Evidence for absorption by exchange-coupled Fe2+-Fe3+ pairs in the near infra-red spectra of minerals

Reduction of Fe³⁺ to Fe²⁺ by heating in hydrogen reduces the absorbance of the bands at 9,000 and 13,800 cm^?1 in the E ⊥ c spectrum of tourmaline, and the 9,000 and 11,000 cm^?1 bands in the E ∥ (001) spectrum of biotite. This behaviour is consistent with the presumed d-d origin of these bands (which seems well established) only if they gain much of their intensity from exchange-coupling with neighbouring Fe³⁺ ions. Intensification of spin-forbidden bands in sapphire by Fe³⁺-Fe³⁺ exchange-coupling was recognized by Ferguson and Fielding (1971, 1972), but exchange-coupling has not previously been thought to intensify spin-allowed d-d bands. Spin-allowed exchange-coupled bands resulting from Fe²⁺-Fe³⁺ pairs have features in common with both normal single ion d-d bands, which they resemble in energy, width and pressure dependence, and Fe²⁺+Fe³⁺→Fe³⁺+Fe²⁺ charge transfer bands, which they resemble in temperature-, heat treatment-, composition-, and polarization-dependence. Distinction between normal d-d, charge transfer, and pair d-d absorptions is thus complicated, and criteria for assigning these bands are discussed. Spin-allowed exchange-coupled pair bands should be sought in the spectra of transition metal clusters (trimers and polymers as well as pairs may be involved) whenever geometry favours their origin. It is possible that the bands near 10,000 and 11,500 cm^?1 in blue sapphire, and at about 5,000 cm^?1 in titanian garnets are of this type, but many other examples are likely to occur. Exchange-coupling may involve ions other than Fe³⁺ (e.g., Mn²⁺, also d ⁵), although Fe²⁺-Fe²⁺ coupling is unlikely to be important at laboratory temperatures.     

Heat extraction from hot,dry, crustal rock

Natural heat stored in the earth's interior represents an essentially inexhaustible energy supply which, at usefully high temperatures, is accessible at practical drilling depths from almost anywhere on the earth's land surface. The problems of extracting and using this heat are those of engineering and economics, and can be expected to vary with the local geology and value of thermal energy. The first major experimental system designed to investigate these problems in one common type of geologic environment has recently been completed in the crystalline rock underlying the Jemez Plateau of northern New Mexico. It consists principally of two boreholes connected at a depth of about 2.7 km by a system of hydraulic fractures produced in granitic rock at a temperature of approximately 185°C. Cool water injected through one hole is heated as it flows through the fractures, and is recovered through the second hole as pressurized, superheated water. In a surface heat-exchange system now being completed, this heat will be extracted and the cool water reinjected to maintain a continuous, closed, pressurized-water energy-extraction loop.     

Petrochemistry and mineralogy of early Precambrian anorthositic rocks of the Limpopo belt, southern Africa

Two sections of the anorthosite ‘complex’ were examined at Messina, South Africa and at Pikwe, Botswana. Thirty XRF whole-rock analyses of samples in stratigraphic order show that alkalies at Messina increase upwards from leuco-gabbros to anorthosites, but no such correlation was found at Pikwe. Electron probe analyses of plagioclases in 33 samples indicate extensive normal zoning and variation (An₃₁-An₈₀) especially at Pikwe. The highest An-contents tend to vary monotonically with stratigraphic position, whereas the lowest values are erratic. Twenty-two amphibole analyses indicate that SiO₂, TiO₂ and MgO/FeO increase, decrease and increase, respectively, with height at Messina, but 22 analyses from Pikwe show no such trends. Their compositions are similar to those from the Fiskenaesset complex. Aluminous chromites are Fe-rich, also like the Fiskenaesset ones.

The Limpopo anorthositic rocks belong to a layered igneous body that was recrystallized by regional metamorphism and subjected to erratic metasomatism. The original Limpopo and Fiskenaesset bodies strongly resemble gabbroic and peridotitic relics in the Peninsular Ranges Batholith in S. California.     

A comparison of the recent eruptions of Mt. Pelée,Martinique and Soufrière,St. Vincent

The simultaneous eruption of Mt. Pelée, Martinique and Soufrière, St. Vincent are regarded as the first recognized examples of Pelean-type and St. Vincent-type pyroclastic eruptions. Both produced nuées ardentes, the former usually laterally directed because of the presence of a dome and the latter vertically directed from an open crater. Both volcanoes have subsequently erupted for a second time this century. The 1902–05 and 1929–32 eruptions of Mt. Pelée produced andesite lava of almost identical composition and mineralogy. Both contain two generations of plagioclase, orthopyroxene, Fe-Ti oxide, corroded brown amphibole and olivine rimmed by pyroxene. In contrast, the Soufrière material is more basic in composition varying from basaltic andesite to basalt in 1902–03 and basaltic andesite in 1971–72. The Soufrière material contains two generations of plagioclase (with those of 1971–72 having additional zones of labradorite), clinopyroxene, orthopyroxene, olivine and Fe-Ti oxide. The pyroclastic deposits are strikingly different, those from the Pelean-type eruption are termed «block and ash deposits» being characterised by poorly vesicular lava blocks up to 7 m in diameter, while the St. Vincent-type eruption produced «scoria and ash deposits» containing vesicular ropey blocks or bombs no larger than 1 m in diameter. The differences in styles of eruption are attributed to differences in viscosity and mechanism of eruption of the magmas. Stratigraphic studies of Mt. Pelée reveal that the volcano has produced basaltic andesite scoria and ash deposits from St. Vincent-type eruptions. It is concluded that the recent eruptions of Pelée tapped a deep level magma during both eruptions releasing magma of similar composition, while the 1971 Soufrière magma is thought to be a remnant of the 1903 basaltic magma which remained at a high level within the volcano where it underwent enrichment in plagioclase and loss of olivine and oxide.     

Experimental studies of sodic microperthites from the Loch Ailsh syenite

Solvus curves under both ‘dry’ and ‘hydrothermal’ (P_H2O = 1 kbar) conditions have been determined for two microperthitic alkali feldspar starting materials from the Loch Ailsh syenite complex, Assynt, Scotland. One sample (Ab₇₄ Or₂₄ An₂) possessed a K-phase showing dominant monoclinic symmetry to both powder and single-crystal X-ray methods, the other (Ab₈₈Or₃₀An₂) had maximum microcline as K-phase. These materials gave essentially identical solvi for both types of experiment, interpreted as indicating similar degrees of local order in the K-phase, but with differing domain size, leading to the different X-ray properties. The ‘dry’ solvus curves confirm the determination of the microcline-low albite solvus of Bachinski & Müller. The hydrothermal curves are used to estimate P_H1O <3.5 kbars, T > 755 °C for emplacement of the Loch Ailsh syenites.     

On the chemical composition of atmospheric aerosols — Part II: Trends between some chemical components of rainwater and physical variables over the United States

Summary Annual average results of rainwater analysis, carried out byC. Junge at about 60 stations of the United States sampling network are used in an attempt to correlate the ionic species present in rain, as a general contribution to atmospheric chemicstry. Correlation with physical parameters is attempted at the same time in some cases, and the resulting trends discussed. The results obtained are presented in the general form of linear and planar correlation, and linear regression coefficients.