24: Penfield — Interpretation of Mineral Analyses. 



that the amount of base is low, SiO„ being assumed as 

 practically correct, or, what is far more likely, that the amount 

 of Si0 2 is too high, as seen best when one-twentieth of the 

 Total Hydrogen is taken as unity. Does the high silica ratio 

 indicate that for these special cases a new type of tourmaline 

 formula is needed, or is it not simpler to assume that the mate- 

 rial from which these analyses were made might possibly have 

 contained a little quartz or other silicate as impurity ? It would 

 take not over two per cent of quartz as an impurity to bring 

 about the extreme amount of variation from the ratio 4 : 20 

 recorded in the foregoing tables. 



Both Clarke and Tschermak seem to place implicit confidence 

 in all of the recent tourmaline analyses : they seem to regard 

 them as perfect, and the material analyzed as necessarily pure ; 

 consequently they try to devise formulas or expressions (how 

 complicated they are will be shown) to suit all of the analyses. 

 Clarke states that a formula in order to be satisfactory u must 

 adequately express the composition of the compound in ques- 

 tion, covering all of its variations." It is evident, however, 

 that a formula should not cover variations due to impurities in 

 the material analyzed, nor possible inaccuracies in analytical 

 work. Both Clarke and Tschermak have their well-known 

 theories concerning the composition of mica, and, seeing in the 

 lithia-, iron-, and magnesia-varieties of tourmaline certain 

 analogies, respectively, to muscovite, biotite and phlogopite, 

 they both endeavor to force the tourmaline formulas to conform 

 to their ideas concerning the constitution of mica. True, as 

 shown by the analyses of Riggs,* there occur at Auburn, Rum- 

 ford and Hebron, Maine, pseudomorphs of muscovite after 

 tourmaline; but the fact need not necessarily be taken to indi- 

 cate that tourmaline is closely related to mica, nor that, by what 

 may be designated as a sort of molecular cleavage, tourmaline 

 is transformed into mica. The author is familiar with these 

 Maine localities, and he does not believe that such alterations 

 are common, nor does he believe that it is common to find 

 similar alterations at other localities where tourmaline is found. 

 Muscovite is evidently a very stable molecular compound: it 

 occurs more or less pure as pseudomorphs after many minerals, 

 and not, as it would seem, because each and every one of these 

 minerals contains as a nucleus the muscovite molecule, but 

 because muscovite has such a tendency to form under a variety 

 of conditions that it develops, provided materials suitable for 

 its formation are at hand. Thus it is supposed that an ancient 

 mud flat becomes converted to a shale and eventually to a mica 

 schist, as the result of indurating and metamorphic processes, 



* This Journal, III, xxxv. p. 41, 1888. 



