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← How do crystals work? - Graham Baird

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Showing Revision 3 created 06/12/2019 by lauren mcalpine .

  1. Deep beneath the geysers and hot springs
    of Yellowstone Caldera
  2. lies a magma chamber produced by a
    hot spot in the earth’s mantle.
  3. As the magma moves towards
    the Earth’s surface,
  4. it crystallizes to form young,
    hot igneous rocks.
  5. The heat from these rocks drives
    groundwater towards the surface.
  6. As the water cools, ions precipitate out
    as mineral crystals,
  7. including quartz crystals from silicon
    and oxygen,
  8. feldspar from potassium, aluminum,
    silicon, and oxygen,
  9. galena from lead and sulfur.
  10. Many of these crystals have signature
    shapes—

  11. take this cascade of pointed quartz,
    or this pile of galena cubes.
  12. But what causes them to grow into these
    shapes again and again?
  13. Part of the answer lies in their atoms.

  14. Every crystal’s atoms are arranged
    in a highly organized, repeating pattern.
  15. This pattern is the defining
    feature of a crystal,
  16. and isn’t restricted to minerals—
  17. sand, ice, sugar, chocolate, ceramics,
    metals, DNA,
  18. and even some liquids have
    crystalline structures.
  19. Each crystalline material’s atomic
    arrangement

  20. falls into one of six different families:
  21. cubic, tetragonal, orthorhombic,
    monoclinic, triclinic, and hexagonal.
  22. Given the appropriate conditions,
  23. crystals will grow into geometric shapes
  24. that reflect the arrangement
    of their atoms.
  25. Take galena, which has a cubic structure
    composed of lead and sulfur atoms.
  26. The relatively large lead atoms
  27. are arranged in a three-dimensional
    grid 90 degrees from one another,
  28. while the relatively small sulfur atoms
    fit neatly between them.
  29. As the crystal grows, locations like these
    attract sulfur atoms,
  30. while lead will tend to
    bond to these places.
  31. Eventually, they will complete the grid
    of bonded atoms.
  32. This means the 90 degree grid pattern
    of galena’s crystalline structure
  33. is reflected in the visible
    shape of the crystal.
  34. Quartz, meanwhile, has a hexagonal
    crystalline structure.

  35. This means that on one plane its atoms
    are arranged in hexagons.
  36. In three dimensions, these hexagons are
    composed of many interlocking pyramids
  37. made up of one silicon atom
    and four oxygen atoms.
  38. So the signature shape of a quartz
    crystal
  39. is a six-sided column with pointed tips.
  40. Depending on environmental conditions,

  41. most crystals have the potential to form
    multiple geometric shapes.
  42. For example, diamonds, which form deep
    in the Earth’s mantle,
  43. have a cubic crystalline structure and can
    grow into either cubes or octahedrons.
  44. Which shape a particular
    diamond grows into
  45. depends on the conditions where it grows,
  46. including pressure, temperature,
    and chemical environment.
  47. While we can’t directly observe growth
    conditions in the mantle,
  48. laboratory experiments have shown some
    evidence
  49. that diamonds tend to grow into cubes at
    lower temperatures
  50. and octahedrons at higher temperatures.
  51. Trace amounts of water, silicon,
    germanium, or magnesium
  52. might also influence a diamond’s shape.
  53. And diamonds never naturally grow into the
    shapes found in jewelry—
  54. those diamonds have been cut to
    showcase sparkle and clarity.
  55. Environmental conditions can also
    influence whether crystals form at all.

  56. Glass is made of melted quartz sand,
  57. but it isn’t crystalline.
  58. That’s because glass cools
    relatively quickly,
  59. and the atoms do not have time to arrange
    themselves
  60. into the ordered structure
    of a quartz crystal.
  61. Instead, the random arrangement
    of the atoms in the melted glass
  62. is locked in upon cooling.
  63. Many crystals don’t form geometric shapes

  64. because they grow in extremely close
    quarters with other crystals.
  65. Rocks like granite are full of crystals,
  66. but none have recognizable shapes.
  67. As magma cools and solidifies,
  68. many minerals within it crystallize at the
    same time and quickly run out of space.
  69. And certain crystals, like turquoise,
  70. don’t grow into any discernible geometric
    shape in most environmental conditions,
  71. even given adequate space.
  72. Every crystal’s atomic structure has
    unique properties,

  73. and while these properties may not have
    any bearing on human emotional needs,
  74. they do have powerful applications
    in materials science and medicine.