Geological origin of alabaster and rock salt deposits
How was alabaster first formed in nature? Why do deposits of the mineral occur in certain areas of the Volterra neighbourhood and along the crest of the hills facing the shore (especially at Castellina Marittima), yet not in other areas right next door to them (for instance, the hills of Livorno or the mountains around Pisa)?
Alabaster is a microcrystalline variety of chalk, from which it is derived via a complex process of structural alteration whose phenomenological development is still not fully known today. To describe the origin of alabaster we first of all have to consider the origin of chalk, a calcium sulphate hydrate whose chemical formula is CaSO4 + 2H2O.
From a mineralogical standpoint chalk is an evaporite rock, in other words it is one of those “rocks that form through the precipitation of seawater when salinity is concentrated through evaporation in at least partially enclosed basins.”
The climate conditions required for the formation of chalk deposits – i.e. the conditions needed to keep seawater at a relatively stable and relatively warm temperature – were precisely the kind of conditions found in the region that is now Italy, and Tuscany in particular, during the Late Miocene (roughly 7 to 8 million years ago); in other words, tropical conditions with high temperatures and low rainfall.
The same cannot be said, however– in the light of our present knowledge– of the process whereby chalk was transformed into alabaster ovules. In other words, there is no consensus of opinion or single explanation for the complex process underlying chalk’s transformation into alabaster.
The only thing that we can say with any certainty is that alabaster is formed through a process of “diagenesis,” a phenomenon which in many ways represents the opposite process to sedimentation. As we have seen, chalk is formed as a result of the evaporation of seawater in semi-closed basins in the presence of suitable climate and environmental conditions. The deposits thus formed sink due to the tectonic activity of the earth’s crust, and so it possible that chalk may subsequently turn into a solution caused by infiltrations in the chalk deposits and that it may therefore sink again.
Yet thanks to “diagenesis”– and this is precisely the most interesting phenomenological aspect of the process – a complete mutation of chalk’s crystalline structure takes place, and that is what generates alabaster.
Source: Website of the Istituto d’Istruzione Statale G. Carducci (iiscarducci.edu.it)