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Encyclopedia Britannica - Main :: BER-BLA |
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BISPHENOIDAL CLASS (Hemihedral). Here there are three dyad axes, but no planes of symmetry and no centre of symmetry. The general form {hkl} is a bisphenoid (fig. 61) bounded by four scalene triangles. The other simple forms are geometrically the same as in the holosymmetric class. Examples: epsomite (Epsom salts, MgSO4.7H20), goslarite (ZnSO4.7H20), silver nitrate, sodium potassium dextro-tartrate (seignette salt, NaKC4H4O6.4H20), potassium antimonyl dextrotartrate (tartar-emetic, K(SbO)C4H406), and asparagine
Hemimorphite
4. MONOCLINIC 1 SYSTEM
(Oblique; Monosymmetric). In this system
axes are right angles, but the third angle is oblique, and the axesare of unequal lengths. The axis
two is taken as 0Y =b (fig. 62) and is called the ortho- axis
diagonal . The choice of the other two axes is arbitrary; the verticalaxis (OZ =c) is usually taken parallel to the edges of a prominently developed prismatic zone, and the clino-axis or clino- diagonal (OX =a) parallel to the zone-axis of some other prominent zone on the crystal.. The acute angle between the axes OX and OZ is usuallydenoted as and it is necessary to know its magnitude, in addition to the axial ratios a: b: c, before the crystal is completely deter- mined
a : b : c and f3, are characteristic of the substance. Thus for gypsum a : Iz : c=o6899 : 1:0.4124; 0=80 42'; for orthoclase a : b : c= 0.6585 : I : OS554; $=63 57'; and for cane- sugar
1.2595 : I : 0.8782; ft =76 30'. End of Article: BISPHENOIDAL If you wish, you can link directly to this article.
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