Zeolites of the heulandite structural type, have been identified in volcanoclastic rocks of northern Sardinia. Samples were collected from two types of volcanoclastics, an epiclastic formation formed in a lacustrine environment and a welded ignimbrite. In both types of rock clinoptilolite was found to be the prevailing mineral, along with minor amounts of opal-CT, quartz, feldspars, and biotite. Quantitative evaluations carried out on ignimbrite and lacustrine deposits gave clinoptilolite mean contents of 50 and 70%, respectively. Clinoptilolite from the epiclastic deposits broke down structurally at about 450°C, a thermal behavior typical of Ca- and Mg-rich clinoptilolite, whereas clinoptilolite from the ignimbritic rocks broke down only > 750°C, suggesting that the latter material was clinoptilolite sensu stricto. As far as cation-exchange capacities are concerned, tests revealed that about 70-80% of Ca and 60-70% of Mg in clinoptilolite was exchangeable; consequently the cation-exchange capacity was less than that expected. Synchrotron Rietveld refinements showed that the difficulty in removing Ca and Mg from the framework was probably due to the specific position of these cations in the framework and to the correspondingly high bond strength of these cations with the framework oxygens.
Mineralogy and crystal chemistry of clinoptilolites from Northern Sardinia. An evaluation of their technological potentiality
CRUCIANI, Giuseppe;
2000
Abstract
Zeolites of the heulandite structural type, have been identified in volcanoclastic rocks of northern Sardinia. Samples were collected from two types of volcanoclastics, an epiclastic formation formed in a lacustrine environment and a welded ignimbrite. In both types of rock clinoptilolite was found to be the prevailing mineral, along with minor amounts of opal-CT, quartz, feldspars, and biotite. Quantitative evaluations carried out on ignimbrite and lacustrine deposits gave clinoptilolite mean contents of 50 and 70%, respectively. Clinoptilolite from the epiclastic deposits broke down structurally at about 450°C, a thermal behavior typical of Ca- and Mg-rich clinoptilolite, whereas clinoptilolite from the ignimbritic rocks broke down only > 750°C, suggesting that the latter material was clinoptilolite sensu stricto. As far as cation-exchange capacities are concerned, tests revealed that about 70-80% of Ca and 60-70% of Mg in clinoptilolite was exchangeable; consequently the cation-exchange capacity was less than that expected. Synchrotron Rietveld refinements showed that the difficulty in removing Ca and Mg from the framework was probably due to the specific position of these cations in the framework and to the correspondingly high bond strength of these cations with the framework oxygens.I documenti in SFERA sono protetti da copyright e tutti i diritti sono riservati, salvo diversa indicazione.