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Gravity
Terms in this set (18)
define crystal faces
planes where build-up of crystal structure ceased
describe reference and symmetry axes of cubic crystal system and name a common cubic gem
3 reference axes, equal length, at 90 to each other, 4 3-fold symmetry axes. fluorite. (foundation 3-15)
what do reference axes indicate?
directions in the crystal structure. their position can be anywhere in the crystal. looking down an axis can reveal repeat distances and repeat directions (foundation 3-15)
what is the difference between reference axes and symmetry axes?
reference axes are the directions where the fundamental crystal structure (what is this called?) repeats. symmetry axes of a perfect crystal indicate where you can turn the crystal by a certain angle and see the same shape again. (needs work!)
describe reference and symmetry axes of a tetragonal system and name a common tetragonal gem.
3 reference axes, 2 of equal length, at 90 degrees to each other. 1 4-fold symmetry axis. zircon is an example. (foundation 3-16)
describe reference and symmetry axes of the orthorhombic system and name a common orthorhombic gem.
3 reference axes, all of unequal length, at 90 degrees to each other. 3 2-fold symmetry axes. topaz is an example. (foundation 3-16)
describe reference and symmetry axes of the trigonal and hexagonal systems, how they are related, and name one gem of each kind.
there are 4 reference axes (including c). the first 3 (a1,a2,a3) are of equal length and lie in the same plane (lateral). they are at 120 degrees to each other. these 3 laterally positioned axes are at 90 degrees to c.
the symmetry axes are that the trigonal system has 1 3-fold axis, whereas the hexagonal has 1 6-fold axis (which is also a 3-fold axis). tourmaline is trigonal, beryl is hexagonal. (foundation 3-17)
describe the reference and symmetry axes of the monoclinic system and name a common monoclinic gem.
3 reference axes of unequal length. 2 of them are at 90 degrees to each other, but one is greater than 90 degrees. there is 1 2-fold symmetry axis. feldspar is an example. (foundation 3-17)
describe reference and symmetry of the triclinic system
3 unequal reference axes none of which are at 90 degrees to each other. no axes of symmetry. (foundation 3-18)
define pinacoid
open form. side pinacoid: 2 parallel forms on the sides of a crystal. basal pinacoid: 2 parallel forms on the top and bottom of a crystal
define habit
overall shape of a crystal. made up of the form or forms with which it has grown, subject to the matrix. development of a form, or relative development of 2+ forms.
what happens to an orthorhombic bipyramid when combined with 1 basal and 2 side pinacoids?
equant habit, a cube (from pinacoids) with notches (from bipyramid), which can become flat and wide (tabular) or tall and narrow (prismatic or acicular). in both cases the equant notches appear.
crystalline features of beryl
hexagonal prism, prismatic habit, sometimes long. etch pits may reveal symmetry. etch pits on prism faces are rectangles, on pinacoid faces are hexagons (revealing symmetry)
crystalline features of calcite
trigonal. DR seen by placing crystal over print on page, double print image. 3 cleavage directions w/iridiscence in incipient and pearly lustre on cleavage faces. under 10x, cleavage features may be visible in marble.
crystalline features chrysoberyl
orthorhombic. pinacoid faces common. 3-fold tiwns with hexagonal angles w/striations and reentrant angles. lamellar twinning on surface, internally as striations.
crystalline features corundum
trigonal. ruby often tabular. short prism faces & small rhombohedron faces. sapphire w/long steep bipyramids, sometimes barrel shaped. may have color zones and fibrous inclusions in trigonal symm. pinacoid faces often have triangular growth marks revealing trigonal symm. lustre often bright vitreous, sometimes sl. metallic.
crystalline features diamond
cubic. octa- & dodacehedron are most common forms. octahedra frq have trigon markings. more complex forms w/rounded faces. 3-fold symm usually discernable. triangular macle twinning may show reentrant angles in corners. adamantine.
crystalline features feldspar
monoclinic. most rough fragmentary w/2 cleavage directions. colorless/bluish/yellow/pale flesh. vitreous, may be pearly on cleav. surf. some cryst. show iri. intern. refl. from crystal plane orientations ie moonstone labradorite.
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