CHAPTER II. 



THE CHEMICAL CHARACTERS OF IGNEOUS ROCKS 



AN igneous rock is not a definite chemical compound. If we except 

 the comparatively rare case in which it takes the form of glass, it 

 is an aggregate of one or more definite compounds (minerals) with or without 

 an amorphous base. The chemical elements which make up the bulk 

 of igneous rocks are oxygen, silicon, aluminium, iron, calcium, magnesium, 

 potassium and sodium. If, however, we consider those which occur in smaller 

 quantities, or as traces, then it is possible that every known element may be 

 represented in igneous rocks. Titanium, phosphorous and manganese, are 

 frequently present in sufficient quantity to be estimated in small portions of 

 the rock. Barium, strontium and lithium may often be detected ; caesium and 

 rubidium occur in leucite. Zirconium occurs in the widely distributed 

 mineral zircon ; cerium and lanthanum are found in allanite, (1) a mineral re- 

 cently detected in many igneous rocks. Chromium occurs as a constituent of 

 the minerals picotite, chromite and certain diopsides. Hydrogen is present 

 in water, an original constituent of certain igneous rocks. Carbon in 

 carbonates; sulphur in sulphates, pyrites, and in the original minerals, hauyn 

 and nosean. Chlorine is present in sodalite, and in the chlorides which 

 occur in the liquid inclusions in many granitic rocks. Boron and fluorine 

 are found in tourmaline, and the former element has recently been detected 

 in many micas. 



The researches of F. SANDBERGER^' have added enormously to the list of 

 elements which must be regarded as occurring in normal igneous rocks. This 

 author has shown that if a sufficient quantity, say from ten to twenty 

 grammes, of the common rock-forming minerals, mica, olivine, augite and 

 hornblende, be taken, it is possible to detect many of the metals which are 

 usually known only as constituents of mineral veins. Thus copper, nickel 

 and cobalt were constantly found to be present in olivine. Tin and arsenic 

 were found only in this mineral from certain localities, and always in very small 

 quantities. Copper and cobalt were constantly found in the augite of gabbro, 

 diabase, melaphyre, augite-porphyrite, augite-aiidesite and basalt. Nickel, 

 lead, tin, zinc, antimony and arsenic were found only in the augites of certain 

 localities. Hornblende from the older rocks was found to contain nickel, copper, 

 arsenic and cobalt ; that from the younger rocks yielded the same elements, and 

 in addition lead, antimony, zinc and bismuth. The minerals of the mica group 



(1) IDDINGS AND W. CEOSS. Occurrence of allanite as a constituent of many rocks. 

 A.J.S., Vol. XXX., p. 108. 



(2) Untersuchungen iiber Ezzgange. 1st Part, 1882. 2nd Part, 1885. Wiesbaden. 



