Stibnite and CalciteFilled Geode Fossil Belemnites 49 THIRD FLOOR EXHIBITS Bladed Sand Selenit e Crystals S elenite is the clear colorless variety of gypsum occur ring in dis tinct transparent monoclinic crystals or in large crystalline masses Stibnit e and Calcit eFilled Geode L ong dark crystals of stibnite and compact white calcite fll this geode whic h formed in a layer of limestone Limestone can dissolve into a solution as w ater s mo ve through it so cavities and openings develop fairly eas ily in limestone These open areas mak e it easier for mineralbearing fuids to pass through the roc k The thin ir regular bluegray band near the outer edge of the geode is c halcedon y frequently one of the fr st minerals to line the w all of a cavity Mer cury Or e This specimen from Califor nia contains two sets of di morphs minerals that can crystalliz e in two dif ferent internal ar rangements Cinnabar and metacinnabar are dimorphs of mercury sulfde and pyrite and marcasite are dimorphs of iron sulfde Cinnabar is bright red though impuri ties can color it brown Marcasite is pale brassyellow to almost white but when tarnished it becomes a deeper yellow to brown Marcasite forms from low temperature acidic solutions in metamorphic and igneous set tings When exposed to air it disintegrates easily into a white powder Gift from Edwin Eck el US Geological Surv e y Denv er Colorado and for mer e x ecutiv e secretary of the Society Stibnit e and Cinnabar Stibnite is a leadgray mineral with a brilliant metallic luster It of ten has a blac k or iridescent tarnish Its name is from stibi the Greek word for anti mon y Stibnite is of ten found with cinnabar in hot spring deposits or in veins that were mineraliz ed by h ydrothermal fuids The leadgray stibnite and red cinnabar in this specimen may also contain gold and silver This specimen w as probably collected in Shoshone County Idaho Gift from W illiam J LeV eque F ossil Gr apt olit es Graptolites are colonial animals whose carboniz ed remains are found in P a leoz oic roc ks Although they are thought to have been extinct for more than 40 0 million year s recent disco veries in both the P acifc and Atlantic oceans have raised the possibility that relati ves of the graptolites are ali ve today Graptolites name is deri ved from the Greek graptos painted or writ ten and lithos stone Gift from Xu ZhengLiang East China Geological Institute F ossil Belemnit es These cig ar shaped fossils are the remains of belemnites extinct marine cephalopods that were the ancestor s of the modern cut tlefsh squid and octopus The thic k shell of the belemnite protected its sof t body par ts and its head and tentacles extended from the open broader end of the shell