Mineralogical and microstructure characterization of a Neogene natural building limestone from Western Crete, Greece

Natural building stones are being used in Crete for ages to build masonry structures because of being abundant, relatively easy to cut and shape and good performance in many applications. In Crete the largest comprehensive occurrences of Neogene sediments are found along the north-western coast (provinces of Chania and Rethymon), in the Heraklion depression and in the Sitia district, Eastern Crete. Several quarries for the extraction of Neogene limestone, being used as building and decorative natural stone, are located in Western Crete country. Despite their great variety, relatively few types of stone are suitable for construction materials. In addition to accessibility and ease of quarrying, the stone must satisfy requirements of strength, hardens, porosity, durability and appearance. Mineralogy and microstructure greatly influences engineering properties lime permeability, strength and durability. The present study summarizes the results of a primary investigation of a Neogene fossiliferous fluvio - lacustrine - brackish - shallow marine marly limestone, cropping out thirty km east of Chania, Crete. The combination of macroscopic rock description, mineralogical, chemical and microstructure analyses were used, in order to characterize the natural stones regarding their colour, lithology, microfacies, sedimentary structure and fabric.
   The natural stone is macroscopically described as an unweathered white-yellow to white-grey homogenous compact limestone rock. The mineral composition was investigated by using X-ray diffraction (XRD) analysis. The main minerals present are calcite and quartz, whereas clays, micas and feldspars are present in minor amounts. The chemical composition was determined by wet chemical analyses. The results, illustrate that the composition of the major elements is in general monotonous with typical high contents of CaO (40 % wt.) and SiO₂ (17% wt.), followed by Al₂O₃ (5% wt.). In order to enhance the microscopic analysis of the rock fabric and to observe weathering phenomena, special treatment of the raw samples has been used before the preparation of thin sections. This consists of pore and microcrack staining by a mixture of epoxy resin and fluorescent dye. The prepared thin sections where then observed through a conventional optical microscope (Leica DMLP type), equipped also with a source of UV light. Microscopic investigations showed that, the Neogene formation consists of fossiliferous micrite marly limestones, which appear as moderately-sorted, fine grained packstones, rich in foraminifera, echinoderms, calcareous as well as siliceous sponge spicules and other bioclasts. Some quartz crystals are well observed. Cavities and chambers in some fossils are filled with relatively coarse-grained mosaics of authigenic sparry calcite. Fabric-selective porosity is controlled by primary depositional fabrics that include interparticle and intraparticle pores. A system of secondary pores, developed independently of texture or fabric (channel porosity) is also observed.
   Detailed porosimetric analysis has been conducted using mercury intrusion technique. Through this measurement, pore radii, pore size distribution, pore volume and pore surface area can be evaluated. Porosity in all studied samples is high (29-30 vol. %), the specific surface area of meso-macro-and coarse pores (range of measured pore radii where from 3 nm to 58 μm) varies between 14.04-14.17 m²/g and the volume of all pore size categories ranges from 160.71-163.43 mm³/g. Also values of specific surface area and volume of mesopores (2-50 nm), macropores (50-7.500 nm) and coarse pores (over 7.500 nm) where measured respectively. Mineralogy, chemistry, thin section criteria, together with porosity and microstructure characterization, allow these natural building limestones to be used as compatible replacement materials in reconstruction works of certain buildings (monumental and other), that have been previously built with similar natural stones.