Textural and chemical inheritance during a pseudomorphic double mineral transformation

Abstract

Pseudomorphism, a prominent feature of many fluid-driven mineral transformations, happens when a secondary phase inherits the morphology of the primary one due to a tight coupling between primary phase dissolution and secondary phase precipitation. Pseudomorphic transformations are common in nature, yet the fate of pseudomorphs, once formed, has been largely overlooked. Here, we assess the double pseudomorphic transformation of i) gypsum (CaSO4·2H2O) to celestine (SrSO4), and ii) of the so-formed celestine to strontianite (SrCO3). Through detailed mineralogical and geochemical analyses we document two successive pseudomorphic mineral replacement transformations. Both replacements occur through a coupled mechanism of dissolution-crystallisation, are complete, fast, and occur while preserving the external morphology of the gypsum precursor. The two transformations lead to the development of two generations of pores within the replaced phase(s), caused by the decrease in molar volume associated with both transformations. Interestingly, the final strontianite pseudomorphs inherit not just the texture but also chemical features of both the gypsum precursor and the intermediate celestine. Our findings highlight the ability of pseudomorphs to record and preserve chemical and textural information in successive transformations, underlying their relevance as resilient geological proxies.