Decrespignyite-(Y) is a copper yttrium rare earth carbonate chloride hydrate; With a simple formula of; Cu(Y, REE)4(CO3)4(OH)5Cl · 2H2O Usually found as single pseudohexagonal platelets, often curved, and regularly measuring 10-50μm in size. It appears as royal blue to turquoise blue in colour, with a pale blue streak, and a pearly to vitreous lustre. Regarded as a supergene mineral which is believed to be formed through by mildly carbonated ground waters precipitating through the ore body. It is often associated with malachite, kamphaugite-(Y), donnayite-(Y) and caysichite-(Y)