GEOLOGY 284: MINERALOGY

Transcription

1 Dr. Helen Lang Dept. of Geology & Geography West Virginia University FALL 2005 GEOLOGY 284: MINERALOGY

2 The Six Crystal Systems

3 Minerals are Grouped into Six Crystal Systems based on Symmetry System Characteristic Symmetry Isometric (Cubic) System four 3 or 3 Hexagonal System one 6, 6, 3 or 3 Tetragonal System one 4 or 4 Orthorhombic System three 2 and/or m Monoclinic System one 2 and/or m Triclinic System 1 or 1

4 Isometric System Four 3 or 3 (corner-to-corner of reference cube) All isometric shapes also have three perpendicular 4, 4 or 2 axes These are the crystallographic axes a 1, a 2, a 3 ; all equal length All isometric forms are equidimensional Highest symmetry system

5 The Cube and Octahedron are simple, common Isometric Forms cube octahedron

6 A Form is: A set of similarly shaped faces That are related to each other by the symmetry of the crystal Forms can be open or closed

7 More Isometric Forms dodecahedron hexoctahedron tetrahedron tristetrahedron

8 Isometric Minerals: Fluorite (CaF 2 ) Name Growth Form? Twin Name Cleavage Form? To what mineral group does fluorite belong?

9 Isometric Minerals: Garnet (Ca,Fe,Mg,Mn) 3 Al 2 Si 3 O 12 Name of Garnet Growth Form? All isometric minerals are isotropic which means?

10 Halite Growth Form? Formula? Mineral Group?

11 Isometric Minerals: Pyrite Forms? Formula? Mineral Group?

12 Names of Common Nonisometric Forms: Open Forms Pedion Pinacoid Prisms Pyramids single face 2 parallel faces 3, 4, 6, 8 or 12 faces, all parallel to a common line 3, 4, 6, 8 or 12 faces that intersect at a point Use prefix to indicate System or symmetry

13 Some Open Forms tetragonal pyramid hexagonal prism ditrigonal prism (3- fold symmetry) What mineral?

14 Names of Common Nonisometric Forms: Closed Forms dipyramid two 3-, 4-, 6-, 8- or 12- sided pyramids (top and bottom) related by a horizontal mirror plane disphenoid 4 non-equilateral triangular faces rhombohedron 6 rhomb-shaped faces scalenohedron 8 or 12 scalene triangleshaped faces

15 Non-isometric Closed Forms Hexagonal dipyramid Tetragonal disphenoid Hexagonal scalenohedron

16 Rhombohedra 6 faces related by a 3-bar axis - common form for carbonates

17 Grouped in Six Crystal Systems System Characteristic Symmetry Isometric (Cubic) System four 3 or 3 Hexagonal System one 6, 6, 3 or 3 Tetragonal System one 4 or 4 Orthorhombic System three 2 and/or m Monoclinic System one 2 and/or m Triclinic System 1 or 1

18 Crystallographic Axes Reference axes Conventional ways to hold and refer to faces on crystals Different convention for each system

19 Crystallographic Axes: Isometric System Three perpendicular axes Coincide with three 4-fold or 2-fold axes All equal length +a 3 +a 2 Called a 1, a 2, a 3 We already saw many examples, garnet, halite, pyrite, fluorite +a 1

20 Crystallographic Axes: Tetragonal System Three perpendicular axes Vertical axis, c, coincides with 4 or 4-bar axis One axis, c, is longer or shorter than other two, a 1 and a 2, which are equal +a 1 +c +a 2

21 Tetragonal Examples

22 Tetragonal Examples Wulfenite PbMoO 4

23 Tetragonal Examples - Zircon ZrSiO 4 Jay Vonderhay- Bancroft, Ontario John H. Betts - Brazil

24 Crystallographic Axes: Orthorhombic System Three perpendicular axes - coincide with 2-fold axes or are perpendicular to mirror planes All different lengths called a, b, c +a +c +b

25 Orthorhombic Examples +c Two Rhombic Dipyramids -b -a +b +a Sulfur -c

26 Crystallographic Axes: Monoclinic System All axes different lengths Called a, b, c b axis coincides with 2- fold axis or mirror plane c is parallel to long edges a slants down to the front a b, b c, angle between a and c>90 o β > 90 o +a +c +b

27 Monoclinic Example +c -a -b +b +a -c Orthoclase KAlSi 3 O 8

28 Crystallographic Axes: Triclinic System No perpendicular axes +c All different lengths Called a, b, c +a +b

29 Crystallographic Axes: Hexagonal System Four axes Vertical axis, c, is longer or shorter and coincides with 6-fold or 3-fold axis +a 3 +c Three horizontal axes coincide with 2-fold axes, are to c, and 120 o to each-other +a o +a 2 Three horizontal axes are equal lengths a 1, a 2, a 3

30 Hexagonal Examples Name of form? 3-fold or 6-fold?

31 Hexagonal Examples Name of form? 3-fold or 6-fold?

32 Hexagonal Examples Names of forms? 3-fold or 6-fold?

33 Six Crystal Systems System Axial Relationships? Isometric (Cubic) System Hexagonal System Tetragonal System Orthorhombic System Monoclinic System Triclinic System