GILSONITE
Gilsonite has the appearance of obsidian or anthracite
coal. Gilsonite (a.k.a. uintaite) is solidified oil. It’s an asphaltite - a naturally
solid hydrocarbon - that is jet black in color, very lightweight, brittle, and has
a conchoidal fracture. The largest “deposits” of gilsonite in
the world are in the Uinta Basin of northeastern Utah, USA. Gilsonite
occurs there as vertical NW-SE trending veins intruded within the Uinta
Formation (Eocene). The source of the hydrocarbons is organic-rich beds
in the underlying Green River Formation (Eocene) and Wasatch Formation
(Paleocene).
Gilsonite (uintaite) (4.5 cm across) from northeastern Utah.
Not all gilsonite material from Utah has an
obsidian-like appearance. The sample shown below is a variety called pencillated
gilsonite, characterized by having abundant parallel fractures. The
fractures give the material a splintery appearance (from certain angles, the
stuff looks like hornblende schist). The only pencillated gilsonite
material I've seen also has abundant fracture circlets (conchoidal fracture) -
see below.
Pencillated gilsonite (uintaite) (~4.3 cm across) from northeastern
Utah. This is a commercial sample
mined from the Cottonwood Dike (Cottonwood Vein), a N60-64W striking, 13
to 16 km long, 0.6 to 0.9 meter wide, vertical to subvertical gilsonite dike in
the Willow Creek System. The dike was probably emplaced ~10 to 30 million
years ago. The Cottonwood is mined south-southeast of Ouray (section 35,
T10S, R21E, Big Pack Mountain NE 7.5’ USGS topographic quadrangle) in
south-central Uintah County, Utah.
Pencillated gilsonite sample & locality info.
generously provided by Jim Lekas of the Lexco Gilsonite Company.