In the first part of this thesis, CaCO3 (calcite) powder ball-milled to micro/nano scale were subjected to isothermal firing in the temperature range of 450-600 ¢XC in an open air furnace in order to study specific-surface area reduction as a result of early-stage sintering/coarsening/coalescence/repacking (denoted as SCCR process) of the fine particles. The surface area and pore size distributions were obtained from the BET and BJH methods, respectively. The H1 type adsorption/desorption hysteresis loop of the type IV isotherm was used as an indicator of cylindrical pore formation upon dry pressing and firing of the powder. The apparent activation energy for the onset SCCR of the dry-pressed calcite powder turned out to be 57.5¡Ó1.0 kJ/mol based on t0.5, i.e. time for 50 % specific surface area reduction. The minimum temperature for such an incipient SCCR process was estimated to be 590K (317¢J) by extrapolating the specific-surface-area reduction rate to null. The mechanism of specific surface area change includes the Brownian motion, coarsening and coalescence/repacking of calcite particles besides sintering via synchronizing diffusion of calcium ion and carbonic acid ion along grain surface and boundary. In the second part of this thesis, pulsed laser ablation on micrometer-sized calcite (type I) powder in liquid H2O (PLAL) was conducted to study the structure and optical property change of calcium carbonate under a dynamic high-temperature. high-pressure aqueous condition. X-ray diffraction (XRD) indicated the fragmented calcite I powder via such a PLAL process change predominantly into a metastable CaCO3 II phase presumably by a displacive type transformation from calcite I and/or nucleated from atom clusters. The refined XRD lattice parameters indicate a significant internal compressive stress (up to 1.5 GPa) was retained for the predominant CaCO3 II nanoparticles having well-developed (013), (010) and (013) faces as revealed by transmission electron microscopy (TEM). Minor calcium carbonate nanoparticles were also identified by TEM to be other high-pressure polymorphs (type III and aragonite), hydrated (monohydrocalcite, ikaite), amorphized (amorphous calcium carbonate), and even decomposed as cubic lime (CaO). Monohydrocalcite occasionally occurred as epitaxial intergrowths within the predominant CaCO3 II matrix. Vibrational spectroscopy (Raman and FTIR) indicated the structure units of the overall nanoparticles by the PLAL process were considerably modified as a combined results of size miniature, protonation and internal compressive stress. The UV-visible absorption results further indicate that the minimum band gap of the colloidal solution was narrowed down to ca. 5 eV and 3 eV for the predominant CaCO3 II and minor accessory phases, respectively, thus shedding light on their potential opto-catalytic applications.