Origins of Oakland ocher

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.

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One Response to “Origins of Oakland ocher”

  1. marymactavish Says:

    this is so cool! I want to go poke it!

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