Bulletin . d mine. In most occurrencesit is so intimately intergrown with chalcocite that it is all but impossibleto free the two. Wlien a polished section of bornite is etched and examinedwith the microscope it is seen to be made up of an interlocking mass ofmedium-sized, anhedral grains of different orientation. When the mineralis intergrown with chalcocite the intergrowths, when they involve theindividual grains, are invariably along definite crystallographic directionsof the two minerals. In a recent article* Allen has shown that the chemical composition ofnatural bomite is Cu5FeS4 instead
Bulletin . d mine. In most occurrencesit is so intimately intergrown with chalcocite that it is all but impossibleto free the two. Wlien a polished section of bornite is etched and examinedwith the microscope it is seen to be made up of an interlocking mass ofmedium-sized, anhedral grains of different orientation. When the mineralis intergrown with chalcocite the intergrowths, when they involve theindividual grains, are invariably along definite crystallographic directionsof the two minerals. In a recent article* Allen has shown that the chemical composition ofnatural bomite is Cu5FeS4 instead of CugFeSg as given in most text-booksof mineralogy. In this paper he quotes an analysis of liornite from theVirgilina district by Dr. Chase Palmer^ of the U. S. Geological Survey asfollows: Per cent Cu Fe S Allen, E. T.: The composition of natural bornite. Am. Jour. 1916. Vol. 41,pp. 409-413. .Tmir. Washington Acad. 1915. Vol. 5. p. 351. VIRGINIA GEOLOGICAL SURVEY. PLATE VIIL. _^ (B) Photomicrograph of a polished section of ore from the Seaboard mine. Showstypical development of supergene (secondary) chalcocite in in all respects to (A), except that the alteration in this section isfarther advanced, and the statements made in regard to (A) are equallyapplicable to this section. It may be noted that there are at least two con-stituents in the chalcocite areas. These, so far as could be determined, con-sist of quartz and limonite. They are better shown in Plate XIII (A) and(B), which is a more highly magnified area from a polished section of orefrom the Seaboard mine wliich shows the same phenomena in better de-velopment. Light gray = chalcocite. Dark gray = bornite. Black =ipits. VIRGINIA GEOLOGICAL SURVEY. PLATE VIII.
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