Cavity-Filling Calcite in the Deccan Traps as a Natural Analogue for Mineral Carbon Sequestration and Low-Temperature Deformation

Cavity-filling calcite is among the most widespread secondary phases in the Deccan Traps, and its position late in the amygdale paragenesis, after zeolites and silica phases, is long established. Two questions that remain open are addressed here: whether this calcite retains a mechanical record of deformation imposed after the cavities were sealed, and what a naturally carbonated flood-basalt pile can tell us about the durability of engineered mineral carbon storage. Calcite from three closely spaced amygdaloidal basalt exposures in the district of Maharashtra, Ane Ghat in Junnar taluka, Pune, Shindewadi 1 and Shindewadi 2 in Parner taluka, Ahilyanagar was examined by transmitted-light petrography, scanning electron microscopy with energy-dispersive X-ray spectrometry (SEM–EDS) and micro-Raman spectroscopy, with growth features deliberately separated from post-growth strain features. All three occurrences comprise coarse, optically continuous calcite grown into open voids: rhombohedral cleavage is perfect, birefringence is extreme, planar crystal faces survive in secondary-electron images, and no replacement front is developed against the basalt wall. Superimposed on this growth fabric are closely spaced polysynthetic e-twin lamellae, undulose extinction, cleavage-controlled intracrystalline microfractures, and healed fractures carrying trails of secondary inclusions. Because these features truncate cleavage and crystal boundaries that are continuous elsewhere in the same individual, they post-date crystal growth. Twin morphology is dominated by thin, straight lamellae, which on the Ferrill classification indicates deformation below roughly 170–200 ◦C. EDS spectra confirm a Cacarbonate composition; because light-element quantification in a coated carbonate is not reliable, measured Ca/C atomic ratios of 1.31–1.61 depart substantially from the ideal value of unity and the analyses are treated as semi-quantitative throughout. Micro-Raman spectra are those of well-ordered calcite, with lattice modes near 156 and 279–281 cm−1 and internal modes at 711–712 cm−1 (ν4) and 1085–1087 cm−1 (ν1); differences between sites do not exceed instrument calibration uncertainty. The results are placed in the framework of in situ mineral carbon sequestration. Deccan amygdale calcite is a completed natural experiment in basalt carbonation: divalent cations liberated by fluid–rock interaction was fixed as a thermodynamically stable carbonate that has survived approximately 66 million years in the same pore volume now targeted for engineered CO2 injection. The microstructural record modifies that otherwise reassuring picture in a specific way. Mineralized amygdales were subsequently strained, brittle microfractures propagated through the carbonate cement, and those fractures were later healed by carbonate-bearing fluids. Permanence of mineral trapping in a tectonically quiescent plateau interior is therefore neither unconditional nor catastrophically compromised: it is a balance between fracture generation and carbonate self-sealing that can be read directly from the cement.