Readers of this blog, I am sure, are familiar with terms like Agate, Jasper, Onyx, and Opal. Out of these, Opal is an amorphous variety of silica, where the silica and oxygen atoms are not attached to each other in a regular repeating geometrical pattern. Agate, Jasper, Onyx are varieties of silica that can show gradations from an amorphous form to being cryptocrystalline i.e. made up of tiny crystals. All these substances originate by hardening of a silica gel that congeals out of a silica supersaturated fluid which has separated from a magma, or from hydrothermal groundwater that has become enriched in silica by reaction with surrounding rock or soil.
They occur as banded siliceous deposits, either as layers or as discrete nodules, in volcanic and sedimentary rocks. Amorphous silica can even be of biogenic origin. Planktonic creatures like Radiolarians have the ability to extract silica from sea water and use it to build its skeleton.
Posted below is a photomicrograph of a cavity in a sandstone filled with banded amorphous silica (center of picture). Notice the regular growth bands (left in plain polarized light) and the silica fibers (in crossed nicols) that make up the fabric of the amorphous material. I happen to have this rock thin section in my collection , but unfortunately I don't know its provenance!
Transitions from amorphous to a fully crystalline silica (quartz) often occurs within the same rock cavity. Amorphous silica is quite stable and has been found well preserved in rocks hundreds of millions of years old.
In contrast, amorphous naturally occurring varieties of calcium carbonate seem to be exceedingly rare. In my more than two decades of following literature of sedimentary carbonates I have not come across a report of amorphous calcium carbonate cement or shell material. Until recently that is!
In the December 2020 issue of Geology, Sascha Roest-Ellis, Justin V. Strauss and Nicholas J. Tosca suggest that certain types of microspar cements in Tonian age Neoproterozoic limestones (~650 million years old), formed from an amorphous precursor stage. These microspar cements (fine grained calcite) are quite common in rocks of this time period, yet their mineralogical evolution and the geochemical conditions under which they formed is poorly understood.
In an effort to understand the origin of these cements, synthetic sea water was prepared of a composition that was similar to that measured from fluid inclusions trapped in Neoproterozoic salt deposits. The finding was that the presence of PO4 above a value of 12 micromoles per liter inhibits the nucleation of crystalline forms of calcite and permits deposition of an amorphous Ca-Mg- Carbonate by production of dense liquid droplets once carbonate supersaturation exceeds a threshold value. Neoproterozoic sea water was rich in PO4 as evidenced by the trapped fluid inclusion composition and by calcium phosphate biomineralizing organisms of that age.
The texture and chemistry of these microspars also suggest an amorphous precursor. The crystals have spheroidal cores which are likely remnants of immiscible liquid/gel particles that would have initially separated out from carbonate saturated sea water. The grain size distribution points to crystal growth by Ostwald ripening, a process whereby smaller gel particles or droplets disaggregate and the chemicals are reconstituted into larger growing crystals. Furthermore, these calcites have an enhanced strontium content. Usually that occurs if they have originated from an earlier aragonite phase. But there is no sign of relict aragonite in these cements. An alternate explanation is the incorporation of strontium into an amorphous carbonate which also favors intake of strontium.
The amorphous phase does not exist today in these limestones, having recrystallized to a variety of calcite fairly rapidly, perhaps even within a few days or weeks of it forming. The photomicrograph below shows the microspar calcite hypothesized to have recrystallized from an earlier amorphous phase.
Source: Experimental constraints on nonskeletal CaCO3 precipitation from Proterozoic seawater - Sascha Roest-Ellis, Justin V. Strauss and Nicholas J. Tosca, 2020.
As it happens I've had two strikes in the past month. Subir Sarkar and colleagues in their analyses of the Cretaceous age Garudamangalam Sandstone from Ariyalur in Tamil Nadu mention that some cavity filling calcite cements developed from a gel, by which I assume they mean amorphous calcium carbonate. But they don't pursue this aspect any further in their study.
The absence of amorphous calcium carbonate is limestones, both ancient and recent, is likely because it forms under only very restricted chemical conditions where nucleation of aragonite and high magnesium calcite is inhibited by the presence of ions like PO4, and because its high reactivity results in it transforming quickly to crystalline calcite, erasing itself from the rock record.
Geological discovery relies heavily on direct observation and measurements of rock/mineral material. But what about ephemeral substances? How does one imagine them and tease out their history? This study highlights the importance of experimental work in geology, where careful laboratory reconstruction of past conditions can throw light on mineralization pathways that have left no physical trace behind.
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