THE OCCURRENCE OF Z INNWALDITE IN

A lithium mica extracted magnetically from china stone is compared chemically and by X-ray methods with known lithium micas. This comparison shows a strong resemblance b tween the Cornish mica and zinnwatdite from the Zinnwald. Most publications on the geology of the St. Austell granite have pointed out the occurrence of lithium-bearing micas in the district. De la Beche in 1850 described a lithium-bearing pegmatite vein in the Trelavour Downs near St. Dennis while Ussher, Barrow and MacAlister (1909) noted that some of the Cornish biotites contained lithium. Richardson (1923) went further and divided the St. Austell granite mass into lithionite and non-lithionite types. The granite, which is thought o have been intruded into Lower De, onian rocks during the Carboniferous period, has recently been described, with particular reference to the china-stone r gion between Nanpean, St. Stephens and St. Dennis, in a series of papers by Keeling (1954) and Keeling and Exley (1954). The china-stone type of granite, which is used in the potteries as a flux, is characterized by the absence of iron and boron compounds, particularly tour-maline, and by the presence of lithium and fluorine compounds. This portion of the granite is thought o be either of later date than the remainder or to be the area which crystallized last. Interspaced throughout the whole region in a random manner, however, and forming a considerable proportion of the whole is a granite, known locally as shellstone, which is kaolinized in some areas. While commercial china stone contains a mica of the mus-covite type with very little iron or fluorine, this granite contains a characteristic golden brown mica which, in contrast o muscovite, contains iron and fluorine as well as lithium. The shellstone mica is a primary mica with crystals of average diameter 0.5-1 mm, and if the granite is crushed to below this size it can be extracted by magnetic or flotation methods. Table 1 give