Geological Thermometers

Minerals that yield information as to the temperature of their formation and of their enclosing deposits are termed as geological thermometers.

They are of scientific and practical importance for a proper understanding of the origin of mineral deposits and their classification. Some of the methods by which geological thermometry has been determined are as follows:

DIRECT MEASUREMENTS:

The measurement of temperatures of formation of lavas, fumaroles and hot springs indicate the temperature of formation of the minerals enclosed therein. The minerals contained in less basic rocks form in part above 810°C, and principally between 870°C and 650°C, decreasing with an increase in silica content. The pyrogenetic ore minerals like chromite form within the range of magma consolidation, which contact metamorphosed minerals form at temperatures lower than that of the magmatic emanations that produce them. Gas fumaroles, likewise, indicate the maximum temperatures for the formation of fumarolic minerals. Lava flow fumaroles reach about 800°C. With decreasing fumarolic activity, lower temperature minerals occur.

The temperatures of shallow hot springs extend downwards from the boiling point of water, and maximum temperatures of formation can be assigned to opal, gypsum, cinnabar, stibnite etc.

MELTING POINTS:

The melting points of minerals indicate the maximum temperature at which they can crystallise from the melt. In a supersaturated solution, they may melt at a considerably lower temperature. The presence of other substances also greatly lowers the liquid-solid temperature point.

Some experimentally verified melting points are albite at 1104°C, stibnite at 576°C, and bismuth at 271°C.

DISSOCIATION:

Minerals that lose their volatile components at certain temperatures serve as poor geological thermometers, as the temperature of dissociation is increase by increasing pressure.

Zeolites indicate low temperature of formation, since when heated they lose their water content, provided that pressure is not too high.

Calcite dissociates under atmospheric pressure at 885°C. This dissociation is affected by the mole fraction of CO2 in a CO2-H2O environment. Silica available for combination with CaO lowers the dissociation temperature.

INVERSION POINTS:

Inversion points are very useful temperature indicators, even though they are affected by pressure. Many inversion points are known within the temperature range prevailing in the formation of most mineral deposits. The inversion points of silica are readily utilised. However, the utilisation of its tridymite and cristobalite polymorphs is performed with some difficulty since both occur in volcanic amygdales, having formed at temperatures well below the inversions of 940°C and 1470°C, at 1 atm.

In the graph, the effect of pressure on the inversion point is shown with respect to SiO2. At about 573C and 1 atm pressure, high quartz inverts decisively to low quartz and vice versa. Thus low quartz may have been formed originally below 573°C, or it may originally have been high quartz that has inverted to the low form.

Argentite and acanthite represent the high and low temperature forms of Ag2S, with an inversion point of 175°C. The external form of the argentite crystals is isometric. Hence, it follows that they were formed above 175°, and the anomalous anisotropism ascribed to argentite indicated that such argentite was originally isometric and later inverted to the orthorhombic acanthite.

EXSOLUTION:

Minerals that form natural solid solutions in each other and at lower temperatures unmix to yield distinguishable mineral intergrowths, serve as geological thermometers, indicating a temperatures of formation above that at which exsolution takes place.

For examples, chalcopyrite and bornite unmix at 500°C, cubanite and chalcopyrite at 450°C, cubanite and pentlandite at 400°C, and bornite and chalcocite at 175°C to 225°C. IT has been shown that chalcophrrhotite exsolves below 255°C into chalcopyrite, cubanite and pyrrhotite.

RECRYSTALLISATION:

This change is somewhat similar to inversion and exsolution, but applies more specifically to native metals.

For example, native copper undergoes a recognisable recrystallisation at about 450°C; native silver recrystallised at about 200°C.

FLUID INCLUSIONS:

Fluid inclusions in cavities of crystals indicate the approximate temperature of formation of the crystals by the amount of contraction of the liquid, assuming that the liquid originally filled the entire cavity.

The validity of fluid inclusions studies is based on three major assumptions:

Þ When a mineral crystallises from the medium, the growing crystal is surrounded by the fluid from which it is crystallising.

Þ If an imperfection is filled with the fluid, and if trapped by additional crystal growth, the resulting primary fluid inclusion is a representative sample of the main fluid at the moment of trapping.

Þ Significant quantities of the material are neither lost nor gained subsequent to this trapping.

A geothermal method is based on the Na-K-Ca concentration of fluid inclusions. The test was done on fluid inclusions of various samples of quartz and one fluorite sample. The K:Na ratios of the hydrothermal fluids were controlled by exchange reactions with alkali feldspars and are a function of temperature. Calcium concentrations, however, affect the temperature estimates.

CHANGES IN PHYSICAL PROPERTIES:

At certain temperatures, some minerals undergo recognisable changes in some of their physical properties. For example, smoky quartz and amethyst lose their colour between 240°C and 260°C., which fluorite loses its colour at 175°C.

Þ Mixed Crystals: An indication of temperature is given only when the composition of the mixed crystals is determined by the temperature of formation. A sphalerite-pyrrhotite geothermometer based on the amount of FeS in sphalerite is used as an ore geothermometer.

Þ Mg/Fe Substitution In Biotite: A possible geothermometer for use in the potassic zone of porphyry copper deposits is one that pertains to the differences in molecular ratios of Mg:Fe in the primary biotite of typical granitic/granodioritic host rocks in contrast to the hydrothermal biotite. The former exhibits Mg:Fe ratios less than 1.0, while the latter are characterised by a ratio of more than 1.5.

The substitution or mixing of Mg2+-Fe2+-Fe3+ provides a geothermometer for the potassic zone, which is based on the composition of the biotite coexisting with magnetite and K-feldspar.

CONDUCTIVITY:

It is determined by a pyrite geothermometer for the measurement of thermoelectric potential of pyrite against a metal, to give the temperature of formation of the pyrite.

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