19 GARDEN LEVEL EXHIBITS Garden Le v el Exhibits Starting with the frst case from left to right Gr een Stalactitic Calcit e This palegreen stalactitelik e specimen of calcite probably grew in a cave and w as found in Chihuahua Mexico Calcite is common in caves where long col umns extend down from cave ceilings stalactites and up from cave foor s stalagmites Calcite deposits develop when w ater car rying calcium bicar bonate in solution reac hes a cavern In the lower pressure of the cavern car bon dio xide is released from the solution and calcite calcium carbonate is slowly deposited on the roofs of caverns by dripping w ater and on the foor s of caverns where the dripping w ater falls Gift from the Geolog y Museum Colorado School of Mines Golden Colorado Silicifed W ood The silica in petrifed wood may come from v olcanic ash interlayed with the sediments that buried the wood W ater mo ving through the layer s dissolves silica from the v olcanic ash and car ries it through the sediments Chalcedon y and opal form from the silicaric h w ater when it mo ves into the wood The silica can be deposited as a flling in the woody structure or as a replacement of the woody fber s themselves When original wood fber s remain in petri fed wood the woody structure can still be seen and sof t woody tissue can be reco vered by dissolving the silica in h ydrofuoric acid Gift from the Department of Geolog y Idaho State Univ ersity P ocatello through H Thomas Ore Musco vit e A mineral of the mica group musco vite is colorless to pale brown It is a common roc kforming mineral in igneous and metamorphic en vironments Musco vite is useful in a broad range of applications because it is fexible can be split into tough thin sheets is resistant to heat and is a poor conductor It is used in thermal and electrical insulator s in heatresistant windows as lustrous material in w allpaper and as fller in rubber and plastics Colorless fawless plates of mica are used as wedges in polarizing microscopes Mus co vite s name comes from R ussia where thin sheets of the mica were used as sto ve windows Gift from W illiam J LeV eque Claremont California Neg ativ e Pseudomor phs of Quar tz af t er Bar it e This specimen is composed of quar tz that grew o ver older crystals of bar ite Af ter the quar tz formed the barite crystals dissolved leaving long tabular holes in the quar tz where the barite used to be Gift from the Geolog y Museum Colorado School of Mines Pr ehnit e P rehnite valued as a gemstone typically forms seagreen botryoidal crusts with radiating ridges formed by the edges of its curved crystals It is most fre quently found lining cavities in basalts of ten with z eolite minerals and calcite P rehnite is also found in gray white and colorless forms Green specimens lik e this one can fade when exposed to air and light It is named for Colonel van P rehn of the Netherlands who brought specimens home from S outh Af rica in 1 77 4