Over the years I’ve collected a few sightings in Oakland of actinolite, an uncommon-but-not-rare mineral in the Coast Range and Sierra foothills. This is my reference specimen, one I bought at a Russian River rock shop when I asked the owner for something local.

Actinolite is a metamorphic mineral, forming when basalt and similar rocks are altered by moderately high temperatures and pressures, or invaded by chemically active fluids. When faced with a greenish rock, we have several iron-bearing minerals that could account for the color. I tell actinolite by its particular rich green and its radiating, needlelike crystals (the “actino-” part of the name is scientific greek for “ray”). The green comes from divalent iron (Fe²⁺) and can be light or dark depending on the iron content. Its chemical formula is Ca₂(Mg,Fe)₅Si₈O₂₂(OH)₂, in which magnesium always outweighs iron but not completely. (With less than 10% Fe the mineral is tremolite; when Fe exceeds Mg the mineral is ferro-actinolite.) The Si₈O₂₂ part signifies a structure of twin chains of silica units with the other atoms arranged around them to balance charges, and that accounts for the elongated crystals.

I picked this chunk of actinolite out of Peralta Creek in Rettig Canyon. It’s not as pretty but it’s genuine. Under my ultraviolet flashlight, I fancy a faint blue fluorescence, which might reflect the presence of finely disseminated prehnite.

I’ve also encountered it along the trail on the south side of Palo Seco Creek in Joaquin Miller Park. That’s a park so I left it alone.

I’ve seen it around town, once in someone’s artistic driveway and twice as yard boulders, farther down Peralta Creek and up at Mountain and Maynard (though that one may be from a landscaper’s yard). Finally, I have a resident’s report of actinolite on their land east of Mountain View Cemetery.

The thing they seem to have in common is locations mapped as Franciscan melange, which is basically a deep-sea landslide deposit that recycles chunks of whatever different rocks were around. But that just pushes the question back: how did the actinolite form in the first place?

Up top I mentioned two different ways to make this mineral, physical and chemical. The physical method is high temperatures and pressures, which around here means subduction. Actinolite is typical of greenschist, which is what mafic rocks like basalt become when subducted to moderate depths (~20 kilometers) and temperatures (~400°C). The chemical method is invading fluids or metasomatism, which can happen at lesser depths when serpentine rock interacts with mafic rocks, maybe even within the melange. We have both ingredients in Oakland: serpentinite and the Jurassic-age basalt mapped in the same parts of town — I emphasize “mapped” because it’s hard to actually find on the ground.

Clearly I need more data. Have you seen examples?

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