Can thin films of amorphous calcium carbonate in travertine be stained to differentiate from opaline deposits. Can ultra small crystallites of vaterite within ACC be differentiated by staining?
Hello,
As far as I know, the ACC (amorphous calcium carbonate) has not yet been described from the travertines. There is no known abiotic natural formation for ACC, but a biomineralisation component may be present in travertines. Therefore, the presence of the ACC can obviously not be completely ruled out, it would be interesting. ACC is highly reactive and would quickly be converted into calcite or vaterite. I don't have much experience coloring carbonates, but the assumption is that they're all going to color similarly, the ACC hopefully faster.
ACC is observed in some hot spring tufa/travertine deposits in western Canada. For example, see Jones B. and Peng X. (2012) Amorphous calcium carbonate associated with biofilms in hot springs. Sedimentary Geology, v. 269, p. 56-68. One should observe ACC in biotic fabrics of travertines, which are not always abiotic.
I'm not questioning the possibility of ACC biomineralization in travertines at all. There's a stability problem with the ACC. The chemically unstabilized ACC lasts in a truly amorphous state for minutes, hours at the most. Among other things, there is no direct evidence in the cited text that spherical particles are really ACCs. Except morphology, then. I know, of course, that it's extremely complicated.
I read the question again, and I'm sorry. I forgot to mention how to differentiate opal and ACC. An easier procedure than dyeing, in my opinion, will be EDS mapping on polished samples.