The calcite → aragonite transformation in low‐Mg marble: Equilibrium relations, transformation mechanisms, and rates

Abstract

[1] Experimental transformation of a rather pure natural calcite marble to aragonite marble did not proceed via the expected straightforward polymorphic replacement. Instead, the small amount of Mg in the starting material (0.36 wt %) was excluded from the growing aragonite and diffused preferentially into the remaining calcite grains, producing Mg-rich calcite rods that persisted as relicts. Nucleation of aragonite occurred exclusively on grain boundaries, with aragonite [001] oriented subparallel to calcite [0001]. The aragonite crystals preferentially consumed the calcite crystal on which they nucleated, and the reaction fronts developed preferentially along the {010} and {110} planes of aragonite. Each aragonite neoblast that grew was nearly free of Mg (typically <0.1 wt %). The excess Mg was taken up by the calcite grains in between, stabilizing them and causing a few volume percent rodlike relicts of Mg-enriched calcite (up to 10 wt % MgO) to be left behind by the advancing reaction front. The aragonite growth rates are approximately linear and range from $3 Â 10 À11 m s À1 at 600°C to $9 Â 10 À9 m s À1 at 850°C, with an apparent activation enthalpy of 166 ± 91 kJ mol À1 . This reaction mechanism and the resultant texture are akin to cellular precipitation reactions in metals. Similar transformation textures have been reported from high-Mg marbles in Japan and China that disproportionated to low-Mg calcite and dolomite.

Structural architecture and low‐grade metamorphism of the Mikabu‐Northern Chichibu accretionary wedge, SW Japan

“…The inferred a Jd value was used for barometry based on the equilibrium Ab=Jd (in alkali pyroxene)+Qz and the thermodynamic data of Holland and Powell () (Figure a). The result for the Mikabu unit is consistent with the occurrence of chemically pure aragonite (Suzuki & Ishizuka, ) if the calcite=aragonite curve of Hacker, Rubie, Kirby, and Bohlen () is adopted (Figure a). The alkali pyroxene compositional data (Figure b) also suggest that the peak pressures of the low‐grade part of the Sanbagawa belt are higher than those of the Mikabu‐A unit, although aragonite has not been found in the low‐grade Sanbagawa belt possibly due to the higher T retrograde path unsuitable for the aragonite preservation (e.g.…”

“…The alkali pyroxene compositional data (Figure b) also suggest that the peak pressures of the low‐grade part of the Sanbagawa belt are higher than those of the Mikabu‐A unit, although aragonite has not been found in the low‐grade Sanbagawa belt possibly due to the higher T retrograde path unsuitable for the aragonite preservation (e.g. Hacker et al., ).…”

Aragonite and magnesite in eclogites from the Jæren nappe, SW Norway: disequilibrium in the system CaCO3–MgCO3 and petrological implications

“…This is corroborated by the observation that eclogite minerals such as phengite and omphacite are not retrogressed when they are solely in contact with aragonite and magnesite. Assuming isothermal exhumation at 700 °C and applying the aragonite–calcite phase transition of Hacker et al. (2005), the metasomatic replacement of magnesite by aragonite is estimated to have occurred at a pressure higher than 1.9 GPa, i.e.…”

“…Error ellipses indicate estimated 2σ analytical uncertainty. The diamond–graphite and coesite–quartz transitions are after Krogh‐Ravna & Terry (2004) and the position of the aragonite–calcite transition is after Hacker et al. (2005).…”

Phase transformation of Mg-calcite to aragonite in active-forming hot spring travertines

“…Although this geologically important phase transformation from Mg-calcite to aragonite is a relatively common occurrence in nature it is usually simulated in the laboratory (Hacker et al 1992(Hacker et al , 2005Lin and Huang 2004). Despite this, the step-by-step transformation is not usually studied from a morphology point of view.…”