Volcanic Surface Coatings on Apollo 15 Green Pyroclastic Glass Beads

Apollo 15 green and Apollo 17 orange and black beads are the most common volcanicc glasses in the Apollo collections. They formed as a result of fire-fountain eruptions during the same time period as most mare basalt volcanism. Earlier SEM and chemical studies of tiny surface features on the glasses demonstrated that both types of pyroclastic glass have similar coatings of submicrometer micromounds and tiny crystals of soluble salts. We have begun a study of volcanic glass surface coatings in order to constrain the nature and distribution of the surface coatings, to help define the conditions and history of lunar pyroclastic volcanism, and characteristics of the deep lunar interior. We are using high-resolution field-emission TEM (FE-TEM) and EDS X-ray mapping, along with field-emission SEM (FE-SEM) and EDS. Studies of samples in D. McKay's allocations are under way. Pristine samples will also be investigated. FE-TEM X-ray maps demonstrate that green glasses have Si-deficient layered surface deposits, ~20-150 nm thick. Layering has not been previously documented. The inner layer typically consists of amorphous material rich in Fe and minor S while the outer layer seems to be primarily Fe (no detectable S). In places the innermost layer also contains Zn which appears to be correlated with S. Volcanic surface deposits are still present in the non-pristine samples (exposed to air ~30 years ago). They contain complex textures and chemistries, and layering is heterogeneous at the nanometer scale. The composition, stratigraphic sequence, and structure of the vapor-deposited nanolayers may contain information on the eruption sequence, the plume and deposit time-temperature regime, and the composition and source of the volatile species. Previously documented water-soluble crystals on the glass surfaces have not yet been identified; pristine samples will likely be needed to find them.