Infrared spectra[6] show that picropharmacolite contains water molecules H2O, hydroxyl groups (OH)− co-ordinated with Mg2+ cations, and acid arsenate radicals (HAsO4)2−.

Four independent water molecules are sandwiched between adjacent layers, and build up hydrogen-bonded chains which are also parallel to the c axis.

Picropharmacolite is usually found as small to microscopic pearly white botryoidal aggregates with a radiating foliated structure internally.

Less commonly it occurs as silky fibrous aggregates or minute needle-like crystals, that are rectangular prisms elongated along the c axis.

Formed as an oxidation product of arsenic-bearing sulfides in reaction with surrounding calcium-bearing rocks, and as a recent efflorescence in mine workings.