxene

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.

DaG 476 - Olivine Undulatory Extinction