Archive for the ‘Oakland stone’ Category

Oakland building stones: Slate

10 October 2016


Slate offers visual texture and a range of colors to the architect, plus superior performance as an exterior stone. It’s not particularly strong among California stone resources, but slate is deeply engrained in European-American culture and geologically interesting to boot. This photo is from a downtown building where slate is used as a wall finish. It presents a naturally textured surface consisting of very thin layers that aren’t boringly flat.


Slate is well displayed at the Ordway Building, where it makes up the pavement around Oakland’s tallest building. Its dignified gray color and organic texture, reminiscent of wood grain, is an excellent foil for the metal and glass around it.


Slate is a claystone or shale that has been squeezed enough to start remaking its mineral content. The clay begins to convert to mica, but more importantly the minerals realign their crystals in response to the pressure. This change imposes a strong new fabric upon the stone that allows it to be split into thin sheets. In this photo from the Ordway Building, the dim stripes running almost vertically down the image are remnants of the original bedding in the shale protolith. You can also see, at the upper right, the approximate point where the quarry worker struck the slab to split off this sheet.

In Oakland buildings, slate appears mainly as an accent in the outside facades and sometimes as a floor in interiors. And, of course, as rugged fireproof roofing tiles.


The rest of the photos below are from the eastern U.S. where slate has been produced for hundreds of years from occurrences in the Appalachian mountain chain.


Slate is not rare — California has lots of it in the Sierra foothills — but it’s hard to find deposits with good, flat slaty cleavage. The two biggest slate-producing areas in this country are in Pennsylvania and in Vermont and the adjoining area of New York, where I took this photo of a waste pile. There’s a great deal of waste rock in a slate operation. Stone like this is still good for flagstone.


In the slate regions, you’ll sometimes see the stone used in unusual applications like this post office building. Notice the range of colors.


And this staircase in Albany, New York, shows how properly selected slate can perform very well under foot traffic. Another advantage is that it isn’t slippery when wet.

The nearest thing to slate that Oakland has is argillite, which is the same metamorphosed claystone but without the slaty cleavage. I think the stone in the Davie quarry qualifies.

Oakland building stones: Larvikite

3 October 2016

The building that houses Autotrends Body Shop, on Broadway’s Auto Row, is trimmed with larvikite, a remarkable decorative stone from the area of Larvik, Norway, in the southern part of the Oslo Graben.


It’s a steel-blue stone, just under 300 million years old, with flashing highlights from fingernail-sized crystals of feldspar. This closeup from the auto body shop shows the richly textured grains in detail. The many tiny dots are probably either dirt or paint spatter, so ignore them.


Other than the black grains, which are mostly titanium-rich magnetite and possibly augite, this rock is almost totally feldspar. The large feldspar grains are intimately intergrown crystals of orthoclase and alkali feldspar. These separated out (exsolved) from an initial material consisting of anorthoclase feldspar in microscopically thin layers (lamellae) as it very slowly cooled. Because the three feldspars occur together, this is called a ternary feldspar. (Feldspars are complicated minerals — there will not be a quiz.) The visible lines are tiny fractures inside the grains that are typical of feldspar.

The lamellae interfere with light in a way that absorbs red and yellow wavelengths. The remaining blue or green diffraction colors are what give the stone its gleam, a characteristic called schiller. The gemstone labradorite, another feldspar, also displays this kind of schiller.

Here’s a cleaner example from a gravestone I photographed at Evergreen Cemetery a few years ago.


This stone has been produced since the 1880s. Buildings all over Oslo, Sweden, feature green, blue and rare red larvikite. There is a large amount of quarry waste involved, which the locals use for everything from concrete aggregate to seawalls.

Larvikite is scarce in Oakland buildings, but you’ll see it as accent stones or in interiors. The bar at Club 21, for instance, is a gorgeous slab of it that glitters like a cosmos under the disco lighting. Think of happy dancing Swedes when you’re there for the monthly meetings of Nerd Nite East Bay. (I’ll be speaking there December 26.)

The balsawood boulders of the Kaiser Roof Garden

26 September 2016


Ever since 1960 when the rooftop garden on the Kaiser Center’s garage, largest in the world at the time, was built, it’s been heralded by American landscaping pros, Kaiser Center employees and appreciative residents. After the garden’s recent rehabbing, now (specifically, any weekday) is a good time to visit.

Putting a garden on a roof, even one made of the best Kaiser reinforced concrete, takes careful design and execution. The heaviest plantings and features were situated directly over the strongest supporting columns.

Even so, the designers had to minimize the weight of everything. The soil, a blend of lava rock, expanded shale and peat moss, is practically potting mix. The concrete is a lightweight formula. And then there are these impressive boulders.


Each one is a generously sized hunk of pumice, the nearest thing to balsa wood in the mineral kingdom. Well, diatomite is lighter, but so friable you can crumble it in your hands. Pumice, basically a foam of expanded lava, stands up to weather beautifully with its relatively inert glassy composition. And close up, it’s handsome.


If you see this stuff in a landscaping-supply yard, kick it around or pick it up. It’s so light, you’ll feel like Superman — but pumice will cut your bare skin, so be careful.