THE PROPERTIES OF MINERALS. 667 



between tlie faces of diftereiit crystals of the same substance are 

 essentially constant. These features of a crj^stal are illustrated by a 

 series of specimens and models. 



CRYSTALLOGRAPIIIC AXES. 



The crystallographic axes are imaginary straight lines passing 

 thi'ough the center of a crystal. They are assumed as axes in order to 

 describe by reference to them the relative positions of the different 

 planes. One of the axes is called the vertical, the other two or three 

 the lateral. They may be of equal or unequal lengths, and may inter- 

 sect at either right or oblique angles. The relative positions and incli- 

 nations of the planes of crystals are expressed by referring them to sys- 

 tems of axes. They are : 



Isometric. — Three equal and interchangeable axes (a) which intersect 

 at angles of 90 degrees. 



Teirafional. — Two equal and interchangeable lateral axes {a) at 90 

 degrees to each other, and one unequal and dissimilar vertical axis (c) 

 at right angles to them. 



Hexagonal. — Three equal and interchangeable lateral axes [a) inter- 

 secting each other at angles of CO degrees. One unecjual and dissimilar 

 axis, a vertical (c), at 90 degrees to the others. 



Orthorhomhic. — Three unequal and not interchangeable axes at 90 

 degrees to each other. Any one of the three directions may be made 

 the vertical axis (c). Conventionally the longer lateral axis (6) is 

 placed horizontally from right to left and is called the macrodiagonal; 

 while the shorter lateral axis {a), which runs from back to front, is 

 called the brachydiagonal. 



MonocUnie. — Three unequal and not interchangeable axes, two of 

 which {a and c) lie at an angle /i to each other. The third axis {b) is 

 at 90 degrees to both a and c. Conventionally, e is placed verticallj^, b 

 horizontally from right to left, and is called the orthodiagonal, while a, 

 the oblique axis, is called the clinodiagonal. 



TricUnic. — Three unequal and not interchangeable axes at oblique 

 angles, a, /i, ;/, to each other. Anyone of the three directions may be 

 taken as the vertical (c). The longer of the two remaining axes, the 

 macrodiagonal (h), inclines downward toward the right, and the 

 shorter, the brachydiagonal {a), downward toward the front. 



CRYSTAL FORM. 



The term "crystal form" is defined as: The sum of all possible planes 

 bounding a crystal which are geometrically and physically equal. 

 Crystal forms are of three tyjies: Pinacpids, composed of planes par- 

 allel to two axes. Prisms and domes, forms whose planes intersect 

 two axes and are parallel to a third. Pyramids, forms whose planes 

 cut all three axes. 



Crystal forms may be simi)le or in combination. Simple forms are 



