Lunar sample 12002,320.
This low-Ti basalt is typical of the younger mare lavas. High-Ti lavas (9 to 13% TiO2) are generally confined to the period 3.85 to 3.55 Ga, whereas the low-Ti lavas (1 to 5% TiO2) are most common in the period 3.45-3.15 Ga (Meyer, 1987). This basalt is thought to have been derived from an olivine-pyroxene source rock at a depth of ~300 km. The sample was collected from the southeastern end of Mare Procellarum by the Apollo 12 mission.
Magnification key 20x = 8 mm image width 40x = 4 mm image width 100x = 1.6 mm image width 200x = 0.8 mm image width 400x = 0.4 mm image width 500x = 0.32 mm image width |
20X, plane polarized light.
This rock contains mostly brownish elongate pyroxene, colorless lath-shaped plagioclase, colorless blocky olivine, and opaques. The abundance of elongate pyroxene and plagioclase suggests rapid cooling.
20X, cross polarized light.
In this view the elongate colorful minerals are mostly pyroxene, and the blocky ones mostly olivine. They can actually be distinguished better by color in plane light in the image above.
40X, plane polarized light.
Closeup of a some skeletal olivine crystals encased in masses of pyroxene and plagioclase.
40X, cross polarized light.
The skeletal olivines include the pink-and-orange crystal in the center, and the bluish grain in the upper right in which four masses of melt were trapped. Notice how the ends of the skeletal arms are rounded, suggesting partial resorption following skeletal growth.
40X, plane polarized light.
Closeup of elongate pyroxene, just left of center, and a skeletal olivine crystal to the right of center. Notice how the brown color of the pyroxenes becomes darker toward the margins. This change follows increasing iron and titanium contents near the pyroxene rims.
40X, cross polarized light.
The chemical zoning in the pyroxenes and olivines changes their birefringence (both minerals) and optic orientation (pyroxene only). This causes rims of these minerals to be a different birefringent color than the cores.
100X, plane polarized light.
Closeup of radiating bundles of pyroxene, suggestive of high cooling rates.
100X, cross polarized light.
Pyroxene interference color zoning is clearly seen here.
200X, reflected light.
Chromite occurs in this sample, suggesting it is one of the more primitive lavas. In olivine, chromite is pristine. In the matrix, chromite is overgrown (partially replaced by?) ulvospinel. In this image, the dark gray shades are silicates. The medium gray laths are ilmenite, the light gray in the center of grains is chromite, the pinkish-brown overgrowths on the chromite are ulvospinel, the creamy brown bits are troilite, and the bright white grains are iron. Notice how the chromite grain embedded in olivine (right) is euhedral and not overgrown with ulvospinel.
500X, reflected light.
Closeup of a chromite overgrown (replaced?) by ulvospinel. Colors are as above. Notice the large bright white native iron grain, and the small specks of iron in the triangular troilite grain toward the lower right.