mineralogy - module 1-3

Plane of symmetry

divides a crystal into two parts so that one part is a mirror of the other

axis of symmetry

repetition of a crystal face more than once in a revolution

diad

2 fold axis (180 degrees)

triad

3 fold axis

tetrad

4 fold axis

hexad

6 fold axis

center of symmetry

causes every face of a crystal to have a similar face parallel to it repeated by inversion through the center

rotoinversion axis

causes every step of rotation to be followed by a step of inversion through the center

degree one of rotoinversion

bar 1 = center

degree 2 of rotoinversion

bar 2 = mirror plane

symmetry elements in a cube

center of symmetry, 9 mirror planes 3// 6 diagonal,3 tetrads, 4 triads an 6 diads

seven crystal systems

triclinic,monoclinic,tetragonal,hexagonal,trigonal,orthorhombic and isometric

symmetry requirements of a triclinic system

none

symmetry requirements of monoclinic system

one 2 fold rotation axis or one mirror plane

symmetry requirements of a orthorhombic system

three 2 fold rotation axis or 2 mirrors and one 2 fold rotation

symmetry requirements of tetragonal

one 4 fold rotation or rotary inversion

symmetry requirements of hexagonal/trigonal crystal

trigonal- one 3 fold rotation axis, hexagonal- one 6 fold rotation axis

symmetry requirements for isometric/cubic

four 3 fold rotation axes

miller indicies

system for labellin g crystal faces mathematically using coordinate geometry according to unit cell axes a b and c

why are indices always intergers

because the planes and axial lengths of a crystal are related by its class symmetry and unit cell contains the full symmetry of the atomic structure

(100)

intercepts on a axis

(010)

intercepts on b axis

(001)

intercepts on c axis

(bar100)

intercepts on negative a axis

open form

does not enclose space eg. bar 1 class all two faces parallel to eachother

closed form

all faces enclose a space eg. cube

pedion

single face

pinacoid

pair of parallel faces

prism

set of three or more faces parallel to a common axis

pyramid

set of three or more faces equally inclined to a axis

dihedrons-dome

two parallel faces angled down from the mirror plane

dihedrons-sphenoid

two parallel faces angled down from the diad axis

zone

a group of faces whose lines of intersection are parallel to one another, the zone axis is in the direction of the common edge

triclinic system classes

1: all forms pedions. bar : all forms pinacoids

minerals in bar one class

plagioclase feldspar, microcline, wollastonite

plagioclase feldspar

NaALSi3O8-CaAl2SI2O8

Microcline

KALSi3O8

Wollastonite

CaSiO3

Turquois

CuAl6(PO4)4(OH)8~4H2O

Microcline classes

2, m, 2/m

crystals in 2/m

diopside,clinopyroxenes,tremolite,clinoamphiboles,orthoclase,gypsum,realgar,orpiment

diopside

CaMgSi2O6

tremolite

Ca2Mg5Si8O22(OH)2

Orthoclase

KAlSI3O8

Gypsum

CaSO4~2H2O

realgar

AsS

Orpiment

As2S3

2

sphenoidal

2/m

prismatic

domatic

m

Orthorhombic system classes

222,mm2,mmm (2/m2/m2/m)

minerals in mmm class

olivine,enstatite, orthopyrxenes,anthophyllite,orthoamphiboles,aragonite,sillimanite/andalusite,sulfur

olivine (forsterite and fayalite)

forsterite:Mg2SiO4 fayalite: Fe2SiO4

enstatite

MgSiO3

anthophyllite

Mg7Si8O22(OH)2

Aragonite

CaCO3

Sillimanite/andalusite

Al2SiO5

Sulfur

S

222

disphenoidal

mm2

pyramidal

mmm

Dipyramidal