116 



MINERALOGY 



IV. Pinacoids. Three other positions of the poles are possible, 

 which represent the fixed forms, that is when the poles take the 



position of the angles of 

 the triangle, or coincide 

 with the crystallograph- 

 ical axes. 



Basal pinacoid, 

 ooa:oob:c, (001). 



FIG. 224. The Unit Brachydome of Barite. 



FIG. 225. Combination of the 

 Three Pinacoids. 



When the pole coincides with the c 

 axis, the face will be parallel to & and b, 

 the form consisting of two faces, one 



above and one below the equatorial plane, which will produce the 

 basal pinacoid. 



Macropinacoid, a : QO b : oo c , (100). 



Here the pole coincides with the a axis, when the face is parallel 



to b and c. 



Brachypinacoid, oo a : b : oo c, (010). 

 Here the pole coincides with the b 

 axis and the faces are parallel to a and 

 c; Fig. 225 represents the three pina- 

 coids in combination. 

 Forms in combination. The possible forms to combine in this 

 type therefore are : 



FIG. 226. Barite. 



Pyramids, series (hkl). 

 Prisms, series (hko). 

 Macrodome, series (hok). 

 Brachydome, series (ohk). 



Basal pinacoid, (001). 

 Macropinacoid, (100). 

 Brachypinacoid, (010). 



Examples. A large number of, and especially important, rock- 

 forming minerals crystallize in this type : 



