A family of silicate minerals rich in magnesium and water, derived from low-temperature alteration or metamorphism of the minerals in ultramafic rocks (intrusive igneous rocks very rich in iron and magnesium and with much less silicon and aluminum than most crustal rocks, most come from the Earth's mantle). Serpentine minerals are light to dark green, commonly varied in hue, and greasy looking; the mineral feels slippery.
Rocks made up of serpentine minerals are called serpentinite.
The Origin of Serpentinite:
The origin of serpentinite is inferred to be a metamorphic alteration product of mantle rock or oceanic crustal rock.
Grades of Serpentinite:
Serpentinite is considered greenschist, a metamorphic facies associated with low temperature, low pressure conditions relative to other grades of metamorphic rocks. Higher pressure metamorphic grades of serpentinite contain glaucophane (a pale bluish-gray to black serpentine mineral that occurs in fibrous to felted aggregate masses). Higher temperature and pressure metamorphic grades contain garnets, pyroxene- and amphibole-minerals and are grouped into a metamorphic class called "eclogite".
California Coastal Ranges:
The typically green serpentinite that occurs in relative abundance throughout Franciscan rocks in the California Coastal Ranges consists dominantly of "antigorite" (a typically green mineral with a dull, earthy, or frothy texture, but has a soft, soapy feel on some fresh surfaces that may display a conchoidal fracture pattern), lizardite (white to pale gray-green with a platy or scaly texture, typically found on exposed surfaces of antigorite masses), chrysotile (white, pale green to bluish-green, fibrous to frothy-textured masses, commonly occurring in fractures on weathered surfaces), and accessory minerals including chlorite, talc, magnetite, magnesite, and other minerals.
Landscape with serpentinite bedrock tends to have thin or absent soil cover. Serpentinite soil tends to have low levels in all major plant nutrients (particularly calcium), and tend to be rich in magnesium, chromium and nickel - elements that are probably toxic to many plants. Many plants that grow on serpentinite will grow on non-serpentinite soils, but they tend to be crowded out by other species, particularly non-native grasses. However, many non-native grasses and other plants tend not to grow well on serpentinite soil.
-- Excerpt from:
USGS/NPS Geology in the Parks Website, September 2001, and Stoffer and Messina, 2002, Field-Trip Guide to the Southeastern Foothills of the Santa Cruz Mountains in Santa Clara County, California: USGS Open-File Report 02-121.