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## Crystal Structure

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**Crystal Structure**link**The structure**• The expression of a mineral’s internal arrangement of atoms • The structures of crystals depend on • the types of bonds between the atoms and the conditions under which the crystals are formed**Structure**The cubic shape of salt crystals results from the regular arrangement of atoms forming the crystal.**Importance of structure**• Gives minerals their properties • Graphite C sheets • Diamond strongly bonded to 3 other C atoms**What shows us the shape?**• X-rays • Strike a photographic plate • Shows arrangement of ions or atoms that make up crystal**crystals**• 7 types of crystals HEXAGONAL ISOMETRIC TETRAGONAL ORTHORHOMBIC Trigonal TRICLINIC MONOCLINIC**Properties of Minerals:Crystal Structure**• Isometric Group of Crystals • “equal measure”**Isometric**• 90* • Blocky or ball-like**Pyrite**Fluorite Cube Cube with Pyritohedron Striations Garnet Trapezohedron Mineral Identification Basics • ISOMETRIC BASIC CRYSTAL SHAPES Spinel Octahedron Garnet - Dodecahedron**Hexagonal**120* Three horizontal axes meeting at angles of 120o and one perpendicular axis.**Mineral Identification Basics**• HEXAGONAL CRYSTALS These hexagonal CALCITE crystals nicely show the six sided prisms as well as the basal pinacoid.**Pyramid Face**Prism Faces Rhodochrosite Dolomite SCALENOHEDRON RHOMBOHEDRON Quartz Mineral Identification Basics Pyramid Faces • HEXAGONAL CRYSTALS Prism Faces Vanadinite Hanksite**c**a2 a1 c a2 TETRAGONAL Crystal Axes a1 This is an Alternative Crystal Axes Mineral Identification Basics TETRAGONAL Two equal, horizontal, mutually perpendicular axes (a1, a2) Vertical axis (c) is perpendicular to the horizontal axes and is of a different length. • TETRAGONAL CRYSTALS**Mineral Identification Basics**• TETRAGONAL CRYSTALS WULFENITE Same crystal seen edge on.**c**c b b a a An Alternative Crystal Axes Orientation ORTHORHMOBIC Crystal Axes Orthorhombic Three mutually perpendicular axes of different lengths.**ORTHORHOMBIC**• not square in cross section (when viewing the crystal on end), Topaz from Topaz Mountain, Utah.**Mineral Identification Basics**• ORTHORHOMBIC CRYSTALS ORTHORHOMBIC This model shows the alternative axes where the vertical “c” axis is not the longest axis. The model shows the outside “brick” shape of the PRISM and the inner shape is a DIPYRAMID. The top and bottom faces are called PINACOIDS and are perpendicular to the “c” axis. ORTHORHMOBIC Crystal Model**Orthorhombic**BARITE is also orthorhombic. The view above is looking down the “c” axis of the crystal.**c**b a MONOCLINIC Crystal Axes Mineral Identification Basics • MONOCLINIC CRYSTALS MONOCLINIC In this crystal form the axes are of unequal length. Axes A and B are perpendicular. Axes B and C are perpendicular. But A and C make some oblique angle and with each other.**Top View**Gypsum Orthoclase Mineral Identification Basics • MONOCLINIC CRYSTALS Mica**c**b a TRICLINIC Crystal Axes Triclinic all axes are of different lengths none are perpendicular Crystals that form intersect at oblique angles. no symmetry and no mirrored planes.**Mineral Identification Basics**• TRICLINIC CRYSTALS Microcline, variety Amazonite**Crystal Systems**• cubic - not always cube shaped! You'll also find octahedrons (eight faces) and dodecahedrons (10 faces). • hexagonal - six-sided prisms. When you look at the crystal on-end, the cross section is a hexagon • tetragonal - similar to cubic crystals, but longer along one axis than the other, forming double pyramids and prisms • trigonal - possess a single 3-fold axis of rotation instead of the 6-fold axis of the hexagonal division • orthorhombic - like tetragonal crystals except not square in cross section (when viewing the crystal on end), forming rhombic prisms or dipyramids (two pyramids stuck together) • monoclinic - like skewed tetragonal crystals, often forming prisms and double pyramids • triclinic - usually not symmetrical from one side to the other, which can lead to some fairly strange shapes**Topaz**Olivine Sulfur Chrysoberyl Wulfenite Zircon Kyanite Turquoise Axinite Flourite Halite diamond Gypsum Mica Sapphire Ruby Calcite Quartz Emerald Graphite Apatite