Abstract

Denticulated margins (also known in older literature as “sawtooth,” “cockscomb,” or “hacksaw” terminations) are a common feature of pyroxenes and amphiboles, visible in optical and electron microscopes. Denticles are remnants of undissolved material that formerly constituted the walls between elongate etch pits, the characteristic aqueous-dissolution form of chain-silicate minerals. Denticles occur widely in terrestrial near-surface materials that have experienced low-temperature aqueous alteration, including chemically weathered regoliths, soils in a variety of climatic and geomorphic settings, sediments, and sedimentary rocks. Ranges of dissolution forms and dimensions (commonly tens of microns in length) are identical on both pyroxenes and amphiboles in these materials.

Denticles on terrestrial chain silicates are formed by low-temperature aqueous alteration. Denticles on a soil mineral grain (of unknown composition) at the Phoenix Mars Lander site and on terrestrial pyroxenes naturally weathered in terrestrial low-temperature aqueous environments are broadly similar in size, shape, and orientation relationships between denticles. However, quantitative morphometric parameters vary among different terrestrially weathered pyroxenes and even between different images of compositionally identical pyroxenes from the same naturally weathered sample; such variations are too large to allow simple identification of minerals of unknown composition from morphology alone. Nevertheless, the overall morphological similarities are consistent with a low-temperature aqueous corrosion origin of denticles at the Phoenix Mars Lander site.