4 VISIT OR GUIDE Nature s Processes A Brief Lesson in Geolog y Minerals are naturally occur ring c hemical elements or compounds of several elements Eac h min eral species has its own defnite c hemical composition and man y other constant c haracteristics suc h as color hardness luster and specifc gravity whic h permit the mineralogist to identify it Man y minerals form crystals in geometric shapes that refect their internal atomic ar rangement whic h ag ain is par ticular to eac h species Though man y of our crystal specimens appear to be carved by hand they are all naturally formed No gem cut ter could duplicate time af ter time the exact angles between crystal faces that c haracteriz e every crystal of quar tz for example wherever in the world it may have been found S ome minerals have crystalliz ed from molten material or have formed in other w ays but most are formed by precipitation from solutions of various c hemicals in w ater hot or cold muc h as crystals of sug ar form in the bot tom of a syrup pitc her S ome roc ks are masses of single minerals Limestone for example is made up almost entirely of the mineral calcite Most roc ks however are aggreg ates of two or more minerals suc h as granite whic h is basically an intergrowth of the minerals quar tz mica and feldspar R oc ks are not as con stant in c hemical composition as are minerals and of cour se they possess no crystal forms Igneous roc ks suc h as granite are formed by crystallization of minerals from molten material deep in the ear th F or example obsidian is really a natural v olcanic glass rather than an aggreg ate of minerals It w as formed from magma or melt similar to the kind that would have formed granite if it had cooled very slowly R apid cooling of the lava from a v olcano however caused the melt to freez e as glass before it had time to form crystals S edimentary roc ks suc h as sandstone and shale are formed by accumulation of mud and sand grains most commonly in bodies of w ater As more sediments are piled abo ve the lower layer s be come consolidated and form roc k Other sedimentary deposits suc h as limestone are formed directly by precipitation of c hemical compounds from ocean or lak e w ater If preexisting roc ks sedimentary or igneous are contor ted squeez ed or otherwise c hanged by mountainbuilding forces or c hemical action they are called metamorphic roc ks Gneiss sc hist and serpentine are examples With some exceptions roc ks are usually less beautiful and at tracti ve to the layper son than are man y mineral specimens T o geologists and petrologists however roc ks are as fascinating and in structi ve as minerals are to mineralogists or as fower s are to botanists Roc ks of the Roc kies in the W alls The GS A building itself is a display of specimens of most of the roc ks that mak e up the R oc k y Mountainsthe range that sits just two miles west of the building It is reinforced concrete poured