STUDIES OF HEAVY DETRITAL MINERALS

Abstract

This paper embodies the results of a detailed investigation of the heavy detrital minerals in the sands and gravels in streams, beaches, terraces etc. in a wide variety of localities in South Island, New Zealand. Appropriate techniques using screens, bolting silk, electromagnets, and the centrifuge among other devices have been developed for this and earlier work, and some well-known techniques have been modified to suit the special conditions imposed during this investigation. The centrifuge proves superior to the ordinary separating funnel if fine-grained material is handled. Various acids prove effective as solvents for calcium carbonate. Attention has been focussed on the inadvisability of using Clerici solution under certain conditions. Heavy mineral fractionation by heavy liquids in flat-bottomed porcelain dishes, rather than in ordinary filter funnel or separating funnel, is fully described, as well as the preparation of pure mineral samples. Detailed optical, density, and chemical studies of over 50 placer minerals are fully reported. Geological occurrence of the economically important tantalian cassiterite, gadolinite, monazite, thorite and its varieties, xenotime, scheelite, and zircon and its varieties is emphasized. Some varieties of thorite exhibit properties that are absolutely distinct from those previously recorded. The nature and causes of the metamict state in radioactive minerals are considered in the light of modern data on crystal structure, and although this question is not fully answered the opinion is ventured that bombardment of unstable structures by alpha-particles is a vital factor. Micro-fissuring of radioactive minerals is discussed and it is concluded that the web of fracture therein may be attributed to expansion resulting from transformation to the metamict state. An inquiry into the location and nature of the source rocks of the heavy detrital minerals concludes the report.