Thomsonite


Thomsonite Series		Thomsonite-Ca \|( Ca₂Na)(H₂O)₆\| [Al₅Si₅O₂₀] Thomsonite-Sr \|((Sr,Ca)₂Na)(H₂O)₆\| [Al₅Si₅O₂₀]

Morphology:
	Crystals commonly prismatic, acicular, or bladed, flattened on {010}, elongated to 12 cm; radial spherical or columnar aggregates; smooth globular botryoidal; compact, massive.

Physical properties:
	Cleavage:Perfect on {010}, good on {100}. Hardness: 5 – 5.5. D = 2.23 – 2.39 gm/cm³. Luster: vitreous to somewhat pearly. Streak: white.
			Thomsonite, 8 mm crystal cluster group, from Monte Somma, Vesuvius, Naples, Italy. © Volker Betz.
Optical properties:
	Color: colorless to white, yellowish, pink, brown, greenish; maybe concentrically zoned; colorless in thin section. Thomsonite-Ca. Biaxial (+). α 1.497 - 1.530, β 1.513 - 1.533, γ 1.518 - 1.545. δ 0.007 - 0.017, 2Vz 42 - 75°. X = a. Y = c, Z = b, O.A.P. \|\| (001). Thomsonite-Sr. Biaxial (+). α 1.528, β 1.532, γ 1.540. δ 0.012, 2Vz 62°. X = a. Y = c, Z = b, O.A.P. \|\| (001).

Crystallography:
	Unit cell data:
		Thomsonite-Ca	a 13.104, b 13.057, c 13.246 Å Z = 4, Space group Pncn Ståhl et al. (1990)
		Thomsonite-Sr	a 13.123, b 13.050, c 13.241 Å Z = 4, Space group Pncn (Pekov et al. 2001)

Names:
	Thomsonite was recognized as a distinct species by Brooke (1820) in a study of mesotype, an early name that included all the fibrous zeolites (natrolite, etc.). The name honors Dr. Thomas Thomson (1773-1852), who analyzed the material a few months later. The type area is in the vicinity of Old Kilpatrick, Dumbartonshire, Scotland. Brewster (1821) determined that the blocky crystals in cavities of lavas from Vesuvius were not apophyllite, and thought they were a new species, giving them the name comptonite. Only later was it recognized as thomsonite. A third, distinctive habit of thomsonite, translucent waxy balls of very thin fibers, commonly called faroelite, was also proposed for a time to be a separate species. Now these latter two terms are occasionally used to refer to the distinct habits of thomsonte. Thomsonite with dominant contents of Sr has recently been discovered on Yukspor Mountain within the Khibiny Massif, Kola Peninsula, Russia (Pekov et al. 2001), and the name thomsonite-Sr was approved by the Commission on New Minerals, Nomenclature and Classification of the International Mineralogical Association. With the discovery of thomsonite-Sr the name thomsonite is now raised to series status, which therefore includes the two species, thomsonite-Ca and thomsonite-Sr.

Crystal structure:
	The structure was first determined by Taylor et al. (1933), and solved in the space group Pbmn with a = 13.0, b = 13.0, c = 6.6 Å. The doubling of c (Alberti et al. 1981, Ståhl et al. 1990) is a result of (Si,Al) ordering in the T₅O₁₀ chains, which are parallel to the c-axis (see THO). Gottardi and Galli (1985) and Tschernich (1992) gave Pcnn as the space group setting for thomsonite.
	Pcnn and Pncn differ by the choice of the a- and b-axes. For Pcnn, a<b, whereas for Pncn, as used here, a>b. The framework of thomsonite-Ca consists of Al5Si5O20 chains, that if ordered, have a repeat distance of 13.2 Å. Ordered linking of the chains avoids Al-O-Al bridges. Therefore, in order to bring a Si-tetrahedron in juxtaposition with an Al-tetrahedron, the chains are translated and linked two ways. Parallel to the b-axis the chains are translated by 6.6 Å (c/2) and parallel to the a-axis chains are translated c/8 (as in natrolite group). The composition ranges from T_Si = .5 to .56. In those crystals with T_Si = 0.5 the framework must be highly ordered to avoid Al-O-Al bonds, but with higher Si contents the framework is partially disordered. Cations occur in two different sites, labeled Ca and CaNa by Alberti et al. (1981), Ståhl et al. (1990), and in the figure here.
	Within unit cell there are four Ca (red) and eight CaNa (orange) sites. Because the two positions for the Ca site are only 0.52 Å apart, both cannot be occupied simultaneously. Each is in six-fold coordination with four framework oxygen anions and two H₂O molecules. The CaNa site is eight-fold coordinated, bonded to 4 framework oxygen anions and to four H₂O. Typically all the CaNa sites are fully occupied with all the Na and part of the Ca, the Ca-site has the remainder Ca and any Sr. Pekov et al. (2001) show that the Sr in thomsonite-Sr occupies a position between the two Ca positions.

Chemical composition:
	The compositions of many samples of thomsonite-Ca cluster near the generalized formula Ca8Na4(H2O)24[Al20Si20O80](see the plot here), but other samples have higher Si-content. Many of these maintain Ca:Na at about 2.0, forming a trend toward mesolite (Wise and Tschernich 1978). However, other sets of analyses, such as Hey (1932), show a spread toward gonnardite and natrolite. Compositions of thomsonite-Ca intergrown with gonnardite at Magnet Cove, Arkansas, lie along a line between natrolite and thomsonite-Ca (Ross et al. 1992). R²⁺ - R⁺ - Si compositional diagram of thomsonite analyses from Hey (1932) and Wise and Tschernich (1978). The unit cell has positions for 12 non-framework cations and 24 H₂O molecules. Although available complete analyses, mostly from Hey (1932), show some variation, the average is close to 12 cations and 24 H₂O molecules, regardless of the Si content. Cations other than Ca and Na are absent or rare. Sr comprises as much as 20% of the cation positions in some samples, and of course, is the dominant cation in thomsonite-Sr. K rarely exceeds 10%. Hey (1932) and Wise and Tschernich (1978) showed that there is a relationship between the crystal habit of thomsonite-Ca and the composition, particularly the Si-content. The blocky crystals, like those at Vesuvius, have TSi near 0.50, while the very fine-grained fibers forming the waxy balls of the variety faerolite have the highest Si, near TSi of 0.54. The common bladed habit generally has intermediate Si contents, that is, TSi between 0.51 and 0.53.

Occurrences:
	Thomsonite-Ca is a fairly wide-spread alteration product of basaltic rocks, especially lavas. Because it is a low-silica zeolite, it is found mostly in saturated or undersaturated lavas and intrusive rocks. Common associated zeolites are chabazite, analcime, and gonnardite. Diagenesis and burial metamorphism of sediment and sedimentary rocks Thomsonite-Ca is a very rare constituent of the authigenic minerals formed in sediments from arc-source terrains, mostly because the composition of these rocks is too silicic. However, Boles and Coombs (1984) report thomsonite-Ca, chabazite, and gonnardite with calcite as cement in coarse sandstone derived from gabbro in the Foveaux Formation at Bluff Hill, New Zealand. They speculate that decomposition and solution of calcic plagioclase and possibly olivine favored the crystallization of the low-silica zeolites. Thomsonite-Ca is among several zeolites that fill vesicles of dikes and lavas within the Lower Permian volcaniclastic sequence of the Takitimu Group, western Southland, New Zealand (Houghton 1982). The composition of the host dike controls the local vesicle zeolite-assemblages. Cavity and vein filling thomsonite-Ca in dikes and lava flows within sedimentary sections are formed under conditions similar to those in thick sequences of lavas only. Minor thomsonite-Ca occurs with laumontite of Zone II of the metamorphosed volcaniclastic sections of the Tanzawa Mountains of central Japan (Seki 1969). Diagenesis or very low grade metamorphism of basalt and other kinds of lava flows There are many localities in which thomsonite-Ca is an important constituent in amygdules and other cavities and veins in basalt. Some of these are: eastern Iceland (Walker 1960b, Betz 1981); County Antrim, Northern Ireland (Walker 1960a); the Fareo Islands (Betz 1981); Old Kilpatrick, Dumbartonshire, Scotland; Monte Somma-Vesuvius, Naples, Italy; many localities in Bohemia, Czech Republic (for example, Rychlý, et al. 1992); the Table Mountains, Colorado (Kile and Modreski 1988); Goble, Oregon, USA (Wise and Tschernich 1978). The conditions under which thomsonite-Ca occurs in altered basaltic lavas is fairly well understood from the exposures and descriptions in Northern Ireland (Walker 1960a) and eastern Iceland (Walker 1960b). In both areas thomsonite-Ca, chabazite, and levyne are the key zeolites in a zone highest in the exposed sequence of flows. Also in both areas the analcime and natrolite zone is overlain by the thomsonite-Ca-bearing rocks. In Iceland the mesolite-scolecite zone is exposed near sea-level. These assemblages are not the same found in geothermal areas, where thomsonite-Ca has a wider distribution. Nonetheless, chabazite and levyne appear to crystallize below 70°C in geothermal systems, and this may represent the conditions in thick lava piles. These diagenetic zones grade into low grade metamorphism in areas of deep burial or high heat flow. For example, the mineral assemblages in the 8 km thick sequence of Keweenawan metabasalt of the North Shore Volcanic Group, Minnesota (Schmidt and Robinson 1997) progress from zeolite to greenschist facies. Flows of the upper most zone typically have amygdaloidal assemblages of thomsonite-Ca, scolecite, and smectite with some heulandite. Typical minerals of the next deeper one are heulandite, stilbite, and smectite with sporadic laumontite. Epidote, chlorite, and albite form the amygdale assemblage, as well as replace much of the groundmass. The thick sections basaltic lava exposed on Disko Island and Nuussuaq Peninsula, central West Greenland, exhibit the effects regional low grade and metamorphism and hydrothermal alteration Neuhoff et al. 2006). Regional metamorphism of the upper Paleocene lava formation, the Maligât Formation, produced early mixed dioctahedral–trioctahedral smectite followed by chabazite and thomsonite. This same assemblage persists into the upper portions of the underlying Vaigat Formation, where the chabazite–thomsonite assemblage is replaced at depth by an assemblage dominated by mafic phyllosilicates, thomsomite, chabazite, analcime, natrolite, and gonnardite. Ocean-floor metamorphism The metamorphic effect on ocean-floor basalt has been studied in ophiolite sequences and in drill core from spreading ridges. Thomsonite-Ca, though quite rare, occurs in the lowest temperature parts of affected rocks. Thomsonite-Ca occurs throughout the Zeolite Zone of the slightly altered pillow lava part of the Horokanai ophiolite, Hokkaido, Japan (Ishizuka 1985). The Zeolite Zone is divided into the chabazite, laumontite, and wairakite subzones, each formed at progressively higher temperatures. Thomsonite-Ca is most common in the two higher temperature zones, and interestingly formed at a higher grade than analcime, common in the chabazite subzone. In the East Taiwan Ophiolite (Liou 1979) thomsonite-Ca occurs as an alteration product of pillow basalt matrices associated with laumontite and analcime. Small amounts of thomsonite-Ca was identified by microprobe analysis in one flow in Deep Sea Drilling Project hole 504B, Costa Rica Ridge (Kurnosov et al. 1983). Flows both above and below the thomsonite-Ca flow (290 m below the seafloor) contain phillipsite, and several much deeper contain analcime. Hydrothermal systems. Thomsonite-Ca occurs only in those active hydrothermal systems that are hosted by basalt, such as Iceland. Kristmannsdóttir and Tómasson (1978) surveyed the zeolite occurrences in drill core from many of the Icelandic geothermal areas. By correlating occurrence with temperature from down hole measurements or geothermal gradients they found that thomsonite-Ca probably grows between 70 and 110°C. It is interesting to note that in the diagenetic occurrence of thomsonite-Ca in the thick lava sequences of eastern Iceland, it is associated with chabazite and levyne, while in geothermal areas the occurrences do not overlap. Westercamp (1981) found that the distribution of zeolites and other amygdale minerals the lavas of Martinique, French West Indies, is not related to regional burial. Instead they are in a zonal distribution around roots of deeply eroded volcanoes. Thomsonite-Ca and analcime characterize zone II, which is well outside the innermost, laumontite-rich zone IV. The interpretation here is the zeolite zonation represents isotherms of hydrothermal systems centered in the areas of volcanic conduits. Zeolites, particularly thomsonite-Ca, occur in veins formed by hydrothermal alteration of metamorphosed ophiolitic rocks along the Sestri-Voltaggio Zone, Liguria, Italy. Argenti et al. (1986) describe the minerals from two occurrences along this zone. In one area thomsonite-Ca is commonly associated with pectolite and prehnite in veins cutting meta-basalt, and thomsonite-Ca−chabazite−aragonite or thomsonite-Ca−gismondine assemblages filling veins in serpentinite near the meta-basalt. In the second area thomsonite-Ca−stilbite−apophyllite or thomsonite-Ca−chabazite occur in intensely sheared meta-ophiolite rocks and serpentinites. Thomsonite-Ca has been found in retrograde assemblages associated with calc-silicate bodies formed by contact metamorphism. Jamtveit et al. (1997) found fine-grained scawtite, giuseppettite, hydrogrossular, phillipsite, and thomsonite-Ca locally replaced high grade calc-silicate minerals in the Permian Oslo Rift, southern Norway. This alteration probably occurred as a very late stage hydrothermal alteration. Thomsonite-Ca has also been found in similar circumstances at the Crestmore Quarry, Riverside County, California. Thomsonite-Ca associated with gonnardite and natrolite, is a deuteric alteration product of nepheline in some syenite bodies. Ross et al. (1992) describe the replacement of nepheline by analcime, gonnardite, and thomsonite-Ca in jacupirangite and by gonnardite and thomsonite-Ca in ijolite at the Magnet Cove igneous complex, Arkansas, USA. Strontian thomsinite-Ca occurs in the central zone of pegmatite body in Koklukhtiuai river valley, Lovozero massif, Kola Peninsula, Russia (Pekov 2000). Thomsonite-Sr occurs in hydrothermal veinlets cutting natrolite cores of ristschorrite pegmatite in the Khibiny alkaline massif (Pekov et al. 2001). Elsewhere at Lovozero thomsonite-Ca replaces nepheline and sodalite (Vlasov et al. 1966), and occurs with natrolite in strongly zeolitized murmanite lujavrite (Bussen 1962). Semenov (1967) also describes thomsonite-Ca from a pegmatite body at Kuivchorr Mountain, where it forms white prismatic crystals in cavities with apatite, catapleiite, and labuntsovite. Rare thomsonite-Ca occurs as tufts of very fine-grained fibers in late stage cavities in the breccia of syenite at Mont Saint-Hilaire, Quebec (Horvath and Gault 1990). Textural evidence indicates that these tufts grew from solutions rather than as a replacement of a prior mineral. The experiments by Wirsching (1981) may approximate these reactions, and if so, indicate that they took place between 100° and 200°C.

References:
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