Our navigation bar is loading . . .

Helping San Diego, California and beyond since 1997.

Do you need volunteer, community service, work, military or court hours?

Click here and add this page to your favorites!

Encyclopedia Britannica

ASYMMETRIC

This article appears in Volume V07, Page 580 of the Encyclopedia Britannica.

Encyclopedia Britannica - Main :: ARN-AUD

ASYMMETRIC CLASS
(Hemihedral, Pediad).
Crystals of this class are devoid of any elements of symmetry. All the forms are
pedions, each consisting of a single plane; FtG. 65.-Crystal of they are thus hemihedral with respect to

Axinite

AXINITE

. crystals of the last class. Although there is
a total absence of symmetry, yet the faces are arranged in zones on the crystals.
Examples are

calcium

CALCIUM [symbol Ca, atomic weight 40.0 (o= x6)]

thiosulphate (CaS2O3.6H20) and hydrogen strontium dextro-tartrate ((C4H40EH)2Sr5H2O) ; there is no example amongst minerals.
6. HEXAGONAL

SYSTEM

SYSTEM

Crystals of this

system

SYSTEM

are characterized by the presence of a single

axis

AXIS (Lat. for " axle ")

of either triad or hexad symmetry, which is spoken of as the "

principal

PRINCIPAL

" or " morphological "

axis

AXIS (Lat. for " axle ")

. Those with a triad axis are grouped together in the rhombohedral or trigonal division, and those with a hexad axis in the hexagonal division. By some authors these two divisions are treated as

separate

SEPARATE

systems; or again the rhombohedral forms may be considered as hemihedral developments
of the hexagonal. On the other hand, hexagonal forms may be considered as a combination of two rhombohedral forms.
Owing to the peculiarities of symmetry associated with a single triad or hexad axis, the crystallographic axes of reference are different in this system from those used in the five other systems of crystals. Two methods of axial representation are in common use; rhonlbohedral axes being usually used for crystals of the rhombohedral division, and hexagonal axes for those of the hexagonal division; though sometimes either one or the other set is employed in both divisions.
Rhombohedral axes are taken parallel to the three sets of edges of a rhombohedron (fig. 66). They are inclined to one another at equal oblique angles, and they are all equally inclined to the

principal

PRINCIPAL

axis; further, they are all of equal length and are interchangeable. With such a set of axes there can be no statement of an axial ratio, but the

angle

ANGLE (from the Lat. angulus, a corner, a diminutive, of which the primitive form, angus, does not occur in Latin; cognate are the Lat. angere, to compress into a bend or to strangle, and the Gr. ayKOS, a bend; both connected with the Aryan root ank-, to

between the axes (or some other

angle

ANGLE (from the Lat. angulus, a corner, a diminutive, of which the primitive form, angus, does not occur in Latin; cognate are the Lat. angere, to compress into a bend or to strangle, and the Gr. ayKOS, a bend; both connected with the Aryan root ank-, to

which may be calculated from this) may be given as a constant of the substance. Thus in calcite the rhombohedral angle (the angle between two faces of the fundamental rhombohedron) is 74 55', or the angle between the normal to a face of this rhombohedron and the principal axis is 446,,
Hexagonal axes are four in number, viz. a vertical axis coinciding with the principal axis of the crystal, and three

horizontal

HORIZONTAL

axes inclined to one another at 6o in a plane perpendicular to the principal axis. The three

horizontal

HORIZONTAL

axes, which are taken either parallel or perpendicular to the faces of a hexagonal prism (fig. 71) or the edge of a hexagonal bipyramid (fig. 70), are equal in length (a) but the vertical axis is of a different length (c). The indices of planes referred to such a set of axes are four in number; they are written as {hikl{, the first three (h+i+k=o) referring to the horizontal axes and the last to the vertical axis. The ratio a : c of the parameters, or the axial ratio, is characteristic of all the crystals of the same substance. Thus for beryl (including

emerald

EMERALD

) a : c = I : 0'4989 (often written c =0.4989) ; for zinc c =1.3564.
Rhombohedral Division.
In the rhombohedral or trigonal division of the hexagonal system there are seven symmetry-classes, all of which possess a single triad axis of symmetry.

End of Article: ASYMMETRIC

If you wish, you can link directly to this article.

<a href="http://jcsm.org/StudyCenter/Encyclopedia/ARN_AUD/ASYMMETRIC.html">
ASYMMETRIC
</a>

(Previous)
ASYLUM, RIGHT OF (Fr. droit d'asile; Ger. Asylr...

(Next)
ATACAMA

JCSM was founded in 1997 and exists to help the community and bring people into a life-changing and productive relationship with Jesus Christ. JCSM offers over 200,000 free web pages, including its weekly inspirational emails that were sent continuously for over a decade.

Jesus Christ Saves Ministries
P.O. Box 9297
San Diego, CA 92169
1-888-887-0417 or Email