Page 56 - visitorGuide

■ Pyrite with Quartz
Tiny quartz crystals, scattered pyrite crystals, and a few larger
quartz crystals encrust this specimen. Pyrite is a minor ore of
iron and is the most widespread and abundant of the sulfide
minerals. It occurs in many kinds of igneous, metamorphic,
and sedimentary rocks.

■ Calcite Cavern Crystals Calcite Cavern Crystals

The yellowish exterior surface of this

specimen is typical of calcite that forms from

slowly dripping solutions in limestone caves.

Its interior shows coarse white crystals of

calcite growing outward from a central

core.This specimen is from Crystal Cave,

Pennington County, South Dakota.

Gift from the Geology Museum, Colorado School of Mines.

■ Septarian Concretion
Many concretions, or mineral masses, form
around a piece of shell, bone, wood, or other
organic material.The organic matter causes chemical
reactions that help concentrate and precipitate certain
minerals from the water contained in the pore spaces be-
tween sediment grains. Septarian concretions are common in fine-grained
sediments deposited in marine settings.The concretions are cut by inter-
secting cracks filled by calcite, siderite, or other minerals.The concretions
may begin as a gelatinous mass that hardens first on the outside, then
cracks as the core of the mass loses water.

■ Chrysotile Asbestos
Asbestos is a commercial term for a group of silicate minerals that readily
separate into thin, strong fibers that are flexible, heat resistant, and chemi-
cally inert. Once widely used in a variety of industrial products, it is now rarely
used commercially because asbestos has been found to be carcinogenic.

■ Barite-Crested Blades
Barite, the mineral that cements “desert roses,” is the principal
ore of barium. It is a soft, heavy nonmetallic mineral that gets
its name from the Greek barys (“heavy”). It is used in drilling
muds where its weight helps prevent blowouts from high
pressure in drillholes. It is also used to make glass and
paint, as a filler for paper and textiles, in cosmetics, and
in medicine.

■ Oncolite
Oncolites are rounded structures with concentric internal
bands formed by the addition of organic or mineral material
around a core.This specimen, collected in Sevier County, Utah,
from the Paleocene lower Flagstaff Formation, formed from the
gradual buildup of layers of algae around a snail shell that is now
fossilized at the oncolite’s center. The algae lived in an ancient lake,
but the snail itself was a land snail that apparently washed into the lake
from a stream or a flood. Today, this type of snail is found only in warmer
tropical climates. The bands in the oncolite are formed from calcium carbon-
ate produced by the life processes of the algae.They also contain silt that was

46 VISITOR GUIDE