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| Group Igneous | Origin Intrusive | Grain size Coarse | Crystal shape Anhedral, Euhedral |
| Classification Ultrabasic | Occurrence Orogenic belts | Color Dark | waxy luster |
Easily seen coarse grain crystals and dark coloring. It is a plutonic rock with an ultra basic composition, with less than 45% total silica. It is composed almost entirely of serpentine minerals, such as antigorite and chrysotile. relics of olivine are often present. Other ferro-magnesian minerals such as garnet, pyroxene, hornblende, and mica are also commonly found, as are chromite or chrome spinels. It is dark in color, with areas of black, green, or red. The texture is a coarse- to medium-grained rock in which most crystals are easy to see with the naked eye. This is a compact, often banded rock, commonly veined by fibrous serpentine. The Origin occurs as dikes, stocks, and lenses. Serpentinites are formed by the serpentinization of other rocks, principally peridotite. It commonly occurs in folded metamorphic rocks, probably form altered olivine-rich intrusions. |
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| Serpentine is a major rock forming mineral and is found as a constituent in many metamorphic and weather igneous rocks. It often colors many of these rocks to a green color and most rocks that have a green color probably have serpentine in some amount. Serpentine is actually a general name applied to several members of a polymorphic group. These minerals have essentially the same chemistry but different structures. The following is a list of these minerals, their formulas and symmetry class: |
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Their differences are minor and almost indistinguishable in hand samples. However, the chrysotile minerals are more likely to form serpentine asbestos, while antigorite and lizardite form cryptocrystalline masses sometimes with a lamellar or micaceous character. Asbestos had been used for years as a fire retarding cloth and in brake linings. Its links to cancer however has led to the development of alternative materials for these purposes. Serpentine's structure is composed of layers of silicate tetrahedrons linked into sheets. Between the silicate layers are layers of Mg(OH)2. These Mg(OH)2 layers are found in the mineral brucite and are called brucite layers. How the brucite layers stack with the silicate layers is the main reason for the multiple polymorphs. The stacking is not perfect and has the effect of bending the layers. In most serpentines, the silicate layers and brucite layers are more mixed and produced convoluted sheets. In the asbestos varieties the brucite layers and silicate layers bend into tubes that produce the fibers. Serpentine can be an attractive green stone that takes a nice polish and is suitable for carving. It has been used as a substitute for jade and is sometimes difficult to distinguish from jade, a testament to the beauty of finer serpentine material. Non-fiberous serpentine is not a cancer concern. Asbestos serpentines should be kept in closed clear containers, but makes an attractive specimen. Sometimes with a golden color as the name chrysotile in greek means golden fibers. |
PHYSICAL CHARACTERISTICS:
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| Rocks and Minerals eyewitness Handbooks by Chris Pellant Minerals by Name located at http://mineral.galleries.com/minerals/by_name.htm Rocks and Minerals Back to Sunol home page |
| Sandstone (gray wacke), fine- to corarse-grained, greenish-gray where fresh, yellowish-brown to yellowish-orange where weathered; and shale, olive-gray. |
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Information noted on the Preliminary Geologic Map of La
Costa Valley Quardrangle by the U.S Geologyical Survey Open-File Report. |
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| Chert is a specil kind of sedimentary rock conspicuous
in the San Francisco Bay Area Region. Most comonly it is of marine origin, but is found in some lake deposits as well. The rock is dense and hornlike and may be almost any color. Red, green, and light-gray hues perhaps are most common. Silica in the form of chalcedony, opal, or jasper is the principal consituent, but the silica may be intermigled with clay, iron oxide, microscopic shells, or even silt. It is believed to form by chemical preipitation form bodies of water into which volcanic mineral waters have been introduced. Micro-organisms may play some part in the process of accumulation. Cherts are generally found thinly and distinctly bedded. Commonly they are interbedded with much thinner layers of shale, but in many instances the chert beds are simply piled one on another. Indivdual beds commonly are one to two inches thick. Thick-bedded and massive chert bodies are relatively rare in the San Francisco Bay region. In cherts of the Franciscan group of rocks, red iron oxide and black manganese oxide are common constituents of the shale interbeds. In the Corral Hollow district of eastern Alameda County, the manganese-bearing chert shales are extensive enough to be sources of manganese ore. There is well over two-thirds of the land surface of the San Francisco Bay region with an underlain by marine deposits, these are seen everywhere. Rocks and Minerals of the San Francisco Bay Region, 1966. Rocks and Minerals, A Golden Nature Guide, 1963 |
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Above right, You can see the toe of the slide to the left of the tree. Below, Looks like a nice place to build WRONG. This is a slide area. |
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http://geohazards.cr.usgs.gov/elnino/elninols.html http://www.usgs.gov/elnino.html |
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