Mesolite |Na2Ca2(H2O)8| [Al6Si9O30]
  Monoclinic, m. Single crystals are slender pseudo-tetragonal prisms terminated by a pyramid, also as fine hairlike tufts. Sizes range from a few millimeters to 20 cm. Common forms: {110} {010}, and {131}. Mesolite
Physical properties:
  Cleavage:  {110} and {110} perfect.
Hardness:  5. 
D = 2.26 gm/cm3.
Luster: vitreous. Streak: white.
Optical properties:            
  Color: colorless to gray, yellowish, colorless in thin section.
Biaxial (-).  α = 1.5048, β = 1.5050, γ = 1.5053.  δ = 0.0005.  
X = a, Y = c, Z = b.  2Vz = 80°.
Dispersion: r > v, strong.
  Unit cell:  18.405,  b  56.655,  c  6.544Å
Z = 8.  Space group: Fdd2.
(Artioli et al. 1986)
  The earliest names for the natrolite group, which included natrolite, mesolite, scolecite, and thomsonite, was some form of fibrous zeolite, such as Faserzeolithe of A.G. Werner and mesotype of Haüy (1801). Mesolite was recognized as a separate species and named by Gehlen and Fuchs (1813) for its intermediate composition, but no type locality was specified.
Crystal structure:  
  The framework arrangement of mesolite is the same as natrolite with the same space group and similar unit cell parameters, except the b cell edge is tripled (Adiwidjaja 1972 and Artioli et al. 1986). There is complete (Si,Al) ordering in the framework (Alberti et al. 1995). 
Mesolite has two distinct channel types: one contains two Na’s and two H2O molecules and is similar to the channels in natrolite, and the other channel contains one Ca and three H2O molecules and is similar to the channels in scolecite. One layer of Na channels alternates with a double layer of Ca channels. These layers are parallel to (010), and this symmetric arrangement channels results in the tripling of the b cell edge. 
Structural distortions and extraframework cation arrangement upon dehydration are discussed by Ståhl and Thomasson (1994). Above 598 K mesolite becomes X-ray amorphous.
Chemical composition:
  Critical reviews of the chemical compositions of various members of the natrolite group have been published by Hey (1932), Foster (1965a and 1965b), Alberti et al. (1982), Ross et al. (1992), and Deer et al., 2004.
TSi (the fraction of Si in all tetrahedral sites) averages slightly higher than 0.600, but the amount is, in many cases, within analytical error. A consequence of ordered structures is intermediate compositions between those of natrolite, mesolite, and scolecite do not occur. For example, where mesolite occurs as an epitaxial overgrowth on natrolite, both minerals have the usual compositions (Gunter et al. 1993).
  Mesolite has limited occurrences worldwide with most occurring in cavities and fracture of mafic volcanic rocks.

Diagenesis of sediment and sedimentary rocks
Mesolite rarely occurs as an authigenic mineral in sedimentary rocks. Such occurrences tend to involve unusual chemical compositions of the host rocks or interstitial waters.

Diagenesis of marine sediment from arc-source terrains. Jurassic to Cretaceous of the Eugenia Formation exposed on the western Vizcaino Peninsula, Baja California Sur, Mexico, contain authigenic natrolite and mesolite, heulandite-Na, and analcime (Barnes et al. 1984). These zeolites replace plagioclase and glassy vitric fragments, as well as form cement, in the heterogenous marine volcaniclastic rocks.

Diagenesis and very low grade metamorphism of basalt and other kinds of lava flows.
Cavity and vein filling in altered basaltic lavas is the setting for most occurrences of mesolite. The occurrences in Iceland provide relationships that indicate the general temperatures and depths of formation of fibrous zeolites. In eastern Iceland the lowest exposures are rich in zeolites and define a zone from below sea level to about 800 m, which is overlain by the analcime zone (Walker 1960).

Well known mesolite occurrences in basalt include cavities in Tertiary shoshonite flows in the Table Mountains at Golden, Jefferson County, Colorado, where it is closely associated with thomsonite (Kile and Modreski 1988); as 4 cm long needles in altered basalt at the Bear Creek quarry, Drain, Douglas County and in pockets between two basalt flows east of Ritter Hot Springs, Grant County, Oregon; as long prisms as much as 10 cm from cavities in Eocene basalt at Skookumchuck Dam near Tenino, Thurston County, Washington (Tschernich 1972), all in the USA; as fibrous masses in Triassic basalt in the western part of the Bay of Fundy, Nova Scotia, Canada (Walker and Parsons 1922); as 10 cm long crystals in vesicular Tertiary basalt at Naalsoy, Stremoy, Faeroe Islands (Betz 1981); fine fibers with natrolite in vesicular basalt at Neubauerberg, Bohemia, Czech Republic; and as long prisms in the Deccan basalt in the Pune (Poona) district, India (Currier 1976).

Active and fossil hydrothermal systems.
Mesolite (and scolecite) are known only in the Icelandic geothermal areas. Kristmannsdóttir and Tómasson (1978) define Zone 2 as the Mesolite/Scolecite Zone, probably forming in the temperature interval between 70° and 90°C. The controlling factor is the effect that the basaltic walls rocks have on the geothermal fluids from which the zeolites crystallize.

Fossil hydrothermal systems are exposed in eastern part Martinique, French West Indies. Westercamp (1981) describes concentric zoning of alteration around several centers, that suggest hydrothermal alteration around earlier volcanic conduits. Within the outermost zone chabazite, heulandite or clinoptilolite, mordenite, and mesolite are common in cavities and veins of altered lavas. Inward these zeolites are replaced by analcime and thomsonite.
  Adiwidjaja, G. 1972. Struturbeziehungen in der Natrolithgruppe und das Entwässerungsverhalten des Skolezits. Dissertation. Univ. Hamburg.

Alberti, A., Cruciani, G., and Dauru, I. 1995. Order-disorder in natrolite-group minerals. Eur. J. Mineral., 7, 501-508.

Alberti, A., Pongiluppi, D., Vezzalini, G. 1982. The crystal chemistry of natrolite, mesolite and scolecite. Neues Jahrb. Miner. Monatsh. 1982, 231-248.

Barnes, D.A., Boles, J.R., and Hickey, J. 1984. Zeolite occurrences in Triassic-Jurassic sedimentary rocks, Baja California, Mexico. In Olson, D. and Bisio, A. (eds). Proceedings of the Sixth International Zeolite Conference, Reno, USA., Butterworths, 905-913.

Betz, V. 1981. Zeolites from Iceland and the Faeroes. Min. Rec. 12, 5-26.

Currier, R.H. 1976. Production of zeolite mineral specimens from the Deccan Basalt in India. Min. Rec. 7, 248-264.

Deer, A., Howie, R., Wise, W.S., and Zussman, J. (2004). Rock Forming Minerals. vol. 4B.
   Framework Silicates: Silica Minerals, Feldspathoids and the Zeolites. The Geological Society, London.

Foster, M.D. 1965a. Composition of zeolites of the natrolite group. U.S. Geol. Surv., Prof. Paper  504-D, 7 pp.

Foster, M.D. 1965b. Compositional relations among thomsonites, gonnardites, and natrolites. U.S. Geol. Surv., Prof. Paper  504-E, 10 pp.

Gehlen, A.F. and Fuchs, J.N. 1813. Über Werner’s Zeolith, Haüy’s Mesotype und Stilbite. (Schweigger’s) J. Chem. und Phys. 8, 353-366.

Gunter, M.E., Knowles, C.R. and Schalck, D.K. 1993. Composite natrolite-mesolite crystals from the Columbia River Basalt Group, Clarkston, Washington. Can. Min. 31, 467-470.

Haüy, R.-J. 1801. Traité de minéralogie 3. Chez Louis, Paris, France.

Hey, M.H. 1932. Studies on the zeolites. Part III. Natrolite and metanatrolite. Min. Mag. 23, 243-289.

Kile, D.E. and Modreski, P.J. 1988. Zeolites and related minerals from the Table Mountain lava flows near Golden Colorado. Min. Rec. 19, 153-184.

Kristmannsdóttir, H. and Tómasson, J. 1978. Zeolite zones in geothermal areas in Iceland. in Natural Zeolites, Occurrence, Properties, Use, Pergamon Press, Oxford., 277-284.       

Ross, M., Flohr, M.J.K., and Ross, D.R. 1992. Crystalline solution series and order-disorder within the natrolite mineral group. Am. Mineral. 77, 685-703.

Ståhl, K. and Thomasson, R. 1994. The dehydration and rehydration processes in the natural zeolite mesolite studied by conventional and synchrotron X-ray powder diffraction. Zeolites, 14, 12-17.

Tschernich, R.W. 1992 Zeolites of the World, Geoscience Press, Phoenix, Arizona. 563 pp.

Walker, G.P.L. 1960. Zeolite zones and dike distribution in relation to the structure of the basalts of eastern Iceland. Jour. Geol. 68, 515-528.

Walker, T.L. and Parsons, A.L. 1922. The zeolites of Nova Scotia. Univ. Toronto Studies, Geol. Ser., 16, 13-73.

Westercamp, D. 1981. Distribution and volcano-structural control of zeolites and other amygdale minerals in the Island of Martinique, F.W.I. J. Volc. and Geotherm. Res. 11, 353-365.