New Mexico Geological Society Annual Spring Meeting — Abstracts

Hydromagnesite, a platy hydrated magnesium carbonate mineral commonly forming sticky pastes (moonmilk) in dolostone-hosted caves, is the primary component of a rare speleothem type referred to as a cave balloon. Hydromagnesite cave balloons form from moonmilk and have an interior likely carbon oxygen. A carbon dioxide interior would be counter-productive to carbonate mineral growth. Hydromagnesite moonmilk may be closely associated with microbial activity helping to explain the source for a gas-filled balloon interior. To further understand the origin of this rare and fascinating speleothem, we conducted a cross-section examination of a hydromagnesite balloon wall using high-resolution transmission electron microscopy (HRTEM). HRTEM reveals fibrous amorphous-like material between the hydromagnesite plates. Elemental analysis reveals that this material contains silicon. It is known that magnesium silicates such as trioctahedral smectite form in magnesium-rich cave environments. In addition, in laboratories hydromagnesite has been used as a template for the synthesis of smectite. We believe that the amorphous-like material between the hydromagnesite platelets may be a hydrated magnesium silicate like trioctahedral smectite. Such a material could contribute to the formation of hydromagnesite balloons in two ways: (1) material could act as a lubricating medium allowing the platelets to move with expansion and contraction of the balloon, and (2) the material could provide a shield for the hydromagnesite from the possibly corrosive nature of the balloon's interior environment.