Stilbite is the name of a series of tectosilicate minerals of the zeolite group. Prior to 1997, stilbite was recognized as a mineral species, but a reclassification in 1997 by the International Mineralogical Association changed it to a series name, with the mineral species being named:

  • Stilbite-Ca
  • Stilbite-Na, sometimes also called stiblite

Stilbite-Ca, by far the more common of the two, is a hydrous calcium sodium and aluminium silicate, NaCa<sub>4</sub>(Si<sub>27</sub>Al<sub>9</sub>)O<sub>72</sub>·28(H<sub>2</sub>O). In the case of stilbite-Na, sodium dominates over calcium. The species are visually indistinguishable, and the series name stilbite is still used whenever testing has not been performed.

History

At one time heulandite and stilbite were considered to be identical minerals. After they were found to be two separate species, in 1818, the name desmine ("a bundle") was proposed for stilbite, and this name is still employed in Germany. The English name "stilbite" is from the Greek stilbein = to shine, because of the pearly luster of the {010} faces.

Stilbite shows a wide variation in exchangeable cations: silicon and aluminium ions occupy equivalent sites and can substitute for each other. Since silicon and aluminium have a different charge (Si<sup>4+</sup> and Al<sup>3+</sup>) the ions occupying the sodium/calcium site have to adjust to maintain charge balance. There is a continuous series between stellerite, whose formula can be written as Ca<sub>4</sub>(Si<sub>28</sub>Al<sub>8</sub>)O<sub>72</sub>·28(H<sub>2</sub>O), and stilbite, and another continuous series between stilbite and barrerite, Na<sub>8</sub>(Si<sub>28</sub>Al<sub>8</sub>)O<sub>72</sub>·26(H<sub>2</sub>O).

Epistilbite is a distinct zeolite species unrelated to stilbite.

Crystal class

Stilbite is usually monoclinic 2/m, meaning that it has one twofold axis of rotational symmetry perpendicular to a mirror plane. The twofold axis is the crystal axis b, and the a and c crystal axes lie in the mirror plane. For a monoclinic crystal a and c are inclined to each other at an angle β which is not a right angle. For stilbite β is nearly 130°. Stilbite crystals, however, appear to be almost orthorhombic, and a larger unit cell can be chosen, containing two formula units (Z = 2) such that resembles an orthorhombic cell, with all three crystal axes very nearly mutually perpendicular. The mineral is said to be pseudo-orthorhombic.

Non-endmember forms of stilbite may be triclinic or even truly orthorhombic, indeed the framework can have symmetry ranging from orthorhombic to triclinic in a single crystal.

  • a = 13.60 to 13.69 Å, b = 18.20 to 18.31 Å, c = 17.78 to 17.86 Å, β = 90.0 to 90.91°