endmember

noun2

1. •
   A member at one end of a range or series made up of similar members.
2. •
   A mineral that occurs at one end in a range of solid solutions.

   • Both the endmembers, epidote and clinozoisite, form the epidote sequence, in which part of the aluminium is progressively replaced by Fe³⁺; epidote is a typical mineral of the greenschist facies. — 1997