286 Blackwelder — Th\ Geologic Role of Phosphorus. 



of speculation. If the planetesimal theory of the earth's 

 origin, — elaborated by Chamberlin and Moulton, — is true as 

 regards its major points, the original material of the earth may 



/ be comparable to the meteorites which arc still falling upon its 

 surface from time to time. Nearly all modern meteorites con- 

 tain minute quantities of phosphorus,— chiefly in the form of 

 the iron-and-nickel phosphide (schreibersite). Individual 

 meteorites vary in the amount of phosphorus which they con- 

 ■ tain. On the whole, the iron meteorites are generally richer 

 in the phosphide than are the stony varieties, the range being 

 from a trace in the latter to 1"25 per cent, expressed as P 2 0,,, 

 in the former. , 



On the basis of the planetesimal theory, Chamberlin has 

 suggested that more or less of the original cosmic matter of the 

 globe has been melted and worked its way outward to the sur- 

 face. Meanwhile it should have differentiated until the upper- 

 most part became a familiar volcanic magma. While there 

 are weighty considerations in favor of this concept, it is ad- 

 mittedly speculative. Whether or not it be true, it is a fact 

 that the nearest approach to a primary occurrence of phos- 

 phorus actually known to geologists, is its appearance in the 

 igneous rocks, which have crystallized from a hot mineral 

 solution (the " magma " of geologists) rising from the unknown 

 depths of the earth's interior. All igneous rocks contain 

 small quantities of phosphorus. According to Clarke's cal- 

 culation, the average igneous rock of the world contains '29 



\ per cent P 2 5 . The ratio is usually somewhat higher ('oO-l^lS 

 per cent) in the more basic igneous rocks such as gabbro and 

 the peridotites. In all these rocks the phosphorus exists 

 chiefly as the mineral apatite, — an anhydrous tricalcium phos- 

 phate chemically combined with calcium fluoride or chloride. 



When magma crystallizes it emits large quantities of steam 

 and other vapors. At the surface these are discharged into the 

 atmosphere ; but around deep-seated intrusions of magma the 

 liquid and volatile constituents permeate the adjacent rocks. 

 The mineral matter with which these solutions are highly 

 charged crystallizes out selectively upon the walls of fissures 

 and other openings through which the hot solutions pass, thus 

 producing among other things the type of veins known as 

 pegmatites. Most pegmatites contain such minerals as quartz, 

 feldspar and mica; but in rare instances, apatite is the chief 

 constituent. Like the other minerals, it usually occurs as large 

 crystals, sometimes a foot or more in length. The so-called 

 nelsonites* of Virginia, which contain 5-16 per cent of P 2 s , 

 as well as certain vein deposits in Quebec and Norway, are 



* Watson, T. L., and Taber, S., The Virginia Rutile Deposits, Bull. U. S. 

 Geol. Survey, No. 430, 1910, pp. 202-213. 



