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