Sepiolite

Chemical Formula:  Mg4Si6O15(OH)2·6(H2O) 
Composition:  Molecular Weight = 613.82 gm 
Magnesium  15.84 %  Mg   26.26 % MgO 
Silicon    27.45 %  Si   58.73 % SiO2 
Hydrogen   1.97 %  H   　17.61 % H2O 
Oxygen     54.74 %  O 
　　　　　______        ______  
　　　　　100.00 %      102.61 % = TOTAL OXIDE 
Empirical Formula:  Mg4Si6O15·6(H2O) 
Environment:  Secondary mineral associated with serpentine. Can precipitate from alkali saline waters in arid environments.