'plagioclase-deficient' is a compound of 'plagioclase' and 'deficient'. 'Plagioclase' originates from Greek, specifically the elements 'plagios' (oblique) and 'klasis' (breaking), coined in Neo-Latin/modern mineralogy to name a group of feldspars. 'Deficient' originates from Latin 'deficere' meaning 'to lack, fail'.

'Plagioclase' entered scientific usage via Neo-Latin and German mineralogical terms such as 'Plagioklas' and became English 'plagioclase' in the 19th century. 'Deficient' comes from Latin 'deficiens' > Old French 'deficient' and Middle English 'deficient', yielding the modern English adjective 'deficient'.

Originally the Greek roots referred to the oblique cleavage ('plagios' + 'klasis') observed in certain minerals; over time 'plagioclase' became the established name for that feldspar group. 'Deficient' originally meant 'failing' or 'lacking' in Latin and has retained the core sense of 'lacking' in modern English.