Offretite |CaKMg)(H2O)16|[Al5Si13O36]
  Hexagonal prismatic crystals, commonly less than 1 mm, and in radiating hemispherical forms. Offretite
Physical properties:
  Cleavage: prismatic, poor.
Hardness:  4 . 
D = 2.13 gm/cm3.
Luster: vitreous. Streak: white.
  Prisms of offretite, Mont Semiol, Loire, France.  Image width about 1.5 mm.  © Volker Betz.
Optical properties:            
  Color: Colorless, white; colorless in thin section.     Uniaxial (+ or -).
ω 1.489 - 1.495, ε 1.486 - 1.492, δ 0.003 - 0.005
  Unit Cell: a 13.307, c 7.592 Å
(Passaglia and Tagliavini 1994)
Space Group: P6m2, Z = 1
  Offretite was named offrétite by Gonnard (1890) in honor of Professor Albert J.J. Offrét of the Faculty of Sciences, Lyon, France. The type locality is Mont Simionse (Mont Semiol), Loire, France.
Crystal structure:  
  The framework of offretite is a sequence of 6-membered rings in the pattern AABAAB..., where AA forms a double 6-membered ring, and the B 6-membered rings are the bottom and top of 14‑hedra, also known as gmelinite cages (gme, see OFF). Between the double 6-membered rings are cancrinite cages. Along the columns of double 6-rings and cancrinite cages and the columns of gmelinite cages are 12-membered ring channels (aperature 6.7 x 6.8 Å) parallel to the c-axis.
Gard and Tait (1972) suggest that the B 6-membered rings are Al free, while Alberti et al. (1996) and Gualtieri et al. (1998) found random distribution of Al between both kinds of 6-membered rings, similar to the other 6-membered ring zeolite structures
  . Alberti et al. (1996) and Gualtieri et al. (1998) show that the gmelinite cage is occupied by Mg( H2O)62+ clusters (red and blue), the number depending on cell content. The cancrinite cage is fully occupied by K (purple) bonded to six framework oxygens, and Ca (yellow) is located in the channels with H2O molecules.
Because the structure is similar to that of erionite, it is possible to have stacking faults within an offretite crystal. If the erionite domains are very small, they can only be detected with electron diffraction, but large domains of erionite, e.g. as included microcrystals, can be detected with x-ray powder diffraction methods (Passaglia et al. 1998).
Chemical composition:
  Thirteen new analyses of carefully identified and prepared offretite samples were published by Passaglia et al. (1998), TSi is typically near 0.70, but in a few samples ranges to 0.74. The non-framework cations, Ca, Mg, and K, are present in all samples in roughly equal proportions. These new data form a tighter cluster of compositions than from previous compilations, because of confusion in identification and distinguishing between offretite and erionite.
  There are scattered occurrences of offretite in cavities of basaltic lava flows. Notable are those at Mt. Seminol, France; Fittá, Soave; Contrada Re, Vicenza; and Montorso Vicentino, Italy; Vinarice, Czech Republic; Vogelsberg, Hessen, and Sasbach, Germany, among others (Passaglia et al. 1998). Some of the host rocks of these occurrences show pervasive alteration, suggesting hydrothermal alteration and others are altered mostly near vesicles, consistent with groundwater reactions. Common associated zeolites are clinoptilolite, phillipsite, chabazite, and mordenite.
  Alberti, A., Cruciani, G., Galli, E., and Vezzalini, G. 1996. A reexamination of the crystal structure of the zeolite offretite. Zeolites, 17, 457-461.

Gard, J.A. and Tait, J.M. 1972. The crystal structure of the zeolite offretite. Acta Cryst., B28, 825-834.

Gonnard, F. 1890. Sur l’offrétite, espèce minérale nouvelle. C.R. Acad. Sci., Paris  111, 1002-1003.

Gualtieri, A., Artoli, G., Passaglia, E., Bigi, S., Viani, A., and Hanson, J.C. 1998. Crystal structure-crystal chemistry relationships in the zeolites erionite and offretite. Am. Mineral., 83, 590-606.

Passaglia, E., Artioli, G., and Gualtieri, A. 1998. Crystal chemistry of the zeolites erionite and offretite. Am. Mineral., 83, 577-589.

Passaglia, E. and Tagliavini, A. 1994. Chabazite – offretite epitaxial overgrowths in cornubianite from Passo Forcel Rosso, Adamello, Italy. Eur. J. Mineral. 6, 397-405.