PYBITE AND CHLOBITE 27 



occurrence is that of cubes, the corners and edges of which may 

 be more or less modified by secondary planes, and in concre- 

 tionary masses. The second form, marcasite, also called graif, 

 white, or cockscomb pyrites, is of lighter color, inferior hardness 

 and density, and crystallizes in the orthorhombic system. Its 

 most common form of occurrence is that of irregular concre- 

 tionary masses. 



Both forms of pyrite are susceptible to oxidation when exposed 

 to atmospheric agencies, though of the two the pyrite proper 

 is much the more refractory. There is a difference in the char- 

 acter of the products arising from the decomposition of the two 

 compounds, pyrite yielding limonite and perhaps free sulphur, 

 while marcasite, under the same conditions, yields ferrous sul- 

 phate, though it may also yield limonite. The sulphate of 

 iron, resulting from pyritif erous decomposition, is, if present in 

 quantity, injurious to plant growth. This fact was well illus- 

 trated some years ago on the west front of the National Museum 

 at Washington. Several large masses of iron sulphide, too large 

 for exhibition within the building, were placed here upon a 

 floor of cement bordered by a narrow strip of lawn. Under 

 the oxidizing influence of rain and air the sulphide became 

 slowly converted into sulphate which was washed down upon 

 the cement and thence into the soil, which it so poisoned as to 

 kill the grass roots and necessitate an entire resodding. 



Chlorite (Viridite). — Under the general name chlorite are 

 included several minerals occurring in fibres and folia, closely 

 resembling the micas, from which they differ in their large per- 

 centage of water, and in their folia being inelastic. The three 

 principal varieties recognized are, ripidolite, penniniie, and pro- 

 chlorite, any one of which may occur as the essential constitu- 

 ent of a chlorite schist. Chlorite as a secondary product often 

 results from and entirely replaces the pyroxene, hornblende, or 

 mica in rocks of various kinds, and also occurs filling wholly or 

 in part the amygdaloidal cavities of trap rocks. In this last form 

 it is frequently visible oijly with the microscope, and owing to 

 the difficulties in the way of an exact determination of its 

 mineral species is sometimes called viridite. It is this mineral 

 which gives the green color to a large share of the more or 

 less altered eruptives, like the diabases and diorites, the *' green- 

 stones'' of the older geologists. 



Serpentine. — Composition: A hydrous silicate of magnesium 



