5.3 Evidence for Reactions

Most chemical reactions in rocks occur too slowly to observe on a short time scale, or occur under conditions of pressure or temperature that preclude our presence. Petrologists must identify reactions rocks after they have occurred based on observations of hand samples or thin sections. What observations in rock samples might be used to recognize that a reaction has occurred?

Crystals
Cartesian Plot

Figure 5.02. Halite crystal grown in a laboratory. Click on the image to see a movie of it growing.

showflake

Figure 5.01. Snow crystal grown in a laboratory. Click on the image to see a movie of it growing.

Cartesian Plot

Figure 5.04. Garnet porphyroblasts in Grt-Bt-Chl schist. Click on the image to see a larger version.

Cartesian Plot

Figure 5.03. Orthoclase crystals in a granite. Click on the image to see a larger version.

There are a few types of reactions that we can observe in progress and use those observations to help recognize the results of similar reactions in the rock record. Crystal growth from a liquid is an example of a reaction that can be observed, at least in the laboratory (e.g ice crystals and halite crystals, click on Figs. 1 or 2 for crystal growth movies). Crystal growth in magmas or during metamorphism is difficult to observe directly. However, evidence of crystal growth exists in many rocks. Large orthoclase crystals (phenocrysts) occur in igneous rocks whose bulk chemical composition is the same as that of crystal-free, rhyolite obsidian. Large garnet crystals (porphyroblasts) occur in metamorphic rocks whose bulk chemical composition is the same as common, fine-grained, garnet-free shales. It is reasonable to assume that the orthoclase crystals grew due to a chemical reaction during the slow cooling of a rhyolite composition magma, and that the garnet grew as part of chemical reactions during the metamorphism of a garnet-free shale.

Vesicles
Erupting lava can be observed effervescing steam (bubbling of dissolved water and other gasses) due to a decrease in pressure, similar to the effervescence observed in a bottle of seltzer (bubbling of dissolved CO2) when the pressure is lowered by opening the bottle.
Cartesian Plot

Figure 5.05. Vescicular basalt. Click on the image to see a larger version.

Petrologic processes can capture evidence of this chemical reaction by cooling the lava rapidly (“quenching”) to produce a glassy rock with spherical holes (“vesicles”), a record of trapped gas bubbles. If there are spherical holes in a fine-grained igneous rock, it is reasonable to assume that the rock experienced gas effervescence. Many geologists will interpret this observation as reflecting a decrease in pressure during a volcanic eruption and conclude that the vesicular rock formed from a lava erupted onto the earth’s surface. In most cases, this conclusion is a good one and will be supported by other data. However, vesicles have been observed in basaltic dikes (intrusions), so the presence of vesicles does not always occur in as part of a volcanic eruption. Vesicles prove effervescence and are consistent with a rock forming as part of a surface eruption, but do not prove it.

Reaction Zones
thin section image

Figure 5.06. PPL thin section image of hornblende growing around augite. Evidence for a peritectic reaction between augite (90° cleaveage intersections) and magma to produce brown hornblende (60° and 120° cleavage intersections). Click on the image to see a larger version.

If you leave your bicycle out in the rain long enough, you may see a coating of rust (Fe-oxides and Fe-hydroxides) on exposed metal. Even though you did not see the rust crystals growing, their occurrence between the Fe-bearing metal and O2- and H2O-bearing air is consistent with the rust being produced by chemical reactions between the metal and air. Similarly, bands of one mineral that consistently separate two (or more) other minerals
thin section image

Figure 5.07. Garnet necklace around plagioclase. Garnet and hornblende separating plagioclase and augite in a metamorphic rocks from the Tobacco Root Mountains, SW Montana, USA. Click on the image to see a larger version.

are likely to have been produced by a chemical reaction between the separated minerals. For example, Figure 5.06 shows hornblende growing around augite in a gabbroic rock from Massachusetts. This texture is consistent with the hornblende growing by a peritectic reaction between the augite and the host magmatic liquid. Similarly, the garnet crystals that occur between plagioclase and augite in a high grade metamorphic rock from Montana (Figure 5.07), are consistent with the chemical reaction: augite + plagioclase + water = garnet + hornblende.

Pseudomorphs
In some rocks there are minerals that occur in shapes that are interpreted as the shape of one or more reactants in a chemical reaction. Pyrite can be observed in the shape of a fossil in rock found in New York (Figure 5.00b on the previous page). Most shells of fossil trilobites are made of carbonate minerals (calcite or dolomite). The distinctive shape of these pyrite clumps is interpreted by geologists as the product of a chemical reaction between original fossil and a fluid that provided the Fe and S needed to make the pyrite. Observations of trilobite fossils that are only partially made of pyrite is interpreted as supporting evidence showing a replacement reaction stopped prior to completion.
Cartesian Plot

Figure 5.08. Chlorite pseudomorphs after garnet. Click on the image to see a larger version.

Figure 5.08 shows a schist from Vermont with garnet crystals. Some of the garnet crystals have bands of polycrystalline chlorite partially surrounding them. And there are spherical clots of polycrystalline chlorite that do not appear to contain any garnet. These mineral textures may be interpreted as evidence for a chemical reaction in which garnet is a reactant and chlorite is a product.

If a petrologist finds evidence for a chemical reaction in a rock, how can they use that evidence to identify the chemical reaction that may have occurred?