Evolution of Ephemeral Phosphate Minerals on Planetary Environments
Chemistry and Biochemistry
The detection of ammonium-bearing compounds in meteorites, comets, and in Earth’s geologic record is challenging due to the volatilization of ammonia during heating. Struvite (MgNH4PO4·6H2O) is an ammonium-bearing phosphate mineral considered to be relevant to the origin of organophosphates on the early Earth, and it is possible that this mineral may have formed on the early Earth and in meteorites in favorable environments. However, in contrast to other phosphate minerals such as those within the apatite mineral group, there is little evidence of struvite on the early Earth and no detection of it in meteorites, where such high-N (nitrogen) and low-H2O conditions may be more commonplace. Here, we demonstrate that struvite quickly loses ammonia and transforms into a new suite of minerals; hence, this mineral is ephemeral. This ephemerality is demonstrated by the thermal decomposition reactions of struvite that lead to the mineral newberyite (MgHPO4·3H2O), an acidic phosphate mineral. Both struvite and newberyite transform into magnesium pyrophosphate and magnesium triphosphate, which are the final products of thermal decomposition (T > 200 °C). However, magnesium pyrophosphate itself reacts with calcium-bearing minerals such as calcite or gypsum and transforms into orthophosphate minerals and polyphosphate salts. Such reactions could have occurred in meteorites as well as on the early Earth. The present research helps identify how ephemeral—but prebiotically relevant—minerals may be lost from the geologic record, but still could have played a role in the development of life.
ACS Earth and Space Chemistry
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