A Rock Is a Rock Is a Rock

The young man in the mineral shop who dismissed my gray rock as “just a rock” with none of the special characteristics of a recognized mineral was wrong.

Rocks aren’t just rocks. To begin with, there are only eight elements that go into the composition of most: oxygen, silicon, aluminum, iron, calcium, sodium, potassium and magnesium. They represent great permutations on matter.

Basalt is some combination of oxygen, silicon, aluminum, calcium, sodium and magnesium. The percentages of silicon and sodium are often used to define the type.

Granite is formed from magma and contains oxygen, silicon, aluminum, sodium and potassium.

Quartz is essentially oxygen and silicon.

Rocks don’t limit themselves to just the eight fundamental elements. Basalt, pyroxene and perovskite can contain titanium. Zircons are silicon and oxygen with zirconium.

More important for geologists interested in the history of the planet are their trace minerals. The half-lives of radioactive isotopes of elements like uranium, neodymium and samarium are used to date them. Zircon crystals are the oldest found above ground.

Rocks, as pieces of matter, do have definable characteristics. In the early twentieth century, Norman Bowen crushed them so he could heat the powders until they melted, then heat them more until they boiled. He let them cool in controlled steps.

He ended with iron-magnesium bearing olivine which was formed at a high temperature and pressure. At a specific lower temperature it becomes pyroxene. At another temperature, pyroxene becomes amphibole, and that, in turn, becomes biotite. In contrast, plagioclase gradually transitions from a rock rich in calcium to one rich in sodium without the abrupt phase breaks.

At a set point, they become potassium feldspar, which becomes muscovite, which becomes quartz. These are the three major components of granite, with quartz the least likely to weather away because it was formed in conditions closest to those of the present.

In the past few decades, new technologies like diamond-anvil presses have allowed geologists to return to experiments like those of Bowen, only now they are looking at how rocks are altered at the higher pressures and temperatures deep in the mantle. Instead of changes in elemental chemistry they’re interested in changes in crystalline structure.

When pressure increases, the O^2- ions in olivine are altered into a spinel structure. At even higher pressures, the spinel structure converts to one found in perovskite and another found in periclase.

Near the earth’s surface, enstatite combines silicon, oxygen and magnesium. It too converts to a perovskite-like structure at higher pressures, then near the bottom of the lower mantle to a post-perovskite structure.

All of this makes terminology a bit confusing, since the same thing is identified by its elements, its crystalline structure and its origins. What one thought was just a rock isn’t. There are at least 11 forms of olivine, including basalt which is further broken into five groups. There are 24 types of pyroxene, including enstatite, and 37 varieties of mica, including biotite and muscovite.

It doesn’t help that only some of these can be seen. Anything with a post-perovskite structure immediate changes to something else when the pressure relents which it must if it’s to make a journey from the center of the earth where it may end up as simple quartz in my arroyo.

Mica (top) from near Albuquerque; granite with bands of gray quartz and shining specks of mica (middle) and a thin piece of quartz (bottom) from the far arroyo.