Weathering rates of sandstone in a semi-arid environment (Hunter Valley,Australia)

The rate of sandstone weathering in the semi-arid climate of the Hunter Valley, New South Wales, Australia has been estimated from observations of gravestone weathering in the area. The gravestone data points to two distinct stages in the weathering process. The first stage covering the first century of exposure is characterised by a relatively low recession rate of 0.5 mm/100 years. This is followed by a second stage in which the rate of weathering increases sharply to ca 2.5 mm/100 years. The non-linear nature of the weathering trends over time suggests that during the first century of exposure, structural changes took place within the sandstone material, which lay the foundation for accelerated weathering after further exposure. Laboratory trials were also conducted to identify the effectiveness of different weathering processes in the decay of sandstone in this region. Of the four processes examined, only the freeze–thaw cycle produced a significant degree of mass loss and is therefore most likely a strong contributor to the weathering of sandstone in this region.     

Deformation twinning in calcite,dolomite, and other rhombohedral carbonates

Transmission electron microscope (tem) observations of single and multiple twins in calcite and dolomite are presented, and the results are analysed by means of selected area diffraction and trace analysis. Simple twinning in rhodochrosite and kutnahorite is also analysed. It is shown that the ordered carbonates, such as dolomite, have a common twinning plane {01 \(\bar 1\) 2} and this appears to be their only mode of deformation twinning. The carbonates with higher symmetry, such as calcite, have {01 \(\bar 1\) 8} as the primary twinning plane but calcite itself has other twinning mechanisms, of which the most important is illustrated. Crossing and stopping twins are also discussed. It is shown that twinning in calcite, which occurs predominantly at low temperatures, is characterized by the generation of large numbers of glide dislocations.     

The significance of the metamorphic rocks of Timor in the development of the Banda Arc, Eastern Indonesia

The metamorphic rocks of Timor are reinterpreted in the light of reconnaissance mapping of the whole island. All metamorphic rocks that crop out in Timor are allochthonous. Several metamorphic massifs are reported for the first time, the outline of others is revised. On the basis of their grade, three distinct groups can be mapped: lustrous slate, amphibolite-serpentinite, and a granulite-amphibolite-greenschist complex. Each group has distinctive structural relations to other allochthonous elements. The granulite facies meta-anorthosite in Timor must have originated near the boundary between the continental mantle and the crust. These and related high-grade metamorphic rocks may represent slices of an ancient Asian continental basement. These rocks imply that the history of the Mesozoic-Cinozoic fold belt of the Outer Banda Arc extends into the Precambrian Era. The metamorphic rocks of Seram appear to be remarkably similar to those of Timor in grade, distribution and structural relations. The overthrust directions of the metamorphic rocks in Timor is southwards, in Seram it is northwards. As the islands are separated by the 4–5 km deep Banda Sea, these directly opposite thrusts may be explained in terms of the Banda Arc acquiring its sinuosity after the emplacement of the metamorphic rocks.     

Deformation of dolomite single crystals from 20–800° C

Dolomite single crystals of six different crystallographic orientations were tested in compression under confining pressure at temperatures of 20 to 800° C. The chosen orientations favoured slip or twinning on particular systems. The deformed crystals were analysed by optical and high voltage transmission electron microscopy to determine activated deformation systems, dislocation behaviour, etc., and to assist in interpreting stress-strain data. It is shown that slip on c≡(0001) and on \(f \equiv (\bar 1012)\) ), and twinning on f between 300 and 600° C, are the principal modes of deformation. At low temperatures there is considerable cataclasis, and shear-fracturing must be counted as a significant deformation mechanism. The effects of climb become apparent at temperatures ?600° C. A pronounced increase in strength with testing temperature shown by some orientations of the crystals is largely associated with c slip, but f twinning also shows similar although weaker tendencies. The yield stress for f slip decreases markedly with temperature. Values of critical resolved shear stress are obtained for c and f slip, and for f twinning. Major and minor slip systems, dislocation and twin configurations generated by deformation under different regimes are documented and illustrated. Some of the characteristics of the deformation systems are attributed to the details of atomic displacements. In particular, the increase in strength with temperature for c slip is explained by friction of CO ₃ ^2? groups during dislocation movement, which is unique for c slip. This friction increases with thermal vibration, expansion and rotation of the CO ₃ ^2? groups.     

Froth structure in continuous flotation cells: Relation to the prediction of plant performance from laboratory data using process models

The paper discusses the obstacles in the way of making mathematical models of flotation circuits for use in process design; and describes also a route towards simple, but practical, models of full-scale flotation plant performance, which overcome the major problems. The principal difficulties lie in simulating the froth processes at full-scale. Timed batch tests may be used to identify the pulp-froth transfer processes, and steady-state (“equilibrium”) cell tests may be used to identify static froth concentration profiles. Froth mobility in a real cell is different from that in an “equilibrium” cell, and this paper shows how these dynamic patterns have been investigated, and may be used to simulate full-scale circuits. It is shown that it is not possible, at the present time, to completely eliminate judgement and experience from the establishment of parameter values; but the position should improve as experience accumulates.     

Peralkaline nephelinites

Two fractionation trends in sodic alkaline ultramafic liquids have been predicted from experiments in subsystems of the join Di-Ak-Ne-Lc-Qz. The products of these trends are equated with contrasting suites of peralkaline nephelinites from two nephelinite-carbonatite volcanos of the south Gregory Rift, Shombole (southern Kenya) and Oldoinyo L'engai (northern Tanzania). In both trends, peralkalinity is interpreted to result from fractional crystallization of aluminous clinopyroxene. The Shombole trend has olivine nephelinite as its parental magma, and the differentiation products are mildly peralkaline [(Na+K)/Al1.15] nephelinites. It is the most common lineage observed in nephelinite-carbonatite centres. The Oldoinyo L'engai trend has melilitite or olivine-melilite nephelinite as its parental magma, and produces extremely peralkaline [(Na+K)/Al=1.4–2.3] wollastonite- and combeite- (Na₂ Ca₂Si₃O₉) bearing nephelinites. The presence of a reaction relation between wollastonite and liquid to produce combeite, indicated by corroded wollastonite phenocrysts armoured by combeite in some nephelinites from Oldoinyo L'engai, is confirmed by melting experiments. Combeite nephelinites from Oldoinyo L'engai were erupted simultaneously with natrocarbonatite ash, and are very similar in composition to silicate liquids that have been shown by experiment to be immiscible with natrocarbonatite. Because the L'engai trend is rarely expressed at extrusive centres (combeite has been recorded at only three localities), and combeite nephelinites are highly evolved magmas, it is unlikely that natrocarbonatite is primary to other carbonatite types. It is proposed that carbonatite liquid is exsolved at crustal pressures from a wide range of nephelinitic liquids: Mg-rich carbonatite from primitive, olivine-bearing alkaline ultramafic liquids, Ca-rich carbonatite from olivine-free nephelinites of low peralkalinity, and natrocarbonatite from strongly peralkaline combeite nephelinites.