Increasing Quartz Content Research Articles

Elastic-wave velocity in crustal and mantle rocks at high pressure and temperature: the role of the high-low quartz transition and of dehydration reactions

The compressional- and shear-wave velocities, V P and V S, in typical crustal and mantle rocks were determined in a cubic anvil apparatus over a range of constant confining pressure up to 6 kbar and temperatures ranging from 20–750°C. Samples range between acidic and ultramafic with mean atomic weights between 20.4 and 23.0 and bulk densities of 2.66–3.50 g cm −3 at 6 kbar. Compressional-wave velocities are only roughly related to densities and do not generally follow lines of constant mean atomic weights. However, the rocks typical for the upper mantle, the continental and oceanic crust clearly separate into three distinct areas in the velocity versus density plot. In plutonic and metamorphic rocks velocities are strongly dependent on the mineral composition. Both V P and V S increase with increasing amphibole, garnet, pyroxene and olivine content. High feldspar content produces high Poisson's ratios as a consequence of relatively high compressional wave velocities compared to the respective shear wave velocities. Increasing quartz content results in a decrease of V P and an accompanying increase of V S, thereby significantly lowering Poisson's ratio. Elastic-wave propagation is affected to a large degree by flat cracks and pore spaces. High confining pressures act to reduce microcracks; heating causes an increase in porosity. However, microfracturing is increasingly suppressed by the simultaneous action of confining pressure. A pressure increase of at least 10 bar per 1°C is needed to prevent the opening of cracks. Velocities are highly affected by the α-β quartz transition. Due to stress concentrations in polycrystalline material the quartz transition temperature is shifted to higher values than might be expected from single crystal measurements. Dehydration reactions in zeolite-bearing basalts cause the velocities to decrease as a consequence of lowering the effective pressure and of a reconstitution of pore geometry. Velocity anisotropies observed in some plutonic rocks, and most metamorphic rocks, at high confining pressures and high temperatures correlate well with preferred lattice orientation of the constituent major minerals. The effect of dimensional orientation and microcracks seems to be of minor importance for the directional dependence of wave velocities under conditions of high pressure.

Porosity gradients in North Sea oil-bearing sandstones

The relationship between porosity and burial depth of sandstone reservoirs is complex. It depends on primary porosity, which is related to grain-size and depositional process and also on the porosity gradient (the change in porosity with depth) which, in most cases, is linear. This relationship may be expressed as: φ d =φ p − G.D, where φ d is the porosity at depth D below surface, φ p is the primary depositional porosity, and G is the porosity gradient in % porosity per 1000 ft (305 m). Porosity gradients decrease with increasing quartz content, abnormal pressures, and in the presence of hydrocarbons. They increase with increasing geothermal gradient. There are at present insufficient data to quantify these relationships. Empirically derived porosity gradients for North Sea Rotliegendes, Jurassic and Palaeocene reservoir sandstones are 2.2, 2.6 and 2.9 per cent per 1000 ft (305 m) respectively. When calibrated with the porosity spectra of the various facies, graphs can be drawn showing the depth: porosity windows within which economic reservoirs may be found.

Modal Variation and the Form of the Beinn an Dubhaich Granite, Skye

AbstractRegional variations in modal analyses of the Chandos Lake pluton (Ontario) and the Beinn an Dubhaich granite are compared. Trend surface analysis maps for the latter area show a southwest “grain” parallel to the axis about which King (1960) suggested the sheet of granite is folded. The modal data thus appear to confirm King's hypothesis, and to indicate a more mafic base and increase in quartz content upwards through the sheet. The value to petrogenesis of quantitative variation studies of rock masses is briefly discussed.

A Mineralogical Characterization of Some Loess‐Derived Soils in Illinois

AbstractSix soil profiles representing successive stages of development in Peorian loess were studied by X‐ray diffraction to determine any changes in mineralogical composition of the silt fraction in relation to their maturity. The quartz, albite, and microcline contents were determined using NaF as an internal standard. The dolomite content was determined with the carbon‐induction furnace. The quartz content of the fine silt fraction increased progressively in the A horizons, did not change significantly in the B horizons, and increased slightly in the C horizons with distance from the loess source or bluff. Concomitant with the increase in quartz in the A horizons the feldspar content decreased, indicating the greater susceptibility of the latter to weathering. The slight increase in quartz content in the C horizons was due to the presence of dolomite in the younger soils and the absence of dolomite in the more mature soils. Quartz/feldspar ratios, an index of the weathering of the minerals, increased significantly for the fine silt fraction with increasing distance from the loess source. Albite weathered at a faster rate than microcline in the fine silt fractions. No significant change in the quartz and feldspar content of the coarse silt fraction was observed.