Laws of Crystallography: Understanding Crystal Structures
Discover the fundamental laws governing crystal structures including the Law of Constancy of Interfacial Angles, Law of Symmetry, and Law of Rational Indices. Explore how these laws define the angles, symmetry, and indices of crystals, offering insights into their formation and properties.
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Presentation Transcript
Thereare3lawsofCrystallography.Theyare 1.LawofConstancyofInterfacialAngles 2.Lawof Symmetry 3.LawofRationalIndices
1.LawofConstancyofInterfacial Angles -Thsanglebetweentheadjacentfaces isknownasInterfacialangleofthe crystalofaparticularsubstance. -Accordingtothislaw,itisalways constantforthecrystalsofthesame substance.
-Inspite of different sizes and mode of growth of crystal the interfacialangleissame. -The size and shape of crystal formeddependuponthe conditions ofcrystallization. -ThislawisalsoknownasSteno s Law.
2.LawofSymmetry Thereare3typesofsymmetry -Planeof Symmetry -Axisof Symmetry -Centreof Symmetry
1.PlaneofSymmetry Ifitispossibletodivide thecrystalinto2parts,one being mirrorimagetoother,thecrystalissaidto haveplaneofSymmetry. -Acubiccrystalhave a.Rectangularplaneof Symmetry b.DiagonalplaneofSymmetry
Rectangular plane of Symmetryhas3 planes ofSymmetry.
2.Axisof Symmetry Itisanimaginarylineoraxisaboutwhichthe crystalmayberotatedsothatitpresentsthesame appearancemorethanonceinarotationof360 . Thereare3typesinaxisofSymmetry. (a)Axisof 4 foldSymmetry (b)Axisof 3 foldSymmetry (c)Axisof 2 foldSymmetry
Axisof 4foldSymmetry Passingfromthecsntreof2opposite faces. TotalFaces=6 PairsofoppositeFaces=3 Angle= 90 Whenalongthisaxisthecubeisrotatedwewillget 4sameappearancesofthiscell.
Axisof 3foldSymmetry Passingfromoppositecornersalongwithbody diagonal Totalcorners= 8 Oppositecornerpairs =4 Angle=120 Whenalongthisaxisthecubeisrotatedwewillget 3sameappearances.
Axisof 2foldSymmetry Passingfromtwodiagonallyoppositeedges Totaledges=12 Pairsofoppositeedges=6 Angle= 180 Whenalongthisaxisthecubeisrotatedwewillget 2sameappearancesofthiscell.
Centreof Symmetry Passingfrombody center Onlyoneplaneof Symmetry
PlaneofSymmetry-Rectangular=3 - Diagonal = 6 AxisofSymmetry-4 fold=3 -3fold=4 2 fold = 6 - Center of Symmetry = 1 Total=23
