Page 32 - visitorGuide

■ Stibnite
Stibnite is the most common of the antimony minerals. It is often found in
groups of long-bladed crystals with lengthwise striations. The crystals are
slightly flexible and sometimes have angular bends in them or may be twist-
ed along their lengths. Stibnite is a low-temperature mineral found in veins
mineralized by hydrothermal solutions, in hot spring deposits, in pegmatites,
and in deposits where the original minerals have been replaced by others.
China is home to large antimony deposits.

Gift from Xu Zheng-Liang, East China Geological Institute.

■ Quartz Crystal with Tourmaline
Several dark green tourmaline crystals
grew near the base of this large quartz
crystal from Minas Gerais, Brazil.
Tourmaline crystals vary in color, with
either horizontal bands like these or
with concentric rings radiating from the center
of the crystal outward.

Purchased by the Society.

■ Obsidian

Obsidian is volcanic glass. It forms when lava solidifies

very rapidly, usually when hot lava flows into a cool

body of water like a lake or a sea.The lava solidifies

too quickly for minerals to form crystal structures,

so an amorphous glass is formed instead.The

mottled patterns of black and brown in this speci-

men from Ixtepeque Volcano, Guatemala, are

the result of flowing lava layers that left bands

and streaks of color.The small gray spheres are

feldspar fibers cemented by silica; they barely Copper Dragon
had time to form before the lava solidified. Some

obsidian also contains “stone bubbles” with crystals

arranged in concentric shells inside large cavities.These are

left behind by gases escaping from the lava. Obsidian fractures in the same

way as glass, along nested sets of arcs. Razor-sharp edges result where the

fractures intersect.These sharp edges have made obsidian a desirable mate-

rial for tool-making. It is also used in jewelry and art objects.

Gift from Gabriel Dengo, Guatemala City, Guatemala.

■ Stichtite in Serpentine
The lilac stichtite is a rare alteration product of the green serpentine minerals
that form most of this specimen. Stichtite was first described in 1910 from the
site in Dundas, Tasmania where this specimen was collected. It is named for
Robert Sticht, an Australian metallurgist born in the United States.

Gift from the Department of Geology, University of Tennessee, Knoxville.

■ Lyons Sandstone
The sand grains in this sandstone were deposited by wind in sand dunes 250
million years ago. The Permian Lyons Sandstone is resistant to weathering
because it is composed mostly of quartz grains cemented by silica. Most of
the sandstone splits evenly along its flat bedding planes and has been widely
used as a building stone in Boulder and on the University of Colorado, Boul-
der, campus.

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