Category Archives: minerals

What on Earth are you doing with Howard Bannister’s rocks?

whatsupdoc1

The classification of the igneous rocks is a morass from which you would be well advised to steer clear. Even Judy Maxwell said, “I can take your igneous rocks or leave them. I relate primarily to micas, quartz, feldspar. You can keep your pyroxenes, magnetites, and coarse-grained plutonics as far as I’m concerned.”

Personally, I love the igneous rocks. Nevertheless, there is one coarse-grained plutonic up in the mountains above Santa Fe which has given me fits in trying to classify. And it points perfectly to the sort of look-alike confusion which plagues the field identification of these rocks.

Here’s the rock:

The speckled rock along Tesuque Creek

The speckled rock along Tesuque Creek

Ideal countertop material, you might say. I asked a hiking companion what he thought it was and was told “it looks just like the granite back home up in the Sierra” – the Sierra being the Sierra Nevada Mountains in California. And it does look just like those granites, except for the caveat going though my head – the curse of an education – that, as the geologist P.B. King relates, in the Sierra Nevada, “true granites in the technical sense are rather minor, most of them being the somewhat more mafic quartz monzonites, granodiorites, and quartz diorites”.

I thought it might be diorite. Diorite is an interesting construction, a French name built from the Greek root dior izein, ‘to distinguish’. Diorite is a granular igneous rock made up of bright white feldspar and dull black hornblende, with a classic “salt and pepper” appearance that every first year geology student learns to identify on sight.

Unfortunately, diorite is very difficult to distinguish from gabbro, another dark speckled igneous rock, which is what another hiking companion (understandably) always thought it was.

There’s a reason field geologists carry around that little ten-power hand lens, and when you look at this rock up close, you discover that most of the dark minerals are the black mica called biotite, and that there is an awful lot of quartz mixed in with the white feldspar. This throws the ball back into granite’s court, petrologically speaking, and there is a surprising name for the common hybrid between granite and diorite: granodiorite. So that’s where I finally decided to pigeonhole the rock. A very dark granodiorite.

Except that I found out its real name is tonalite.

The point is, the point is… oh god, I’ve forgotten my point.

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Krystallos

I was on a mountain excursion earlier this week with some 14 year-old hikers, high in the ancient rocks that outcrop east of Santa Fe, when their attention was drawn to this luminous rubble along the trail:

Milky quartz float along the trail

Milky quartz float along the trail

I picked up a piece and asked them what they thought it was. “Crystal?” asked one. “Quartz!” asserted the other.

They were both right. The coarse-grained granite that splits the gneiss in the foothills of the Sangre de Cristo Mountains above Santa Fe spills its crystalline contents all along the mountain trails, mixing with the pines, penstemons, and desert plants there. It covers the ground with pink feldspar and adds icy accents of quartz and mica. Everyone notices the quartz.

The components of granite: feldspar, quartz, and mica

The components of granite: feldspar, quartz, and mica

Each of these minerals is composed of chemical elements common in the Earth’s crust. Among these elements, oxygen is by far the most abundant, making up 47 percent of the rocks by weight, and a whopping 96 percent by volume! Oxygen is a big atom. When you look at a granite mountain like Pikes Peak you are basically looking at a big pile of oxygen with some impurities in it. This fact never fails to impress me.

In second place is the element silicon, making up around 27 percent of the crust by weight. Since oxygen and silicon can link together chemically, it follow that their compounds utterly dominate the composition of the Earth’s crust. There is so much oxygen and silicon around that their simplest combination, two atoms of oxygen sharing one atom of silicon, or silica, is exceptionally common. We know it as the mineral quartz.

Under ideal conditions, such as in the cavities of mineral veins, quartz can be found as transparent, six-sided prisms, each terminated by a pyramid with six shining faces. These noble light-forms are perennially fascinating – visit any mineral shop or New Age bookstore – and they seems to captivate everyone who sees them. The ancient Greeks gave the name krystallos (clear ice) to these forms, considering them a kind of ice that had been eternally frozen. Transparent quartz is still called rock crystal, and from that simple beginning, any mineral or chemical substance that develops symmetrical forms bound by planar faces is now known as a crystal.

A crystal of quartz

A crystal of quartz

With the development of the atomic theory of matter, we now realize that the beautiful symmetry of a crystal is simply the reflection of its internal structure, a lattice-work of linked atoms repeating themselves in endlessly in three dimensions. Even when a mineral is confined and unable to develop its ideal outer expression of crystal faces, its internal order may still reveal itself when it is broken. Cleavage planes in feldspar scattered along the forest trail flash back at you like mirrors when the light is right, literally reflecting the mineral’s crystalline structure:

A cleavage plane in feldspar shining along the trail

A cleavage plane in feldspar shining along the trail

So both of my young hikers were right. In two words they accidentally captured an ancient linkage that gave a permanent name to the crystalline nature of minerals.