Mineralogy and physical properties of some Tasmanian cainozoic clays.

Detailed quantitative mineralogical analysis and physical studies were made of twenty nine Cainozoic clays from different parts of Tasmania. The clays are mainly of Tertiary age and are both residual and sedimentary in origin. The geological occurrences of the clays are described and their mineralogy discussed. The two main methods of analysis consisted of X-ray diffraction techniques and differential thermal analysis; iron determinations were made on four selected samples. The differential thermal method of analysis in conjunction with X-ray diffraction examination proved advantageous in detecting an abnormal montmorillonite which was characterised by a lower dehydroxylation peak at about 550°C. The clays can be divided into five types, on the basis of their mIst abundant clay mineral, as follows: 1.Clays rich in kaolinite 2.Clays having abnormal montmorillonite as the dominant clay mineral. 3.Clays characterised by mixed layer minerals of illite and montmorillonite, as both randomly stacked and ordered structures. 4.Clays characterised jointly by kaolinite and abnormal montmorillonite. 5. Clays predominantly kaolinites with gibbsite. Quartz was abundant in all the clays. Other minor constituents included; illite, feldspar, goethite, gibbsite, matahalloysite, calcite, chlorite, haematite, muscovite, mixed layer minerals and organic matter: The clay minerals were used as indicators to interpret tentatively the environment of deposition during the Cainozoic Era. The possibility exists that different clay minerals might have formed in the weathered zones at different periods in the geological history of the area as a result of changes of micro- chemical climate. In the 36' bored at Sandy Bay the mineral composition revealed no diagenetic change with the accumulation of the clays. Most of the clays required a high proportion of tempering water. Drying and firing characteristics were related to clay mineralogy. Of the two chief mineralogical groups, clays which consist primarily of kaolinite are of medium plasticity and shrinkage, light coloured on firing and moderttely refractory. Another group of clays with abnormal montmorillonite, or mixed ATMinerale have high plasticity and sh±inkage, are very slow in drying, orange - red burning and low - refractory.