Page 59 - visitorGuide

■ Bladed Sand Selenite Crystals
Selenite is the clear, colorless variety of gypsum, occurring in dis-
tinct, transparent monoclinic crystals or in large crystalline masses.

■ Stibnite- and Calcite-Filled Geode
Long, dark crystals of stibnite and compact white calcite
fill this geode, which formed in a layer of limestone.
Limestone can dissolve into a solution as waters move
through it, so cavities and openings develop fairly eas-
ily in limestone. These open areas make it easier for
mineral-bearing fluids to pass through the rock. The
thin, irregular blue-gray band near the outer edge of
the geode is chalcedony, frequently one of the first
minerals to line the wall of a cavity.

Stibnite- and Calcite-Filled Geode ■ Mercury Ore
This specimen from Califor­

nia contains two sets of di-

morphs, minerals that can crystallize in two different

internal arrangements. Cinnabar and metacinnabar are

dimorphs of mercury sulfide, and pyrite and marcasite are

dimorphs of iron sulfide. Cinnabar is bright red, though impuri-

ties can color it brown. Marcasite is pale brass-yellow to almost white, but

when tarnished it becomes a deeper yellow to brown. Marcasite forms from

low-temperature acidic solutions in metamorphic and igneous settings.

When exposed to air, it disintegrates easily into a white powder.

Gift from Edwin Eckel, U.S. Geological Survey, Denver, Colorado, and former executive secretary of the Society.

■ Stibnite and Cinnabar
Stibnite is a lead-gray mineral with a brilliant metallic luster. It often has a
black or iridescent tarnish. Its name is from stibi, the Greek word for anti-
mony. Stibnite is often found with cinnabar in hot spring deposits or in veins
that were mineralized by hydrothermal fluids. The lead-gray stibnite and red
cinnabar in this specimen may also contain gold and silver. This specimen
was probably collected in Shoshone County, Idaho.

Gift from William J. LeVeque.

■ Fossil Graptolites
Graptolites are colonial animals whose carbonized remains are found in Pa-
leozoic rocks. Although they are thought to have been extinct for more than
400 million years, recent discoveries in both the Pacific and Atlantic oceans
have raised the possibility that relatives of the graptolites are alive today.
Graptolites’ name is derived from the Greek graptos (“painted” or “written”)
and lithos (“stone”).

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

■ Fossil Belemnites

These cigar-shaped fossils are

the remains of belemnites, extinct

marine cephalopods that were the

ancestors of the modern cuttlefish,

squid, and octopus.The thick shell of

the belemnite protected its soft body

parts, and its head and tentacles

extended from the open,

broader end of the shell. Fossil Belemnites

49THIRD FLOOR EXHIBITS