Archive for the ‘Oakland rocks’ Category

Origins of Oakland ocher

27 March 2017

Before Europeans came into this country, the locals treasured the ocher deposits in the East Oakland hills. Ocher is the name for a variety of clay-like, iron-rich minerals with a color range from yellow to red to brown. For tens of thousands of years, we’ve used ocher as pigments and preservative coatings. Some cultures would bury their dead in it.

Our ocher deposits formed exclusively in the Leona volcanics, because that body of rocks was permeated with pyrite by hydrothermal springs as it rode on the seafloor toward North America, back in the Late Jurassic. Pyrite is pure iron sulfide (FeS2) and looks like this.

You can get nice crystals of it at any rock shop.

In the Leona volcanics, you’ll sometimes see pyrite in fresh exposures, like this roadside boulder along Campus Drive. It’s gray because the crystals are so small.

Oxygen, in air or in water, breaks pyrite down. The sulfur turns into sulfuric acid and leaches away while the iron oxidizes into a range of minerals on the ocher spectrum. This process reliably turns the surface of the Leona orange and red, like here in the former Crusher Quarry.

Pure, straight iron oxide (Fe2O3) is the mineral hematite, or red ocher. It can look black, but when powdered it turns the lovely color shown on the streak plates.

Between pure FeS2 and pure Fe2O3 is a range of hydrated iron oxides that form ochers of different colors. The roadcut on lower Redwood Road, at the site of the former Alma Mine, shows off some of them well. Here’s a hematite crust, which is right near a piece of concrete pavement that’s eaten out by acid.

And here’s a beautiful brown crust.

Most likely this is goethite (“GUUH-tite”), or brown ocher or sienna, an iron oxyhydroxide with the formula FeO(OH). Here’s a specimen I collected in Wisconsin, with a glittering crust of hematite on it.

Yellow ocher has even more water associated with it — the formula is FeO(OH) · nH2O. That’s what I would call this crust in the Crusher Quarry.

There are wild cards in this scheme, namely manganese oxides and jarosite. Manganese oxide, the mineral psilomelane (“sigh-LOW-ma-lane”), is black. Just a few percent turns ocher into umber. (So does carbon.) Jarosite is a hydrated iron sulfate that can form if some of the sulfur lingers instead of turning to acid. It has yellow to brown colors.

So really good ocher, in chunks worth the effort of digging, is hard to find. Oakland once had a large body of it that had slowly gathered on top of the Leona volcanics as the rock beneath was etched away by acid. Such an iron-oxide cap is called a gossan. A little bit of the deposit is preserved on the Holy Names University campus.

All of these ocherous minerals are important ingredients in soil, especially in dry regions. Rarely are they pure, though. Oakland’s ocher patch was the center of a widespread trade, back in the day. But in the late 1800s, Americans mined it out and turned it into red paint.

Two bits of gabbro

20 February 2017

I’ve noted that while the San Leandro Gabbro has a presence in easternmost Oakland, it’s hard to find. The geologic map shows what seems like a lot of it, marked “Jgb” for Jurassic gabbro.

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But if you poke around on the ground, nearly all of those sites are inaccessible due to steep woods, roads or housing tracts. But once last year and once the year before, I found some. The two spots are marked on the map with white asterisks. The northern one is at Seneca Reservoir, right next to the Hayward fault, and the southern one is in Sheffield Village at the north end of Middleton Street where it meets Marlow Drive.

The northern site, Seneca Reservoir, was once the upper pit of the old Catucci quarry. (The lower pit was repurposed as the site of Bishop O’Dowd High School.) Not much of it is accessible, but here and there you can spot pieces of the quarry waste. It looks like nothing else in town and everything like San Leandro’s namesake stone.

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I can’t say the same for the Middleton Street exposure, but on the positive side it’s real easy to visit. Oddly, the last time I came through here, in 2013, I paid it no mind, focusing instead on the other side of the street.

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This site too is very near a strand of the Hayward fault, so it’s been rattled and squeezed for quite some time. It has a battered appearance, even a little fried.

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And no matter how close you get, it doesn’t show much detail. As a whole, though, it has the typical color of the gabbro: light gray with a slight blue-green tinge. This resulted from petrochemical disruption at the time of its eruption, some 165 million years ago, when a pulse of younger magma sent up fluids that changed its black pyroxene minerals into green amphibole and some other greenish minerals — an obscure process known as uralitization.

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I mentioned the Hayward fault being next to the reservoir. We probably would think twice before building that there today.

Museum-quality rocks from Oakland

30 January 2017

I keep saying that Oakland has geological features worthy of being put in textbooks. Today I’m here to show you that Oakland has rocks worthy of being in museums, and I’ve put them there.

In 2012, I was asked to put together a set of teaching rocks for the Chabot Space and Science Center. After all, other planets are made of rocks, right? It took some doing, but some of the rocks were easily available within Oakland’s borders in roadside exposures. The conglomerate of the Orinda Formation was one.

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The red chert from the Franciscan Complex was another.

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And of course there was our serpentinite.

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All told, I made five sets of 15 rock types for the kids.

The next year I got a request from Las Positas College, in Livermore, for a boulder of blueschist. Turns out this little college teaches geology, because every citizen will benefit from a course, and students can get a head start on a 4-year degree there. I struggled one out of this streambed, where it wouldn’t be missed.

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They installed it in their teaching garden as Rock J, on the left. It’s small compared to its mates, but that thing weighs a ton because high-grade blueschist is pretty dense.

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My reward included a visit backstage to see their cool collections.

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Then last year, I got a note out of the blue from the under-construction Maine Mineral and Gem Museum asking my help in building their collection. Maine is well known for its gemstone and mineral mines, but the state has no blueschist. I went to a quiet outcrop where it’s just lying around.

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Got two nice boulders and couldn’t choose between them, so I sent them both. They told me one will go on display and the other will go in their teaching collection.

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None of these are precious collectibles or gemstones. They’re just cool and educational.

I’ve pretty much stopped collecting rocks for myself because I’m not important enough. But museums are important enough.