-clase n combining form (Geological Science) (in mineralogy) indicating a particular type of cleavage: plagioclase. [via French from Greek klasis a breaking, from klan to break]Collins English Dictionary ? Complete and Unabridged, 12th Edition 2014 © HarperCollins Publishers 1991, 1994, 1998, 2000, 2003, 2006, 2007, 2009, 2011, 2014-clase a combining form used in the names of minerals with a particular cleavage, as specified by the initial element: oligoclase; plagioclase. [