Mariposite: The Rock That Made California Famous

July 1987. Wanted: One 10,500-pound rock to represent the Golden State of California in Philadelphia’s “Fountain of Freedom-The Constitutional Monument” celebrating the 200 anniversary of the United States Constitution and the Bill of Rights, to be delivered to Philadelphia Pennsylvania by September 17, 1987, the bicentennial of the signing of the U.S. Constitution.

The El Dorado County Committee on the Bicentennial of the United State Constitution accepted the assignment to select and deliver California’s rock for use in the monument. The Committee first nomination was serpentine, California State Rock. However, local geologist George A. Wheeldon recommended that mariposite, a rock associated with serpentine, be selected to represent California at the Fountain of Freedom.

Wheeldon also searched for the source of the gold that touched off the California Gold Rush. On January 24, 1848, James W. Marshall discovered a gold nugget in the tailrace of Sutter’s sawmill at Coloma, on the South Fork River. Wheeldon located an outcrop of mariposite4 mile upstream from Coloma which may have produced gold. The outcrop can be seen from the head of the Big Canyon, north of Placerville and east of Highway 193. Several Boulders from the outcrop were found on the property of Eugene and Cathy Barnett. The Barnetts donated the mariposite boulder for use in the monument. With this gift, serpentine lost to the harder, more weather-resistant, gold-bearing mariposite rock.

So the beautiful mottled green and white mariposite rock was selected to represent California in the Fountain of Freedom Monument.

Description

Occasionally mariposite rock contains networks of gold-and iron sulfide- bearing quartz veinlets and stringers. Mariposite rock consist of the mineral mariposite (a bright apple green chromium-rich mica) with a white groundmass of fine – grained glassy quartz. Several carbonate minerals are present in some specimens. In 1868, Benjamin Silliman Jr. collected a sample of mariposite from the Josephine mine and named it for Mariposa County, California. Since then, it has been identified throughout the Mother Lode of the Sierra Nevada and in other locations around the world. Today Gold is still mined from mariposite-bearing ores at several Mother Lode localities.

Genesis

Mariposite formed when serpentine was altered under pressure by mineral-laden hot (650oF) water. The water, containing potassium, silica, carbon, oxygen, and other elements, flowed upward from sources deep in the earth’s crust along fractures, faults and fissures in the rocks. When these hydrothermal fluids reacted with the serpentine, they formed deposits of quartz, chromium-rich mica, sulfides, and occasionally gold. At that time these rocks were completely hidden under the surface of the earth. The ancestral Sierra Nevada began to began to rise 100 million years ago (Hill 1975) and the rock above the mariposite slowly eroded. The rich veins were exposed about 45 million years ago. Continued erosion released the gold from the rock and streams carried it down the slopes.

Mariposite rock and the varied quartz and gold veins are found along the mother lode’s Melones fault which separates the Paleozoic Calaveras Complex phyllite from the late Jurassic Mariposa Formation greywacke and volcanic rock.


Uses and Statistics of Mariposite

Mining: Gold Ore

Lapidary: Cabochons, Book Ends, Paper weights, Headstones, Ornamental Objects

Color & Luster: Apple Green, white Emerald Green, Apple Green

Mode of Occurrence: Abundantly distributed in the Mother Lode old belt of the Sierra Nevada, foliated in an assemblage known as mariposite rock.

Environment: Found in Schists, as nests and lenses in talc sericite schists, also found in narrow zones to broad belts in association with quartz and carbonate minerals such as ankerite, dolomite of magnesite, and resulting from hydrothermal alteration of serpentine.

Composition: Variety of Muscovite. Basic potassium aluminum chromium silicate (Murdoch and Webb, 1956).

Crystal Form: Monoclinic. In hexagonal plates and scales, foliated, micaceous (Murdoch and Webb, 1956).

Optical Properties: Straight Extinction and positive elongation, optically negative, of narrow axial or unaxial angle. Index of Refraction: alpha = 1.56 to 1.58, gamma = 1.61-1.63 (Knopf, 1929); beta = 1.624 (Deer and Others, 1962)

Hardness: 2.5-3

Luster: Vitreous

Specific Gravity: 2.78-2.81

Cleavage: Perfect basal

Habitat: Green plates and flakes, tabular. Foliated, micaceous.


By George W. Peabody for the The Rock Across America Project

Posted on June 14, 2016