THE NONMETALLTC MINKRALS. 53T 



The iiiuni^ fullers' earth (W!ilk(>rde, volaorde, terre a foulon, terra 

 da puroiitori, etc.) includes a vari(>ty of clay of a greenish white, 

 greenish gray, olive and oil green or brownish color, very soft, with a 

 greasy feeling. It falls into powder in water, imparts a milky hue to 

 the li(iuid. and appears to melt on the tongue like butter. It was for- 

 merly used by fullers to take the grease out of cloth; hence the name. 



The English beds, according to Geikie^ occur in Jurassic and Cre- 

 taceous formations. Fullers' earth from beds at Nuffield, near Red- 

 hill, Surr(>y, P^ngland, is described" as a heavy blue or yellow clay, 

 with a grcMisy feel and an earthy fracture. 



When examiniHl with a microscope it is found to consist of 

 extrem(>ly irn^gular corroded particles of a siliceous mineral which in 

 its least altered state is colorless, but which in nearly every case has 

 und(M-gon(> a chloritic or talcosi^ alteration whereby the particles are 

 invertcMl into a faintly yellowish green product almost wholly on polar- 

 iz(Kl light. The particles are of all sizes up to 0.07 mm. The larger 

 portion of the mattn-ial is made up of particles fairly uniform in size 

 and a])out the dimensions mentioned. In addition to these are minute 

 colorless fragments down to sizes 0.01 mm. and even smaller. 



The minute size of these colorless particles renders a determination 

 of their mineral nature practically impossible. But the outline of the 

 clea\'age flakes is evidently suggestive of a soda lime feldspar. The. 

 high percentage of silica in the insoluble residue would indicate the 

 presence of a considerable amount of free quartz. This, however, the 

 microscope only partially substantiates, very few of the particles 

 showing the ])rilliaiit polarization colors characteristic of this mineral. 



When the powder is treated with hydrofluorsilicic acid it yields 

 abundant crystals of potassium and aluminum Huosilicate, together 

 with i-adiating forms of calcium Huosilicate. The material differs 

 from that last described in that its particles are much larger and more 

 angular in outline and the various elements in a different state of com- 

 bination. (See Plate 17, fig. 2.) 



A substance recently put upon the American market as a fullers' earth 

 (Specimen No. 62737, U.S.N.M., from Enid, Oklahoma), under the 

 trade name of "glacialite," has the following chemical composition, the 

 material being dried at 100° C. before analyzing: 



Silica 50. 36 



Alumina r 33. 38 



Ferric oxide 3. 31 



Sodiuiii, lithium, potassium oxide 88 



Water 12. 05 



Organic matter Trace. 



Titanium Trace. 



99.98 



iText book of Geology. 3d. ed. p. 133. ^ Geological Magazine, VI, 1889, p. 4F'^ 



NAT MUS 99 22 



