XRD line profile analysis of calcite powders produced by high energy milling

Abstract

Abstract. Natural calcite crystals were ground to powder by high energy mechanical milling. Depending on the grinding conditions, powders can be pure calcite or calcite-aragonite mix-tures, with a complex equilibrium between the two polymorphs of calcium carbonate. In any case, the mechanical milling is effective in reducing domain size to the nanoscale, introduc-ing large microstrains whose exact nature is not yet totally clear. Domain size distribution and possible sources of microstrain were investigated by XRD line profile analysis, with the support of electron microscopy and BET surface area analysis. 1. Introduction Even if calcium carbonate is one of the most known and diffuse minerals on the Earth crust, several features are still object of active research. For example, the crystallographic aspects of the transformation between the two most frequent polymorphs, calcite and aragonite, is well known [1], but the mechanisms are still debated [2-4]. In particular, the role of lattice defects is still to be clarified. In a more general sense, most mineral properties change by mechanical activation. In Earth science, fine dispersion of crystalline domains and incorporation of lattice defects have an influence on surface reactivity [5], uptake of contaminant species and in particular on disso-lution kinetics [6,7] and transport properties [8]. In technological applications involving mineral powders, domain size and defectivity influence nearly any forming process and many properties of ceramic materials [9]. Mechanical grinding is one of the most effective and simple means to comminute and acti-vate mineral powders: grinding can lead to extended solubility, phase transformations, mech-ano-chemical reactions, amorphization and formation of metastable compounds [10]. X-ray Diffraction (XRD) is a powerful tool to study mechanically activated minerals: besides fol-lowing structural evolutions (as in polymorphic phase transformations), XRD can provide a detailed picture of the microstructure by means of Line Profile Analysis (LPA). In the present study we propose some preliminary results on calcite powders produced by high energy grinding. Nature and extent of calcite-aragonite transformation and lattice defect incorporation are discussed.