Geology 284 Mineralogy Fall 2005 Dr Helen Lang West Virginia University Crystallization of Minerals Minerals must be crystalline part of the definition Crystalline having an ordered internal structure Internal order may or may not lead to perfect external crystal shape Shapes of crystals euhedral perfect external shape flat faces controlled by internal structure anhedral irregular no crystal faces visible subhedral in between Amorphous noncrystalline lacks ordered internal structure Where and how do crystals grow Minerals grow gradually from liquids like magmas and aqueous solutions From magmas in igneous rocks From water between grains in sediments and sedimentary rocks From saline water in basins where evaporation is significant By gradual replacement of other minerals in metamorphic rocks Size of Crystals is controlled by Temperature at hi T atoms are very mobile Time Abundance of constituent elements Presence or absence of a flux a substance that speeds up crystallization reaction or melting magma or water vapor can act as a flux Pegmatites have the largest crystals Pegmatites are extremely coarse grained igneous rocks Some single crystals are up to 50 feet long They form from the last liquid in a granite pluton rich in water and other volatiles which act as flux composed mainly of quartz feldspar and micas rich in excluded elements like B Be Li U Sn may contain well formed unusual minerals like tourmaline beryl spodumene and topaz Pegmatite examples Black Hills California New England Mistakes or Defects in Crystals Crystalline substances have an ordered internal structure Real crystals have some mistakes or defects in their internal structures Rapidly formed crystals have the most defects Some defects can be annealed out by heating the substance to a temperature below its melting point and holding it there for a while Types of Defects in Crystals see handout Schottky defect Impurity defect interstitial Screw dislocation Frenkel defect misplaced ion Edge dislocation extra layer Multiple defects point defects line defects Twinning When two or more crystals of the same mineral share common atoms typically along planes Twinned crystals must be symmetrically related Simple twins only 2 members or parts Complex twins more than 2 members or parts Contact twins share only one plane of atoms Penetration twins members share a volume Polysynthetic twins complex twins with parallel planes of shared atoms Cyclic twins complex twins with non parallel planes of shared atoms Twinning see handout for examples