Sunday Sensational Science

Arizona Rockhounding

One thing Arizona doesn't lack is rocks. We've got nearly every type of rock on earth, plus a big one from space. And they're not buried under a bunch of inconvenient vegetation. They're right out in plain sight. For the geologist, Arizona is utter paradise.

I did some rockhounding in my home state, and subjected you to the results in Arts & Cats I, II & III. You even got to see my horrible attempt at photographing the completed collection:

Now comes the science part, which is a hell of a lot more interesting than just looking at someone's random collection of rocks. We'll be discussing a select sampling, delving into deep time and discovering how little (well, mostly big) rocks are made.

And, yes, there will be a cat involved.

We'll leave the photography to others for the most part, though. Blurry snapshots will not do for these grand old rocks.

Let's begin in the deepest of deep time. Let's talk schist.

Alas, I could not find a decent photograph of a Brahma Schist specimen, so this shot of schist in-situ will have to do. I can't prove beyond reasonable doubt that the dark bit of schist I picked up by my house in Flagstaff is, indeed, Brahma Schist, but like any good suspect, it matches the general description. We'll run with it.

The Brahma Schist is found with the Vishnu and Rama Schists, gorgeously exposed in the Grand Canyon. When you hold a piece of this schist in your hand, you're holding nearly two billion years of history. Brahma Schist "[c]onsists of amphibolite, hornblende-biotite-plagioclase schist, biotite- plagioclase schist, orthoamphibole-bearing schist and gneiss, and metamorphosed sulfide deposits." It was created from a variety of volcanic rocks from mafic to intermediate composition. That's a fancy way of saying the Brahma Schist is made from volcanic islands that, due to the vagaries of plate tectonics, got squashed up against the North American continent a long time ago.

As the islands collided and ocean crust sank into the subduction zone, mountains formed and the lower part of the earth's crust got pushed down into the squishy, near-molten mantle. Schist formed in the middle bits of this mass.

Since it came from dark volcanic rocks, the Brahma Schist is fairly dark. But schist is a variable stone, and our next specimen looks like a flaky chunk of the sun when the light hits it right. My darlings, allow me to introduce you to mica schist.

Well, actually, Jim and Ellen of Jim & Ellen's Rock-N-Shop shall be doing the honors:

Mica schist represents the final stage of metamorphism. Just short of actual remelting, mica schist is the penultimate product of alteration, by heat, volatile gasses, and pressure, of the mixture of the hydrated and oxidized minerals in shale and muddy sandstones. The "normal" stages of the metamorphic process due to plate tectonic activities are as follows: Shale - slate - phyllite - Mica schist. Sandy shales, on complete recrystallization, will be compressed and recrystallize to a final rock that appears to be mostly mica. The mica crystals have arranged themselves so that the flat plates have grown at right angles to the crustal pressure affecting the rock. Other volatile compounds produced in this intense metamorphic environment are garnet, staurolite, andalusite and kyanite as well as bits of granite and feldspar which are not very obvious because the flat cleavage planes of mica dominate the outward appearance of the specimens.

My gorgeous hunk of mica schist comes from Mingus Mountain, which is part of Arizona's Black Range. Mingus "exposes Precambrian, Cambrian, Devonian, Mississippian, Pennsylvanian, and Tertiary rocks." A long history, to be sure! It's one of the few mountains near Northern Arizona that wasn't caused by a recent volcano going boom. Further discussions of its geology, alas, shall have to wait until I've finished reading up on Arizona's geology, as no one seems to have been considerate enough to a) put up a long description online b) where Google can easily find it.

Ignorance is not bliss.

Mingus overshadows Jerome, Arizona, where you can come across all sorts of interesting and often valuable rocks. Some of the rocks aren't even rocks as most folks understand them, but metal. Jerome owes its existence to copper, which deposits were "the result of two giant, mineral-rich hydrothermal vents that formed its ore deposits some 1.7 billion years ago." That's right - hydrothermal. As in, deep ocean vents. The ore bodies "occur at the top of a great pile of Precambrian submarine volcanic rocks, now so metamorphosed that they were at one time thought to be an intrusion. Some of the rocks in direct contact with the ore bodies have been dated as about 1,800 million years old." (source) Pretty amazing to think that my little nugget of native copper from Jerome might be about as ancient as my Brahma schist.

Copper usually doesn't appear in shiny little lumps, but in ores. One of those ores, and one of my favorite finds, is conichalcite, calcium copper arsenate hydroxide. What a mouthful, right? Here's what Amethyst Galleries has to say about it:

Conichalcite has a sparkling grass green color that once observed is hard to mistake for any other mineral. It is often encrusted onto limonitic rocks that have a red to yellow color and the two produce a very colorful specimen. Conichalcite forms in the oxidation zone of copper ore bodies. Oxygen rich ground water that might react with copper sulfide and/or copper oxide minerals produce a wonderful assortment of attractive and colorful minerals in a zone called the oxidation zone. Conichalcite is just one of these minerals. Other oxidation zone minerals include malachite, azurite, linnarite, etc.

Conichalcite forms a solid solution series with the mineral calciovolborthite. A solid solution series occurs when two or more structurally identical minerals can interchange elements within their chemistries without dramatically altering the crystal structure. In the case of conichalcite and calciovolborthite the two elements are arsenic and vanadium. Conichalcite is the arsenic rich end member of the series and calciovolborthite is the vanadium rich end member.

My bit comes from Sonora, Mexico, but it's also common enough around Arizona to count as an Arizona rock.

Another copper ore, bornite (copper iron sulfide), is one of the most flamboyant minerals ever. Its spectacular blues and purples are actually tarnish:

The colors are from an iridescent tarnish that forms on bornite upon exposure to air. The tarnish is made of assorted copper oxides or hydroxides that form a mere atoms thin layer over the bornite. The thickness of the layers is close to the wavelength of light. When light waves bounce between the bornite surface and the top of the tarnish layer they will leave with the wavelengths of various colors. This effect is the same as the rainbow effect that occurs with oil on water. In the case of bornite, the tarnish will have a purplish, violet or blue color. Because bornite is often intergrown with chalcopyrite which tarnishes to more greens and yellows, the peacock ore may have many colors ranging from purple to blue to green to yellow.

As you can see, it's also considered a rather tasty treat by my parents' cat Spook. I'm not sure it's part of the recommended daily allowance of minerals, but I don't think he cares.

It's not just attractive to humans and cats, but valuable:

Bornite is an important copper ore mineral and occurs widely in porphyry copper deposits along with the more common chalcopyrite. Chalcopyrite and bornite are both typically replaced by chalcocite and covellite in the supergene enrichment zone of copper deposits. Bornite is also found as disseminations in mafic igneous rocks, in contact metamorphic skarn deposits, in pegmatites and in sedimentary cupriferous shales. It is important for its copper content of about 63 percent by mass and is found in Arizona, Butte, Montana, and Mexico.

I have no idea where my specimen came from, because the gift shop at Gold King Mine didn't say. We'll pretend it's a native, then.

My next copper ore is most definitely an Arizona native, although I had to go all the way home to Washington to get it. Pima County's not one of my usual stomping grounds in Arizona, though I've been down Highway 83 and passed right by the Santa Rita Mountains, where my lovely specimen of azurite hails from. It came from Gunsight Pass, close to the old ghost town of Helvetia.

First, some geology to set the scene:

Here as in many southern Arizona mountains the geologic pattern includes enigmatic thrust faults with slices of Paleozoic sedimentary rocks sitting astride or leaning up against a Precambrian core. The overthrust school subscribes to broad movement of a thin sheet of rocks from as much as 100 miles to the southwest. Thrust faulting in the Santa Ritas occurred 75 to 80 million years ago. Because their sedimentary sequence is relatively complete and only slightly deformed, these mountains contain more clues than most to the geologic history of the region. Both Paleozoic sedimentary strata and Precambrian core are intruded by Tertiary porphyry associated with scattered copper deposits. (source)

And those scattered copper deposits are sometimes given away by the presence of azurite (copper carbonate hydroxide), a gorgeous mineral formed from the weathering of copper ore:

Azurite is a very popular mineral because of its unparalleled color, a deep blue called "azure", hence its name. Azure is derived from the arabic word for blue. The color is due to the presence of copper (a strong coloring agent), and the way the copper chemically combines with the carbonate groups (CO3) and hydroxyls (OH). Azurite has been used as a dye for paints and fabrics for eons. Unfortunately, at times its color is too deep and larger crystals can appear black. Small crystals and crusts show the lighter azure color well. Azurite is often associated with its colorful close cousin, malachite.

In fact, malachite often sneaks in and replaces azurite.

If the emphasis seems a little heavy on copper and its mineral ores, well, that's because copper and Arizona are virtually synonymous. You can't really throw a stone in most of the state without hitting a copper mine, or so it often seems. In fact, here's one now:


Open pit mines became the done thing in Jerome after a little incident with chemistry and the United Verde mine back in 1894. Which, oddly enough, created yet more minerals I plan to someday get my hands on.

But let us move on from copper and all its varieties, and spend some time in Arizona's coastal dunes. In order to do so, we'll have to travel back in time about 262 million years, to the time when the Toroweap Formation was being deposited. The Toroweap's a riot of rocks - sandstones, limestones and mudstones, in places even containing gypsum. Allow Ron Blakey and Wayne Ranney to set the scene (source):

Toward the end of the Early Permian, two marine transgressions finally completed the [Colorado] plateau's Paleozoic section of rocks. Both seas entered the region, this time from the west. The first of these transgressions deposited marine, sabkha, and shoreline sediments known as the Toroweap Formation.... [In the Sedona area,] eolian shoreline environments deposited resistant, cross-bedded sandstone that laterally replaces the softer sabkha deposits. These eolian deposits form an "upper Coconino Sandstone" (or "sandy Toroweap Formation," take your pick) in Northern Arizona (Oak Creek Canyon...)."

Below it, sometimes mingling with it, the Coconino Sandstone is a huge erg (dune field) deposit that indicates Arizona was once a sea of sand. It's 500 to 1000 feet thick, cream or pale golden-colored sandstone formed after the ancient Pedregosa Sea retreated 265 million years ago. In places in Oak Creek Canyon, you can see its exquisite cross-bedding, showing that it's basically petrified dunes. It forms a nice white counterpoint to the tans and pale reds of the Toroweap Formation, and yes, thank you, all of them look wonderful in my collection.

Since the fragments I picked up were rather too wee to photograph well, and online sources are more excited about cliffs than specimens, you'll just have to content yourselves with a photo of me collecting bits of the two formations:

Exciting, right? Just imagine walking alongside a road with no shoulder and hairpin turns just because you're determined to get yourself a piece of Oak Creek Canyon, and you might find it a bit more entertaining. Yes, I'm obsessive.

The final piece in the collection I shall subject you to today, so unique that I must force my foul photography upon you, is a bit of a mystery, because I picked it up from the Dry Beaver Creek stream bed along Highway 179 back in the 90s. Being that it's obviously been swimming, it could be a piece of anything. It could be a hunk of the Hermit Formation, which includes red and white cross-bedded calcareous sandstone and siltstone. The Hermit was formed from fluvial redbeds, a fancy way of saying that large river systems spread a bunch of sediment around an arid landscape. It could be a chunk of the Schnebly Hill formation along with a bit of Coconino Sandstone. Or it might be a bit of the Supai Group, with the white bits bleached by submersion in a less fickle water source than Dry Beaver Creek. You can read up on Sedona-area geology and take your pick. No matter what it is, holding it in hand reminds you that Arizona's gone through a lot of interesting changes in its (in places) 2-billion year history.

And you can read all about it in the rocks.

(As always, click the pics for sources. If all you get is a ginormous version, that means I'm the one responsible. My apologies.)