SANBORNITE
Specimen snb-4

Dims: 2.1 x 1.6 x 1.6" (2.3 x 4.1 x 4.1 cm)

Wt: 3.9 oz. (109.7 g)

El Rosario, Baja, California, Mexico

This small hand specimen consists of an apparently massive host rock that is made up of countless intergrown Sanbornite crystals. These crystals are so heavily intergrown that it is nearly impossible to detect any crystal form from them. The crystals have flat faces, though, and a flaky, micaceous consistency. They have a white color and a bright, subadamantine luster that also strongly resembles those of the mica group. As a matter of fact, the only visual difference between this material and muscovite is that muscovite usually has a lower luster! The matrix rock in which the Sanbornite is concentrated appears to be made of quartz or a feldspar, but there is actually more Sanbornite than matrix. Also trapped in the matrix are several incomplete formless, and deep red crystals of Gillespite, a barium iron silicate. Maybe our mineralogist, Dave, should buy this "namesake" piece!

Dims: 4.0 x 2.7 x 0.5" (10.2 x 6.9 x 1.3 cm)

Wt: 5.68 oz. (161.0 g)

Esquire #1 Claim, Rush Creek, Fresno County, California, U.S.A.

This specimen consists entirely of crystalline Sanbornite. While the piece shows no definite crystal form, it has the flaky, micaceous crystalline tendencies of the species. Its largest surfaces are freshly-cleaved, whereas its thinner, side surfaces are coated with a thin, white crust or are rust-stained. Actually, it resembles mica in many respects, including its pale, greenish-white color and bright pearly luster. It does not flake as easily, however, and is more brittle than mica. It also lacks the transparence of muscovite and most other mica varieties at thin spots. Other than the aforementioned thin white crust, there is no host rock present.

Dims: 2.8 x 1.4 x 0.5" (7.1 x 3.6 x 1.3 cm)

Wt: 1.75 oz. (49.7 g)

Esquire #1 Claim, Rush Creek, Fresno County, California, U.S.A.

This small hand specimen consists of a crystalline piece of Sanbornite. There is a consisderable amount of fresh breakage on the piece, but most of this is in the form of a large cleavage surface that constitutes one side of the flat specimen. It shows no definite visual crystal form, but has the classic crystalline tendencies of Sanbornite, including an almost micaceous cleavage. Its color is pale green, it has a pearly, almost micaceous luster, and is generally translucent, though some areas show impressive clarity. There is a thin crust of a dull, white material on a few of the edges that is moderately rust-stained, but no actual base rock is present.

Dims: 5.3 x 2.6 x 0.9" (13.5 x 6.6 x 2.4 cm)

Wt: 7.5 oz. (215 g)

Big Creek, Fresno County, California. U.S.A.

This piece consists of a large, apparently split slab of crystalline Sanbornite. Though the material shows obvious evidence of cleavage, it has no apparent crystal shape. Its color is milky-white, its luster is bright and pearly, and it is effectively opaque at its present thickness. A small amount of massive quartz is embedded into one side of the slab.

Dims:5.5x3.0x1.5" (14.0x7.6x3.8 cm)

Wt: 15.4oz. (437g)

Big Creek, Fresno cty., California

This is a large specimen of the mineral sanbornite on a bit of matrix material. Although this specimen is crystalline, nothing can be determined of the mineral's orthorhombic structure. This is not surprising, as sanbornite usually occurs as anhedral crystalline masses. This specimen does show the white color, pearly luster, and the nearly micaceous cleavage for the species.