Scolecite


Scolecite		\|Ca(H₂O)₃\| [Al₂Si₃O₁₀]

Morphology:
	Monoclinic, m. Single crystals are slender pseudo-tetragonal prisms terminated by a pyramid. Sizes range from a few millimeters to 30 centimeters. Common forms: {110} {010}, and {111}. Twins on {100} are common.

Physical properties:
	Cleavage: {110}and {110} perfect. Hardness: 5. Density: 2.25 - 2.29 gm/cm³. Luster: vitreous. Streak: white

Optical properties:
	Color: colorless to gray, bluish, yellowish, colorless in thin section. Biaxial (-). α = 1.507 - 1.513, β = 1.516 – 1.520, γ = 1.517 – 1.521. δ = 0.0.08 – 0.010. Z = b, X ∧ c. 2Vx = 36° - 56°. Dispersion: r < v, strong
			Spray of scolecite prisms, Spec. 4 x 5 cm. Ósfjall, Breiðdalsvík, Suður-Múlasýsla, Íceland. © Volker Betz.
Crystallography:
	Unit cell: a 18.508, b 18.981, c 6.527 Å, β 90.64° (Joswig et al. 1984). Z = 8. Space group: F1d1



Name:
	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). Gehlen and Fuchs (1813) separated a species from the mesotype group applying the name skolezit, later changed to scolecite, to the calcium end member. The name from the Greek skolex, worm, is in reference to the tendency of a crystal to curl when heated. The type locality is not apparent from the original reference.

Crystal structure:
	The framework arrangement is the same as natrolite, but the different placement of Ca and H2O molecules in the channels reduces the space group symmetry to F1d1 (for the pseudo-orthorhombic setting, similar to that of natrolite) or Cc (Adiwidjaja 1972, Fälth and Hansen 1979, Smith et al. 1984, Joswig et al. 1984, Kvick and Ståhl 1985). The Al-O and Si-O bond lengths show that occupancy of tetrahedral sites is that of a highly ordered framework. However, ²⁹Si and ²⁷Al MAS NMR spectra (Neuhoff et al. 2002) show that of some scolecite samples exhibit small degrees (10%) of disorder. Each Ca cation, a little off-center in the channel, is bonded to four framework oxygens and to three H₂O molecules. Compared to natrolite, Na₂(H₂O)₂ in the channel is replaced by Ca(H₂O)₃; therefore, every other cation position is unoccupied.
	The W2 H2O site is nearly at the same height as the Ca cation. The protons of the H₂O molecules, determined by neutron diffraction (Joswig et al. 1984, and Kvick and Ståhl 1985), are each bonded to a framework oxygen, and the H₂O oxygens are coordinated with the Ca cation. This tight bonding and well-ordered arrangement accounts for the high dehydration energies. Note that the Ca-W2 bond direction alternates in successive layers perpendicular to the b-axis. Wang and Bish (2012) reported that the dehydration behavior of scolecite strongly depends on the applied PH₂O. Cametti et al. (2015) have investigated the dehydration dynamics and thermal stability of scolecite between 25 and 300 °C by in situ single-crystal X-ray diffraction at 30(5) % RH (high PH₂O) and under dry nitrogen conditions (low PH2O). The authors described the dehydration under high PH₂O as a two-step process: scolecite (3H₂O) -e meta-scolecite (2H₂O) -e X-ray amorphous anhydrous phase. Under low PH₂O, the temperature-dependent phase sequence was: scolecite - metascolecite - x2-phase e x1-phase - amorphization. The x2-phase has CaAl₂Si₃ O₁₀•½H₂O composition (space group Ad11, Z = 8, a = 17.536(4), b = 17.493(5), c = 6.4847(15) Å, α = 88.884(17). The x1-phase is anhydrous CaAl₂Si₃O₁₀ (space group Fd11, Z = 8, a = 16.327(8), b = 17.433(8), c =6.521(3) Å, α = 85.69(3)). Both structures represent strongly compacted and distorted varieties of the NAT framework type. (Cametti et al. 2015). Metascolecite (200 °C, high H₂O) was rehydrated under ambient conditions to scolecite. The resulting rehydrated structure (3H₂O) is different from the original scolecite and shows significant Ca disorder corresponding to the intermediate structure at 150 °C (Cametti et al. 2015). Under compression, weakening and broadening of the diffraction peaks reveals increasing structural disorder (Vezzalini et al. 2001; Ballone et al. 2002). With increasing pressure, the tetrahedral chains parallel to c rotate along their elongation axis and display an increasing twisting along a direction perpendicular to c (Ballone et al. 2002; Comodi et al. 2002).

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. A few scolecite samples contain up to 1.36 Na cations per unit cell of 80 oxygen anions, but K is less than 0.02. TSi tends to average slightly higher than 0.600, but like natrolite, the variation may be mostly analytical error.

Occurrences:
	The scolecite is an uncommon zeolite, but occurs worldwide in cavities and veins cutting altered basaltic rocks, for example, lavas or shallow intrusives, including ophiolite sequences. There are hydrothermal occurrences in fissures and fractures in granite and gneiss terrains, such as the Central Swiss Alps. Diagenesis and very low grade metamorphism of basalt and other kinds of lava flows. Cavity and vein fillings in altered basaltic lavas is the setting for most occurrences of scolecite. Some of the classic occurrences are the fine sprays, 8-10 cm long in Tertiary basalt cavities at Teirgarhorn, Berufjord, Iceland (Walker 1960, Betz 1981); in the Tertiary basalt at Ben More, Isle of Mull and at Talisker Bay, Isle of Skye, Scotland (Heddle 1901); in cavities in the Deccan basalt at Nasik and Pune, India (Currier 1976); in large pockets in basalt of the Antas Railway Tunnel near Bento Goncalves, Rio Grande do Sul, Brazil (Lieber 1978). Regional occurrences of scolecite zones, like those of Iceland (Walker 1960), occur in Rio Grande do Sol, Brazil (Murata et al. 1987). Active and fossil hydrothermal systems. Scolecite has been found 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. Deuteric to hydrothermal alteration. Scolecite occurs as a low temperature alteration product along fractures in several kinds of metamorphic rocks, for example the alpine-clefts in the granitic gneiss of the Gottard and Aar Massifs, Switzerland (Parker 1922, Armbruster et al. 1996, and Weisenberger 2009). Of the many occurrences in Switzerland described by Weisenberger (2009) a few notable ones are in the walls of the Gotthard road tunnel and the Gotthard-NEAT tunnel, Gibelsbach near Fiesch (Stalder et al. 1973 and Armbruster et al. 1996); and Schirstock, Fellithal, Uri (Parker 1922).

References:
	Adiwidjaja, G. (1972) Struturbeziehungen in der Natrolithgruppe und das Entwässerungsverhalten des Skolezits. Dissertation. Univ. Hamburg. Alberti, A., Pongiluppi, D., Vezzalini, G. 1982. The crystal chemistry of natrolite, mesolite and scolecite. Neues Jahrb. Miner. Monatsh. 1982, 231-248. Armbruster, T., Kohler, T., Meisel, T., Nägler, T.F., Götzinger, M.A., and Stalder, H.A. (1996) The zeolite, fluorite, quartz assemblage of the fissures at Gibelsbach, Fiesch (Valais, Switzerland): crystal chemistry, REE patterns, and genetic speculations. Schweiz. Mineral. Petrogr. Mitt. 76, 131-146. Ballone, P., Quartieri, S., Sani, A. and Vezzalini, G. (2002) High-pressure deformation mechanism in scolecite: a combined computational experimental study. Am. Mineral., 87(8-9), 1194-1206. Betz, V. 1981. Zeolites from Iceland and the Faeroes. Mineral. Rec. 12, 5-26. Cametti, G., Danisi, R. M., Armbruster, T., & Nagashima, M. (2015). De-and re-hydration of scolecite revisited: An in situ single-crystal X-ray study under low and high humidity conditions. Microporous and Mesoporous Material., 208, 171-180. Comodi, P., Gatta, G.D. and Zanazzi, P.F. (2002) High-pressure structural behaviour of scolecite. Eu. J Mineral., 14 (3) 567-574. 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. Fälth, L. and Hansen, S. (1979) Structure of scolecite from Poona, India. Acta Crystallogr. 35, 1877-1880. 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. Haüy, R.-J. (1801) Traité de minéralogie 3. Chez Louis, Paris, France. Heddle, M.F. (1901) Mineralogy of Scotland. Edinburgh, v. 2, 110-112. Hey, M.H. (1932) Studies on the zeolites. Part III. Natrolite and metanatrolite. Min. Mag. 23, 243-289. Joswig, W., Bartl, H. and Fuess, H. (1984) Structure refinement of scolecite by neutron diffraction. Z. Kristallogr. 166, 219-223. Kvick, Å. and Ståhl, K. (1985) A neutron diffraction study of the bonding of zeolitic water in scolecite at 20 K. Z. Kristallogr. 171, 141-154. Kristmannsdóttir, H. and Tómasson, J. (1978) Zeolite zones in geothermal areas in Iceland. in Natural Zeolites, Occurrence, Properties, Use, Pergamon Press, Oxford., 277-284. Lieber, W. (1978) Das Antas, Brasilien. Lapis, 3, 24-27. Neuhoff, P.S., Kroeker, S., Lin-Shu Du, Fridrikson, T., and Stebbins, J. (2002) Order/disorder in natrolite group zeolites: A 29Si and 27Al MAS NMR study. Am. Mineral. 87, 1307-1320. Parker, R.L. (1922) Über einige schweizerische Zeolithparagenesen. Schweiz. Min. Petr. Mitt., 2, 290-298. Smith, J.V., Pluth, J.J., Artioli, G., and Ross, F.K. (1984) Neutron and x-ray refinements of scolecite. In Olson, D. and Bisio, A. (eds). Proceedings of the Sixth International Zeolite Conference, Reno, USA., Butterworths, 842-850. Stalder, H.A., de Quervain, F., Niggli, E., Graeser, St. and Jenny, V. (1973) Die Mineralfunde der Schweiz, revised edition of Parker, R.L. Die Mineralfunde der Schweizer Alpen, Wepf and Co, Verlag, Basel. 433 pp. Vezzalini, G., Quartieri, S., Sani, A. and Levy, D. (2001) The structural modifications induced by high pressure in scolecite and heulandite: in-situ synchrotron X-ray powder diffraction study. Studies in surface sciences and catalysis, 135. Walker, G.P.L. (1960) Zeolite zones and dike distribution in relation to the structure of the basalts of eastern Iceland. J. Geol. 68, 515-528. Wang, H. W., & Bish, D. L. (2012). Infrared spectroscopic characterization of dehydration and accompanying phase transition behaviors in NAT-topology zeolites. Phys. and Chem. of Minerals, 39(4), 277-293. Weisenberger T. (2009) Zeolite in fissures of crystalline basement rocks. PhD thesis, University of Freiburg, 178 pp. Updated: December 2025.