Evolution of Ephemeral Phosphate Minerals on Planetary Environments

Abstract

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.

Iron Silicides in Fulgurites

“…This fulgurite also bore reduced iron silicides, the first reported occurrence of such material in a Type I fulgurite, which to date remains the only report of iron silicide in sand fulgurites. However, we report here the presence of elemental silicon in a Type I fulgurite from Polk County, Florida (collection details in [15], Raman details in [5,42]), based on a match between the Raman spectra of material within the fulgurite and the known spectrum of Si (Figure 1). In such a fulgurite, it seems likely that iron silicide should also have formed, but due to the paucity of iron in the target material (which is nearly pure SiO 2 ), elemental Si was the only highly reduced phase formed.…”

“…Iron silicide minerals are hence "rarer" silicon sinks in meteorites. Considering that early Earth atmospheres may have been highly reducing [69], it is possible that iron silicide may have been present on early Earth a bit more frequently than present day and may have served as a prebiotic mineral during the first billon years of Earth's history [42,70,71], perhaps as a driver of reduction. We note here that most fulgurites do not bear iron silicide minerals, and have highlighted the literature occurrences of these materials to the best of our knowledge.…”

Oxidative Phosphorus Chemistry Perturbed by Minerals

“…Phosphorus species pertain to a great many minerals, of various physical properties and reactivities, valuable to prebiotic chemistries [4,5,[28][29][30][31][32]. Not only has reduced phosphorus been detected in meteorites, but also in rocks and minerals originating here on Earth.…”

A global network model of abiotic phosphorus cycling on Earth through time

“…The solids investigated included CaCO 3 , CaHPO 4 (monetite), CaHPO 4 × 2H 2 O (brushite), Ca(OH) 2 , Ca 3 (PO 4 ) 2 , Ca 5 (PO 4 ) 3 F and Ca 5 (PO 4 ) 3 OH (solid solution), CaSO 4 × 2H 2 O and CaSO 4 (solid solution), FeCO 3 , FeO, Fe 2 O 3 , Fe 3 O 4 , an iron oxyhydroxide solid solution system (consisting of Fe(OH) 2 , Fe(OH) 2 (brucite), Fe(OH) 3 , Fe 2 O 3 × H 2 O, FeO × OH, and FeO × OH (limonite)), FePO 4 × 2H 2 O (strengite), and iron sulfate solid solution system (consisting of Fe 2 (SO 4 ) 3 , FeSO 4 × H 2 O, FeSO 4 × 4H 2 O, and FeSO 4 × 7H 2 O), a liquid acid solution consisting of H 3 PO 4 , H 3 PO 4 × 0.5H 2 O, and H 2 SO 4 , MgCO 3 , MgFe 2 O 4 , MgO and Mg(OH) 2 (solid solution), Mg 3 (PO 4 ) 2 , MgSO 4 , Fe 3 (PO 4 ) 2 × 8H 2 O (vivianite), MgNH 4 PO 4 × 6 H 2 O (struvite), MgHPO 4 × 3H 2 O (newberyite), FeS and FeS 2 (solid solution), Fe, S, Na 2 S (added with H 2 S to adjust pH to 8), NaHCO 3 and Na 2 CO 3 (solid solution), a solid solution of chloride salts (NaCl, MgCl 2 , FeCl 2 , CaCl 2 , NH 4 Cl, and KCl), CaF 2 , and NaOH (to adjust pH as needed). Thermodynamic data for these species was used in the HSC module and was supplemented by data for struvite, strengite, vivianite, and newberyite from Feng et al 94 . Model results are shown in Figure SI11 .…”