The Elements of economic geology

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Monograph

Identifikator:: 1773832379

URN:: urn:nbn:de:zbw-retromon-172798

Document type:: Monograph

Author:: Gregory, John W. http://d-nb.info/gnd/11683014X

Title:: The Elements of economic geology

Place of publication:: London

Publisher:: Methuen

Year of publication:: 1928

Scope:: XIV, 312 S.; graph. Darst.

Digitisation:: 2021

Collection:: Economics Books

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Document type:: Monograph

Structure type:: Chapter

Title:: Part III. Earthy minerals

Collection:: Economics Books

CLAY [71 present in the granite; and fluor-spar is conspicuous where the granite contained a lime felspar. Cassiterite is generally present and many of the china-clay masses were discovered by the Pheenicians and worked for tin. The conversion of granite into china-clay was formerly attributed to the percolation downward of water containing carbon dioxide, the removal of the potash of the felspar, and the recombination of the silica and alumina as kaolinite. G. Hickling (Tr. I.M.E., xxxvi, 190g, p. 2I) regards the kaolinite as formed from altered mica. The formation of the kaolinite by weathering, though adopted by W. Lindgren (Min. Dep., 1913, p. 305) is invalid for English china-clay. If it were due to the water containing carbonic acid the acces- sory minerals should include carbonates; they are however compounds of boric and fluoric acids, and include tourmaline (a variable boro-silicate), axinite (a boro-silicate of aluminium and calcium), and topaz (fluosilicate of alumina, AlF,, SiO). That china-clay was formed by hot deep-seated acids is also indicated by minerals which are clues to weathering; for biotite, which is readily thus destroyed, remains in china- clay formed from biotite-granite, while zircon, which resists weathering, has disappeared... No zircons could be found at the Carpella Mine in Cornwall, whereas they are abundant in the soil on the surrounding granite. Further, the clay- slate or killas beside a china-clay mass, as at the Carpella Mine, often contains much tourmaline due to the entrance of boric acid. Moreover, cassiterite, the metallic mineral characteristic of deep-seated acids, is a typical associate of china-clay. The distribution of the china-clay blocks is inconsistent with their formation by weathering. They are absent from many granites as in Scotland, and are confined in the British Isles to the roots of the Hercynian Mountains. There is no china-clay in many extensive areas of granite and gneiss in Scandinavia, although it occurs in Baltic islands that have been disturbed by the Hercynian movements; but as the kaolinization has there acted upon basic rocks such as dia- base, it has produced a material commercially of little value. R. H. Rastall (Tr. R.G. Soc. Cornwall, xv, 1925, pp. 415-38) suggests the formation of the Cornish china-clay by the action of steam on felspar without fluorine or boron ; but the

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