Sentence:  This group of tetragonal silicates seems to form a graduated series from akermanite to gehlenite, the constitution of the two <WSD>end</WSD>[-]<WSD>members</WSD> being (where R = Ca, with less Mg, Mn, Fe, Nasub 
Meaning of the word marked with <WSD> tags:  A mineral that occurs at one end in a range of solid solutions.