Minerals in rock sections; the practical methods of identifying minerals in rock sections with the microscope, especially arranged for students in technical and scientific schools . Fig. 21. — Sphrerulites in felsite. Ground-mass shows aggregate nicols. chemical composition (the successive zones in the plagioclasesgrowing more acid towards the exterior), or by ultra-microscopictwinning,* Fig. 20. ^?, Petrology for Students, 1895, p. 14. 38 IXVESTIGATION OF CHARACTERS OF MINERALS. (r) Aggregate structure, being a confused mass of separate littlecrystals, scales or gra
Minerals in rock sections; the practical methods of identifying minerals in rock sections with the microscope, especially arranged for students in technical and scientific schools . Fig. 21. — Sphrerulites in felsite. Ground-mass shows aggregate nicols. chemical composition (the successive zones in the plagioclasesgrowing more acid towards the exterior), or by ultra-microscopictwinning,* Fig. 20. ^?, Petrology for Students, 1895, p. 14. 38 IXVESTIGATION OF CHARACTERS OF MINERALS. (r) Aggregate structure, being a confused mass of separate littlecrystals, scales or grains all extinguishing at different times, Fig. 21. {d) Sphcerulitic structure, produced by the aggregation, in aradiate form, of crystals or ciystallites. It is generally easily per-ceiv^ed by the dark cross, resulting from the extinguishing of thelight in those crystals whose directions of vibration are parallel tothe planes of vibration of the nicols. When the stage is revolvedthe arms of the cross do not rotate, Fig. 21. {c) PscudojHorphic structure, which may be partial or completeand is noticed by the changed portions producing different optical. Fig. 22. — Olivine decomposed to serpentine. The pseudomorphism has been quitecomplete, only small portions of the original olivine remaining. The outline of theparent crystal can be quite distinctly seen. Crossed nicols. effects from those of the original mineral. Sometimes, althoughthe pseudomorphism has been quite complete, the form of theoriginal mineral or crystal may still be seen. Fig. 22. Characters Observed by Convergent light is obtained by passing the ra)\s of polarizedlight through a strong condensing lens, which generally fits like acap over the top of the polarizer. By means of a suitable adjust-ment the condensing lens can be brought very close to the lowersurface of the section on the stage. The lens thus sends a coneof light through the section, and used in connection with erossednicols a seri
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