Complex mineral zoning patterns caused by ultra-local equilibrium at reaction interfaces

Textural and compositional complexities resulting from coupled dissolution–reprecipitation reactions in geomaterials

“…The coarsening of porosity is driven by the reduction of surface area per unit volume via Ostwald ripening (Ratke and Voorhees 2002). A similar process was observed by Borg et al (2014) where tube-like structures develop within the As-apatite formed via replacement of calcite (Fig. 10b).…”

“…13b). Borg et al (2014) observed that an As-rich (up to 8 times bulk solution concentration) hydroxylapatite forms as a result of localized increase in the As concentration (Fig. 10a), moving the equilibrium solid composition back towards the hydroxylapatite endmember (dotted arrow, Fig.…”

“…Kasioptas et al (2008Kasioptas et al ( , 2011 investigated the replacement of calcite by hydroxylapatite via a phosphate-rich solution. Borg et al (2014) found that the addition of arsenate (AsO 4 3-) moieties to the solution initially results in a complex zoning pattern of As within the apatite structure ( Fig. 10a,b), but with time the zoning disappears and a homogeneous As-bearing hydroxylapatite forms as the final product.…”

“…b) Ideal Lippmann diagram for the hydroxylapatite-johnbaumite-H 2 O SSAS system developed using solubility products of Zhang et al (2011). The reaction path follows that described by Borg et al (2014). Tielines connect equilibrium fluid (square) and solid (circle) compositions.…”

“…(b) Heterogeneous As distribution in the apatite rim after 5 hours. (c) Homogeneous distribution within a fully replaced grain after 48 hours.Adapted fromBorg et al (2014).…”