A hunt for silica-carbonate

The geologic map of the northern East Bay that I rely on has a few rock units that are very small and hard to notice. One of them is the ultra-purple unit designated “silica-carbonate rock.” The map shows only three small exposures — one in Oakland and two in Berkeley — but they’re close enough to each other to visit in an afternoon.


So that’s what I did back in August, hiking from the lower-right corner to the upper-left through all three areas.

“Silica-carbonate rock” is what happens when serpentine rock is invaded by superhot carbonated fluids, which replace the serpentine minerals with quartz and magnesium carbonates (dolomite and magnesite). The spectacular mercury deposits of the New Almaden and McLaughlin mines are of this type.

I wasn’t too sure what to look for, except that a rock made of hydrothermal quartz and Mg carbonates would be white and messy. Fortunately, the U.S. Geological Survey library, in Menlo Park, has a boulder-size specimen of mercury ore sitting around.


Unfortunately, that’s not a very informative specimen; moreover it’s labeled “calc-silicate rock,” which would be quite different (it’s what happens when lime rocks are invaded by silica-rich fluids). So who knows.

To traverse the first locality, I started at the end of Chabot Road.


It’s a highly disturbed place where railroads, streets, culverts and freeways have come through over the years, and it’s hard upon the Hayward fault. There is little promise of bedrock here, but I kept a close eye out anyway. There was some float, or loose rock, that was likely local: some brecciated stuff from the Leona “rhyolite,” tumbled down from its exposures above Tunnel Road.


Some more of the “rhyolite” plus gray sandstone from the Franciscan melange mapped to the west.


And at the north end, during a strenuous climb, some Franciscan chert from the melange.


The next locality is at the top of the UC Berkeley campus. That was hopeless, given all the buildings and landscaping. Except for Founders Rock.


This excellent knocker has a plaque on the back that reads, “Founders Rock / College of California / April 16, 1860 / Inscribed May 9, 1896” but there are no geological notes. Close up, the rock is enigmatic.


Not much to do here but scratch your head, and feel sorry for any geology students assigned to write a report on this rock.

Onward through Berkeley’s steep hills to Keith Street, the third locality. That’s a residential street with all of its bedrock hidden, but I scrutinized the stone walls, in case the builders had used local rocks. You never know.

Some of those were could-bes. (A reminder: all of my photos click through to a 600- or 800-pixel image.)



What I see in these is a uniform light color, suggesting pervasive alteration to siliceous material; brecciation and deformation, typical of an active hydrothermal environment; hints of channels and fractures such as you’d expect from hydrothermal replacement; and bits of iron staining from weathering sulfides. Without chemical tests and petrographic thin sections to examine, none of that is definitive. I did drop acid on them, but there was no reaction, nor would you expect one.


That’s OK, I still had fun. And North Berkeley neighborhoods are famous for their integration of stone with stylish dwellings of all vintages.

Because it was a one-way walk, from the Rockridge BART station to the 67 bus line, this qualifies as a ramble.

2 Responses to “A hunt for silica-carbonate”

  1. brian oregan Says:

    Further to the above: I have found an visible outcrop of what could be called “silica carbonate”; at the front of 1021 Kieth. It is about 8 feet above the street on a fairly steep slope. The outcrop can be seen on google street view just above the Pride of Madeira which is in turn just above the wooden retaining wall. It’s not a very convincing outcrop in street view, but after the recent rains there is a lot less dirt on the outcrop and, in person, it seems like “bedrock” to me. whatever that really means in berkeley. The rock has a dark grey matrix with <1mm to ~1 cm white calcite veins that occupy ≤5% of the volume. (or so I estimate). The white veins bubble rapidly in RT 10% HCl. The matrix is at least partially crystalline. Through a 20x lens I can make out some small platy, curved, xls. If it's serpentinite it is quite a dark version, but not darker that some of the images on the web.
    After etching for 1 hour in 180F 10% HCl, some areas show deeply etched out veins. Some of the black areas show only the very slightest of bubbling and no significant etching. I will send Andrew some photos and happy to show and/or share the samples I have to anyone interested.

  2. brian oregan Says:

    I’ve been working my way through your blog, starting a few months ago.
    I just reached “the hunt for silcon-carbonate”. Given I live about 2 blocks
    from Spruce and Kieth streets, I thought I go have another look. I went up Keith, to Euclid and
    back down Cragmont to Santa Barbara to Spruce, carefully looking for anything “bedrocky”.
    There is a quite large knocker in the backyards of 1065 and 1061 Keith. (most
    easily viewable down the side yard of 1030 Cragmont.) But by the 2010 Graymer map, that
    would be in Jsv, whatever that might mean in Berkeley.
    I have a photo of it if you are interested. There also appears to be some
    bedrock underneath the deck of 1011 Keith , way above the road. I couldn’t tell what it was,
    but I am pretty sure I can get access to that. I think I know the owner vaguely.
    Last there is an outcrop of something on Spruce under the back stairs of 974 Santa Barbara.
    It isn’t far off the sidewalk, and it sure doesn’t look like silt or clay shale of Kjk. (knoxville)
    I didn’t climb up to it today, but perhaps I will. I took a picture of a piece
    that seems to have fallen off, which I have sent to your email. I am happy to show you where it is
    if you are back this way sometime. Feel free to contact me. Also I’d like to have your opinion of the large knocker in the front
    yard of 1161 Oxford. Given just 4 houses down the road, I am sitting on 20+ feet of landslide deposits, I find
    that knocker quite intriguing.

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