Decomjiositioii of Iron Pyrites. 219 



but it does not always follow as a necessary consequence that the 

 presence of pyrites in stone is of necessity a disadvantage. As 

 a general rule, the presence of marcasite is. Of the ordinary 

 pyrites some varieties do not decompose, while others do ; the 

 presence of such varieties as decomi)ose may disfigure the stone, 

 if in small ([uantities only, or may cause it to swell and disin- 

 tegrate if in large quantifies. In compact stones its presence 

 has but little influence ; in porous ones it is generally objection- 

 able. But no absolute rule should be laid down, for, while as a 

 general thing it is to be avoided, it may be harmless. In gen- 

 eral it may be said that the presence of much pyrites makes the 

 stone unfit for use in the exterior of buildings." 



In general, therefore, where a perishable form of pyrites 

 is unequally distributed throngh the stone, in isolated spots 

 and bands, it inevitably produces the well-known deplorable 

 results in injury to color and appearance which might be 

 readily avoided by a previous test of the true nature of 

 the enclosed mineral. In the freestones — the sedimentary 

 sandstones and limestones — the pyrites is almost always 

 liable to decomposition and staining, not merely on ac- 

 count of the porosity of the rock, but chiefly the marcasitic 

 nature of the pyrite ; in the crystalline rocks — granytes, tufas, 

 crystalline marbles and dolomytes, etc., — the nature of the 

 pyrites is uncertain without trial, in some cases readily oxidiz- 

 able, in others perfectly unobjectionable. It has also been 

 maintained by some architects, and is especially true of the 

 stable varieties of pyrites, that a uniform distribution of any 

 species may be entirely innocuous, or result, after long weather- 

 ing, only in the production of a slight mellowing of color which 

 may be attractive. 



A few instances may be recorded in which the stable form of 

 pyrite has been found in building-stones. In a crystalline dol- 

 omyte of great compactness and blue-gray color, occurring at 

 Great Harrington, Mass, I have found the pyrite (No. 41, Sp. 

 Gr. 4-991), after a half century of exposure to the weather in 

 masonry, remaining perfectly bright, and the enclosing rock en- 

 tirely free from any evidence of corrosion or even discoloration. 

 A similar observation has been made by J. E. Wolff' in reference 



Report on Building Stones of the U. S., op. cit., 290. 



