124 



UNITED STATES MINERAL RESOURCES 



six categories both on the product rather than on the 

 raw material. They are kaolin, ball clay, fire clay, 

 bentonite, fuller's earth, and miscellaneous clay. 

 Major difficulties are met in statistical records be- 

 cause different types of clay products are made from 

 the same clay minerals. Kaolin, ball clay, and fire 

 clay are composed chiefly of kaolinite, and all have 

 the same properties, especially when fired. Bentonite 

 consists primarily of montmorillonite. Fuller's earth 

 may be composed of montmorillonite or palygorskite 

 or both. Miscellaneous clay is composed of illite as 

 the dominant mineral, particularly in the shales, but 

 some deposits contain kaolinite and montmorillonite 

 as the major constituents. 



The structural formula of kaolinite is (OH)s 

 SijAljOio, and its theoretical composition is 39.50 

 percent AI2O3, 46.54 percent SiOa, and 13.96 per- 

 cent H2O. Other minerals of the kaolinite group 

 are metahalloysite and its hydrated form halloysite. 

 Metahalloysite has the same formula and composi- 

 tion as kaolinite, but halloysite contains two more 

 molecules of water. Halloysite is more common than 

 metahalloysite, but inasmuch as dehydration takes 

 place at room temperature, halloysite converts irre- 

 versibly to metahalloysite which may be indistin- 

 guishable from kaolinite. Kaolinte is a potential 

 source of aluminum because it contains as much as 

 39 percent alumina. Furthermore, some kaolin de- 

 posits contain gibbsite (Al(OH).,), and some fire- 

 clay deposits contain diaspore (AIO(OH)) or boeh- 

 mite (AIO(OH)), which increases the alumina con- 

 tent of these deposits to between 40 and 70 percent. 

 (See "Aluminum.") 



Kaolinite and halloysite are formed by hydro- 

 thermal, katamorphic (weathering in place), and 

 sedimentary processes, either alone or in combina- 

 tion. Hydrothermal clay is formed by ascending hot 

 solutions dissolving the country rock and precipitat- 

 ing a mineral of the kaolinite group. These clays 

 are sometimes associated with sulfide deposits. Re- 

 sidual clay is formed in place by the action of 

 chemical weathering, a katamorphic process that 

 alters minerals such as feldspars and muscovites to 

 kaolinite or halloysite. In general, the residual clays 

 are well crystallized and coarse grained. Deposits 

 of unconsolidated sedimentary kaolinite are formed 

 by the weathered debris being eroded, transported 

 by streams, and deposited in fresh- or brackish- 

 water lakes and lagoons. The kaolin deposits con- 

 tain large flakes of well-crystallized kaolinite, and 

 the ball-clay deposits contain very fine flakes of 

 poorly crystallized kaolinite and impurities of illite 

 and (or) montmorillonite. The size and crystallinity 

 of the kaolinite depend upon the source material, 



amount of breakup during transportation, and to 

 some extent the amount of diagenesis or recrystal- 

 lization after deposition. Deposits of consolidated 

 sedimentary kaolinite are formed by the weathered 

 debris being eroded, transported by streams, and 

 deposited in swamps. After deposition, leaching by 

 organic and carbonic acids removes iron, potassium, 

 and other ions so that silica and alumina are con- 

 centrated, and then postdepositional diagenesis 

 causey recrystallization of the kaolinite. Kaolinite 

 formed in this manner occurs in deposits of fire 

 clay or underclay which contain four types of clay 

 differing primarily in the degree of crystallinity and 

 the amount of impurities. The four types are plastic, 

 semiflint, flint clay, and nodular flint. Plastic clay 

 is composed of poorly crystallized kaolinite and con- 

 tains illite as the major impurity; flint clay is com- 

 posed of well-crystallized kaolinite and contains no 

 impurities. Nodular flint clay is composed of well- 

 crystallized kaolinite and contains diaspore or boeh- 

 mite, because advanced leaching has removed some 

 silica and has concentrated alumina. 



Bentonite, by geologic definition, is a product of 

 the devitrification and alteration of volcanic ash or 

 tuff. The chemical alteration of the fine-grained ash 

 that accumulated in a marine environment probably 

 began while the ash was settling through the sea 

 water. Most bentonites consist chiefly of montmoril- 

 lonite (OH) 4Si8(Al3.34Mgo. 66)020; however, some 

 montmorillonite clays not formed from ash or tuff 

 are still classified as bentonite under commercial 

 usage of the term. 



Fuller's earth produced in the Attapulgus, Ga.- 

 Quincy, Fla. district is predominantly palygorskite 

 (attapulgite), (OH2)4Mg5Sis02o-4H20; most of the 

 fuller's earth produced from deposits elsewhere in 

 the United States is montmorillonitic clay, and sev- 

 eral of these deposits are actually bentonitic clays. 

 The origin of palygorskite is speculative, but the 

 general agreement is that it formed in sea water 

 that had a high content of magnesium and silica. 

 Evidence from samples taken from the bottom of 

 the Atlantic Ocean indicates that palygorskite can 

 form as a chemical precipitate from the reaction of 

 hydrothermal solutions with sea water. 



Illite is the dominant mineral in the miscellaneous 

 clay, especially in the shales, and its approximate 

 formula is (OH)4K2(Si6-Al2) Al.Oso- Illite can be 

 formed in many ways, but as the major constituent 

 of shales, it is the result of weathered debris' being 

 eroded, transported by streams, deposited in a near- 

 shore marine environment, and subjected to diagene- 

 sis and lithification. 



Materials such as talc, silica, and calcium car- 



