Page 29 - visitorGuide

Garden Level Exhibits

Starting with the first case, from left to right:

■ Green Stalactitic Calcite
This pale-green stalactitelike specimen of calcite probably grew in a cave and
was found in Chihuahua, Mexico. Calcite is common in caves where long col-
umns extend down from cave ceilings (stalactites) and up from cave floors
(stalagmites). Calcite deposits develop when water carrying calcium bicar-
bonate in solution reaches a cavern. In the lower pressure of the cavern, car-
bon dioxide is released from the solution, and calcite (calcium carbonate) is
slowly deposited on the roofs of caverns by dripping water and on the floors
of caverns where the dripping water falls.
Gift from the Geology Museum, Colorado School of Mines, Golden, Colorado.

■ Silicified Wood
The silica in petrified wood may come from volcanic ash interlayed with the
sediments that buried the wood. Water moving through the layers dissolves
silica from the volcanic ash and carries it through the sediments. Chalcedony
and opal form from the silica-rich water when it moves into the wood. The
silica can be deposited as a filling in the woody structure or as a replacement
of the woody fibers themselves. When original wood fibers remain in petri-
fied wood, the woody structure can still be seen, and soft woody tissue can be
recovered by dissolving the silica in hydrofluoric acid.
Gift from the Department of Geology, Idaho State University, Pocatello, through H. Thomas Ore.

■ Muscovite
A mineral of the mica group, muscovite is colorless to pale brown. It is a
common rock-forming mineral in igneous and metamorphic environments.
Muscovite is useful in a broad range of applications because it is flexible; can
be split into tough, thin sheets; is resistant to heat; and is a poor conductor.
It is used in thermal and electrical insulators, in heat-resistant windows, as
lustrous material in wallpaper, and as filler in rubber and plastics. Colorless,
flawless plates of mica are used as wedges in polarizing microscopes. Mus-
covite’s name comes from Russia, where thin sheets of the mica were used
as stove windows.
Gift from William J. LeVeque, Claremont, California.

■ Negative Pseudomorphs of Quartz after Barite
This specimen is composed of quartz that grew over older crystals of bar-
ite. After the quartz formed, the barite crystals dissolved, leaving long tabular
holes in the quartz where the barite used to be.
Gift from the Geology Museum, Colorado School of Mines.

■ Prehnite
Prehnite, valued as a gemstone, typically forms sea-green botryoidal crusts
with radiating ridges formed by the edges of its curved crystals. It is most fre-
quently found lining cavities in basalts, often with zeolite minerals and calcite.
Prehnite is also found in gray, white, and colorless forms. Green specimens
like this one can fade when exposed to air and light. It is named for Colonel
van Prehn of the Netherlands, who brought specimens home from South Af-
rica in 1774.

19GARDEN LEVEL EXHIBITS