Geology, mineralogy, and geochemistry of Greek perlites: Industrial applications.
2015-11-19T09:05:47Z (GMT) by
The most important Greek perlite deposits crop out on the islands of Milos (Provatas and Trachilas areas), Kimolos, and Kos, Aegean Sea, which are currently under exploitation. More than 200 specimens of raw perlite were collected from these perlite deposits and were subjected to physical, chemical, petrographic and expansion tests. The perlite deposits have been formed by a phreatomagmatic eruption, which is the last event of volcanic activity on the islands. Most of the deposits comprise the sand, ash, and perlitic fragments' formation, the perlitic lahar, the perlite beds, and the perlitic dome. The first formation is the precursor of the eruption, and is followed by the perlitic lahar, which is result of lava fragmentation, and the perlite beds, which are slow-cooled lavas. The perlitic dome penetrates into the perlitic succession. Vesiculation has taken place in the perlite beds, producing the pumiceous textures of perlite. Dacite fragments are found in the perlitic lahar, as well. Perlite consists of more than 70% of high silica and alkalies glass. Phenocrysts of quartz, feldspar (mainly plagioclase of oligoclase composition), biotite, opaque minerals (magnetite, ilmenite) and apatite are also present. Various types of perlite, rich in quartz, feldspar, tridimite, and cristobalite, were identified. Two main modes of water are found in perlite. Differences in chemical composition of perlites indicate four individual parent magma sources. The main chemical variation is present in alkalis and Ba, Sr, Rb, and the R.E.E. The Eu anomaly is a common characteristic for the studied perlites. The relationship between glass and perlite composition is given. The specific gravity of Greek perlites ranges between 1.50 and 3.00 gr/cm3, the p.H. between 6.5 and 8.5, the surface area is around 3.00 m2/gr. All the studied perlites indicate good expansibility, obtaining low loose bulk density values (20-240 kgr/m3), low percent of nonexpansibles (1-7%), large particle size, low compaction resistance, high water (15-35%) and oil (400-2200cc/100gr) absorption, and surface area values (6-13 m2/gr). The expanded perlite is chemical inert. A new phase rich in Cr and Fe, a pyroxene, and cristobalite are determined in the expanded product. Four different stages in the expansion process are determined by the scanning electron microscope. The phenocryst proportion and grain size contribute to the expansion results. On the other hand, the chemical and the mineral composition does not affect the expansibility of perlite, perhaps because the range of composition is small. The studied perlite is of good quality and is therefore suitable for filter aid, concrete and plaster aggregate, loose fill insulation, cryogenic, horticulture, animal feed, fire protection, and thermal insulation purposes.