Montesommakite |K9 (H2O)10| [Al9Si23O64] montesommaiteMontesommaite, 0.3 mm bipyramid crystal, from Trapolino quarry, Monte Somma, Vesuvius, Italy. ©Volker Betz.    
     
Morphology:  
  Tetragonal pyramidal crystals, commonly less than 1 mm.
 
Physical properties:
  Cleavage: none.
Hardness:  4. 
D = 2.34 gm/cm3.
Luster: vitreous. Streak: white.
 
Optical properties:
  Color: Colorless; colorless in thin section.
Biaxial or uniaxial (-). 
α 1.498, β 1.506, γ 1.507, d 0.008
       
Crystallography:  
  Unit Cell: a 10.099, b 10.099 c 17.307 Å.
Space Group: Fdd2, Z = 1 (Rouse et al. (1990).
  
   
   
       
Name:  
  Montesommaite was named and described by Rouse et al. (1990). The name is for the type locality at the Pollena quarry in the northwest flank of Monte Somma, Vesuvio, Napoli, Italy.
       
Crystal structure:  
  Montesommaite was described as being closely allied to merlinoite and gismondine (Rouse et al. 1990). However, in the more rigorous classification used by Armbruster and Gunter (2001), montesommaite is assigned to the group with corner-sharing four-membered rings, not the group with edge-sharing four-membered rings. Montesommaite is most likely orthorhombic with the space group Fdd2, a = b = 10.099 and c = 17.307 Å, but is nearly tetragonal in the space group I41/amd with a = 7.141 and c = 17.307 Å.
 
The structure can be constructed by linking (001) sheets, formed by four- and eight-membered rings (MON). The structure has straight eight-membered channels running parallel to the a- and b-axes (aperture 3.6 Å). K (orange) is in eight-fold coordination by six framework oxygens and two H2O molecules.		 montesommaite
   
Chemical composition:
  The only published chemical analysis of montesommaite is in Rouse et al. (1990). The usual calculation of zeolite analyses shows there is some deficiency of non-framework cations. The water content was calculated by difference, giving 10 molecules per unit cell. Only eight molecules were found in the structure determination. It is likely that missing non-framework cations would reduce the “by difference” water, possibly to match the number found by the structure work.
   
Occurrences:
  Montesommaite is known only from three quarries (Pollena, San Vito, and Trapolino) in Monte Somma, Campania, Italy. It occurs in vesicular latite lava and latite scoria, and is associated with calcite, dolomite, phillipsite-K, and chabazite-K. These minerals occur very near the surface, and probably originated by reaction of meteoric water with the latitic glass at near surface temperatures.
   
References:
  Armbruster, T. and Gunter, M. E. 2001. Crystal structures of natural zeolites. In D.L. Bish and D.W. Ming (eds) Natural Zeolites: Occurrence, Properties, Applications. Rev. in Mineral. and Geochem. Vol. 45, Washington, D.C., 1-67.

Rouse, R.C., Dunn, P.J., Grice, J.D., Schlenker, J.L. and Higgins, J.B. 1990. Montesommaite, (K,Na)9 Al9Si23O64•10H2O, a new zeolite related to merlinoite and the gismondine group. Am. Mineral. 75, 1415-1420.