that have clinopyroxene crystals within. The crystals could be used for thermobarometry of the komatilite mineral. Cpx represents clinopyroxene, ol represents
olivine, and gl represents
glass.
Thermobarometry uses
equilibrium constants to calculate information about the environmental conditions present during the rocks' formation. While each rock is forming, it
reacts with the surrounding
elements until it cools down enough to become inert. Each mineral within the rock will cool and crystalize at different points; a
petrogenetic grid is a useful way to visualize each mineral crystalizing in sequence. Individual reactions of specific minerals can be used to calculate either the temperature or pressure. Therefore, two different reactions are needed to calculate both the temperature and pressure of the magma for a single rock. Some reactions are better for pressure and others are better for temperature, based on
thermodynamics and
Le Chatelier's Principle. This technique requires each reaction to be
calibrated, which is done through
experimentation and
data analysis. Experimentation involves simulating the temperatures and pressures at which these rocks form and observing how the reaction proceeds at those conditions, while data analysis relies on amassing a large
database of rock samples with pressure and temperature information. Experimental data tends to have significant
variation, so using data from natural formations is more
accurate, if it's available.
Pressure that determines the
calibration curve of the change in
volume of
diopside at different
pressures. The
equilibrium constant for this reaction can be calculated based on the data. The reactions best for pressure (
geobarometers) are ones that have a large change in
molar volume during the reaction. Higher pressures cause the reaction to decrease in total volume, and lighter pressures allow reaction to increase in total volume. Therefore, based on the proportion of minerals that have larger volumes versus the proportion of minerals that have smaller volumes, the pressure of the environment during the reaction can be calculated, as a function of temperature. Experiments must be done to
calibrate each reaction and determine the rate at which the volume changes with changes in pressure.
Temperature The reactions best for temperature (
geothermometers) are ones that have a large
enthalpy of reaction, which means they release or consume a lot of
heat. Higher temperatures allow the reaction to consume that heat while lower temperatures cause the reaction to release heat. Similarly to geobarometers, the proportion of minerals that are formed by releasing heat versus consuming heat can be used to calculate the temperature, as long as the reaction is calibrated.
Reaction Types There are three types of reactions that clinopyroxene is involved in and can be used for thermobarometry.
Univariant reactions or displaced equilibria reactions either create or destroy
phases within the magma. Each phase will eventually crystalize as a unique mineral. Based on the temperature and pressure conditions, different
proportions of these phases will emerge in the final rock. An example reaction is
jadeite and
quartz reacting to make
analbite. Jadeite is a type of
pyroxene, so this reaction is used for clinopyroxene barometry. NaAlS2O6 + SiO2 -> NaAlS3O8 This particular reaction involves a large change in
volume between the
reactants and the
products, so the reaction is very sensitive to pressure changes.
Exchange Reactions occur when there are minerals with similar
structures, and
ions switch places with each other within that structure. This is a common method to calculate the temperature because most exchange reactions have a high
enthalpy. One example reaction is an exchange of
Fe2+ and
Mg2+ within
garnet and clinopyroxene. That causes
pyrope and
hedenbergite (pyroxene) to change into
almandine and
diopside (pyroxene). Mg3Al2Si3O12 + 3CaFeSi2O6 -> Fe3Al2Si3O12 + 3CaMgSi2O6
Solvus Equilibria reactions occur when two phases
dissolve into each other based on the temperature, so it is usually useful for geothermometry. One such reaction is when clinopyroxene and
orthopyroxene dissolve into each other. This changes the distribution of
calcium and
magnesium throughout the mineral. == Applications ==