Sentence:  For example , there are persons who are in physical science , in the field of mineralogy , trained in crystallography , who use only X-rays , applying only the powder technique of X-ray diffraction , to <WSD>clay</WSD> minerals only , and who have spent the last fifteen years concentrating on the montmorillonites ; or persons in the social sciences in the field of anthropology , studying the lung capacity of seven Andean Indians . 
Meaning of the word marked with <WSD> tags:  a very fine-grained soil that is plastic when moist but hard when fired