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Sect. VIIL] 



MINERALOGY. 



233 



must consult works and memoirs dedicated especially to 



these researches. 



With respect to the characters of minerals, they have 

 been arranged under the following heads by M. Du- 

 frenoy : 



1. State cf Aggregation.— While minerals are commonly solid, some, 

 like native mercury and certain bitumens, are liquid; so that they may 

 be distinguished as liquid, friable, and solid. 



2. CoZowr.— Colours are either constant or accidental : when the former, 



and 



important 



term 



peroxide of iron is red, sulphuret of lead a peculiar blue-grey, and so on. 

 Accidental colours are chiefly due to the mixtures of mineral substances. 

 The peculiar appearance known as chatoi/ant depends upon the struc- 

 ture^ and is referred to the cleavage-planes, the refl_ect*id light from 

 which changes according to their position. Labradorite is a good 



example of this property. 



3. Form.— This term is not intended to include the geometric form of 

 a mineral, which is considered under the head of its crystaliographic 

 characters, but comprises only common, imitative, pseudo-morpuous, and 



pseudo-regular forms. The firsi 

 rence of the mineral in mass, fragments, plates, or in an amorphous 

 condition. The second to its occurrence in grains, nodules, &c. The 

 third, when a mineral takes the form of a pre-existing body, whether 

 organic or inorganic. The term pseudo-regular is applied to such 

 arrangements of parts as are presented by basaltic columr. 3 and other 

 prismatic forms of igneous rocks, apparently also extending to the 

 parallelepipeds arising from the intersection of the divisional planes, 

 commonly termed the joints aiid cleavage of rocks. 



4. Zz/.^fre.~Such as vitreous, waxy, silky, nacreous, adamantine, semi- 

 metallic, and metallic. 



5. Transparency.— Varying from diaphanous through semi-diapha- 

 nous, translucent, and translucent at the edges, to opaque. Rock crystal 

 is- diaphanous, chalcedony translucent— both different aggregations of 



the particles of silica. 



6. Fracture.— This is distinguished as lamellar, granular, fibrous, 



radiated-fibrous, schistose, and compact. 



7. Hardness.— Thh character is relative. The following is a scale 

 of hardness proposed by Mohs, and somewhat comm.only adopted: 



1. Lamellar Talc. 2. Selenite (crystallized sulphate of lime). 

 3. Iceland spar (carbonate of lime). 4. Fluor spar (fluate of 



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