Lecture 5 Notes (Geoscience 331)
Lecture 5 Notes (Geoscience 331) Geoscience 331
Popular in Gems: The Science Behind the Sparkle
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This 2 page Class Notes was uploaded by Hannah James on Monday October 5, 2015. The Class Notes belongs to Geoscience 331 at University of Wisconsin - Madison taught by Huifang Xu in Fall 2015. Since its upload, it has received 47 views. For similar materials see Gems: The Science Behind the Sparkle in Geology at University of Wisconsin - Madison.
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Date Created: 10/05/15
Crvstglloaranhv Lecture 5 1 Internal arrangement 0 All minerals and crystalline and have a unique crystal structure 0 The crystal structure of a mineral is often the same some minerals can have multiple crystal structures these are polymorphs O A pseudomorph is a mineral that forms in the shape of another crystal 0 The shape of a crystal shows the internal composition of a mineral 0 Ex cubeshaped crystals have cubeshaped molecular structure hexagonal crystal have hexagonal minerals etc 2 Symmetry 0 All minerals have a distinctive crystal shape some have more than one 0 Since crystal shape is often dependant upon chemical makeup crystals can be classified and described based on the symmetry of their crystal shape 0 All crystals have symmetry some more than others 0 There are six groups of crystals based on symmetry called systems 0 The groups are listed below in decreasing order of symmetry I Cubic I Tetragonal I Orthorhombic I Hexagonal includes Trigonal I Monoclinic I Triclinic 0 The Unit Cell It All Begins Here 0 The unit cell is the building block that all crystals are made of I A unit cell can be an atom a molecule or a group of molecules I It is the smallest unit that keeps the symmetry of the crystal 0 It s repeated over and over again to make a crystal 0 Unit cells are repeated in various patterns to make crystal 0 Ex a cubic shape unit cell can be stacked together to form other shapes for example cube blocks stacked together to create pyramids in Egypt I In this case the unit cell is the pyramid not the cube 3 Anatomy of 3 Crystal 0 Axes ex north and south pole axis of Earth 0 In a cube the axes are the same length in other crystal shapes one axis is longer than the others I The longest axis is the caXis 0 Faces 0 While talking about crystals we ve been referring to wellformed single crystals 0 Euhedral crystals are rare minerals grow in the space available 39 Sometimes crystals have grown together so that some lattice directions are parallel while the other lattice directions are in reverse twinning 0 Faces and facets cut gems versus crystals 0 Crystal faces are at surfaces that form naturally as a result of the structure and growth of the mineral O Facets are at surfaces created by gem cutters to increase the value of a gemstone I Crystal faces don39t always match up with gem facets partially because gems are cut not only to increase its value but also to protect the gem from breaking or chipping 0 Many times the gem minerals that are fashioned into cut stones are anhedral crystals that do not have much value for the inherent crystal shape I Sometimes a perfectly formed crystal is worth more untouched than as a faceted gem 0 There are many other factors in uencing how and why a gem is cut will be covered later
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