Langbeinites are a family of crystalline substances based on the structure of langbeinite with general formula M2M'2(SO4)3, where M is a large univalent cation (such as potassium, rubidium, caesium, or ammonium), and M' is a small divalent cation (for example, magnesium, calcium, manganese, iron, cobalt, nickel, copper, zinc or cadmium).
The sulfate group, SO2−4, can be substituted by other tetrahedral anions with a double negative charge such as tetrafluoroberyllate (BeF2−4), selenate (SeO2−4), chromate (CrO2−4), molybdate (MoO2−4), or tungstates.
The crystal structures of langbeinites consist of a network of oxygen vertex-connected tetrahedral polyanions (such as sulfate) and distorted metal ion-oxygen octahedra.
In the cubic form the tetrahedral anions are slightly rotated from the main crystal axes.
[1] A diammonium dicadmium selenate langbeinite could not be crystallised from water, but a trihydrate exists.
[1] Potassium members are absent, as are zinc and copper containing solids, which all crystallize in different forms.
[14] Sodium barium diiron tris-(phosphate) (NaBaFe2(PO4)3) is yet another variation with the same structure but differently charged ions.
[15] Most phosphates of this kind of formula do not form langbeinites, instead crystallise in the NASICON structure with archetype Na3Zr2(PO4)(SiO4)2.
[23] Dipotassium dimagnesium sulfate doped with dysprosium develops thermoluminescence and mechanoluminescence after being irradiated with gamma rays.
[24] Since gamma rays occur naturally, this radiation induced thermoluminescence can be used to date evaporites in which langbeinite can be a constituent.
[28] Different cations can be substituted so that for example K2Cd2(SO4)3 and Tl2Cd2(SO4)3 can form solid solutions for all ratios of thallium and potassium.
Properties such as the phase transition temperature and unit cell sizes vary smoothly with the composition.
[136] Similarly the ammonium vanadium Tutton's salt, (NH4)2V(SO4)2, heated to 160 °C in a closed tube produces (NH4)2V2(SO4)3.
Langbeinite itself can be used as an "organic" fertiliser with potassium, magnesium and sulfur, all needed for plant growth.
The phosphate langbeinites are insoluble, stable against heat, and can accommodate a large number of different ions, and have been considered for immobilizing unwanted radioactive waste.
[137] Zirconium phosphate langbeinites containing rare earth metals have been investigated for use in white LEDs and plasma displays.