LUSTER OF MINERALS. 55 



third, pseudomorphs by infiltration ; the fourth, pseudomorpJu 

 by incrustation* 



Pseudomorphous crystals are distinguished by having a 

 different structure and cleavage from that of the mineral 

 imitated in form, and a different hardness, and usually little 

 luster. 



A large number of minerals have been met with as pseu- 

 domorphs. The causes of such changes have operated very 

 widely and produced important geological results. 



CHAPTER III.— PHYSICAL PROPERTIES OF 

 MINERALS. 



CHARACTERS DEPENDING ON LIGHT. 



The characters depending on light are of five kinds, and 

 arise from the power of minerals to reflect, transmit, or emit 

 light. They are as follows : 



1. Luster; 2. Color; 3. Diaphaneity; 4. Refraction; 

 5. Phosphorescence. 



LUSTER. 



90. The luster of minerals depends on the nature of their 

 surfaces, which causes more or less light to be reflected. 

 There are different degrees of intensity of luster, and also 

 different kinds of luster. 



a. The kinds of luster are six, and are named from some 

 familiar object or class of objects. 



1. Metallic : the usual luster of metals. Imperfect me- 

 tallic luster is expressed by the term sub-metallic. 



2. Vitreous : the luster of broken glass. An imperfect 

 vitreous luster is termed sub-vitreous. Both the vitreous and 

 sub-vitreous lusters are common. Quartz possesses the 

 former in an eminent degree ; calcareous spar often the lat- 

 ter. This luster may be exhibited by minerals of any color. 



3. Resinous : luster of the yellow resins. Ex. opal, zinc 

 blende. 



4. Pearly : like pearl. Ex. talc, native magnesia, stil- 

 bite, &c. When united with sub-metallic luster, the term 

 metallic-pearly is applied. 



How are pseudomorphous crystals distinguished 1 What characters 

 depend on light } Explain the varieties of luster, metallic, vitreous, &c. 



* This subject is farther treated of by the author in the Amer. Jour. 

 *f Science, vol. xlviii, pp. 68, 81, 397. 



