Min Lecture 11.06.ppt

8/10/2019 Min Lecture 11.06.ppt

1/17

Lecture 11 (10/18/2006)

CrystallographyPart 4:

Crystal Forms

Twinning


2/17

Crystal Forms

Habit:the general externalshape of a crystal (e.g.,prismatic, cubic, bladed...)

Form: refers to a group offaces which have the same

relation to the elements ofsymmetry.

Crystals with higher degreesof symmetry tend togenerate more form faces.

openform

closedform


3/17

Unique Attributes of Crystal Forms

NaClFaces of particular forms commonlyshare unique physical or chemicalattributes

Quartz

Anthophyllite

Pyrite


4/17

Form Indexes

defined by the Miller

index (hkl) of the face

in the positive

quadrant enclosed in brackets

{hkl}

a{100},r {111},c {001}

Tetragonal 4/m 2/m 2/m

a1

a2

c


5/17

Pyramids and Dipyramids

Prisms

Low Symmetry Forms


6/17

Trapezohedron, Scalehedron,

Rhombehedron, Disphenoid


7/17

Isometric

Forms


8/17

Twinning

Symmetrical intergrowth of two or more crystalsrelated to a symmetry operation (twin element)that is otherwise absent in a single crystal.

Twin elements includes mirrors, rotation axes(usually 2-fold) and roto-inversion that usually donot align with symmetry elements in the crystal.

Twin Laws define the twin element and itscrystallographic orientation (twin planes areidentified by its Miller index (hkl), twin axes aredefined by a zone symbol [hkl]).


9/17

Twin TypesPotential

Twin Plane (111)

Mirror

Reflection

Composition

PlaneContact TwinsCompositionplanescorrespond to twinplanes (mirrors)

Penetration TwinsCompositionplanes areirregular;formed by

rotational twinaxis operation

Twin

Axes


10/17

Multiple TwinsFormed from 3or more twinnedcrystals

- Polysynthetictwins wherecomposition

planes areparallel

- Cyclic twinswherecomposition

planes are notparallel


11/17

Twin Laws in the Triclinic System

Albite Law{010} twin plane

Albite combinedwith Pericline Law[010] twin axis

results in tartantwinning in microcline(K-feldspar)


12/17

Twin Laws in the Monoclinic

System

Note that twin planes do

not align with mirror planes


13/17

Twin Laws in the Orthorhombic Systemcontact & cyclictwinning

Contact or Penetrative??


14/17

Twin Laws in the Tetragonal

System

T i L i th


15/17

Twin Laws in the

Hexagonal System{0112} is most commonand may produce singlecontact or polysynthetictwins

exception:

twin axis || C


16/17

Twin Laws in the Isometric System

Penetrativetwins withtwin axesparallel torotationalaxes


17/17

Next Lecture

No Lecture Next WeekYeah!!!

Documents

Min Lecture 11.06.ppt