Solid-state reactions in “salt X”

Abstract Two solid-state reactions can occur from a compound known in literature as “salt X”. The first is a topotactic reaction which leads to a salt having similar a and b parameters but with c three times the c of salt X. The second produces ferrinatrite, that is quite different both for its chemistry and structure from the parent compound and its topotactic product. The structure of salt X is known. Recently it has been refined to understand some structural details and the causes which lead salt X to change sometimes into ferrinatrite, sometimes into another compound. No appreciable crystallochemical differences have been noticed between salt X crystals showing different solid-state transformations, even if the presence of a merohedric twin has been pointed out. The presence or absence of this twin, seems the main cause which allows to understand the strange behaviour of salt X.