Wairakite


Wairakaite		\|Ca(H₂O)₂\| [Al₂Si₄O₁₂]

Morphology:
	Trapezohedra in sizes ranging from millimeters to centimeters

Physical properties:
	Cleavage: {100} distinct Hardness: 5.5 - 6. Density: 2.26 g/cm³. Luster: milky to vitreous. Streak: white.
			Wairakite, Bandaiatami,Koriyama, Fukushima Prefecture, Honshu, Japan. Specimen is 4 x 6 cm. @ Volker Betz.
Optical properties:
	Color: colorless to white, colorless in thin section Fine, cross-hatched lamellar twinning on {110} Biaxial (- or +). α = 1.498 - 1.500, γ = 1.502, δ = 0.000 - 0.002, 2V = 70 - 105°
Crystallography:
	Unit cells:
	tetragonal	a 13.72, c 13.66 Å, space group I4₁/acd
	monoclinic	a 13.692, b 13.643, c 13.560 Å, β 90.5°, space group I2/a Z = 8 (Mazzi and Galli 1978)
	Z = 8

Name:
	Steiner (1955) discovered and described wairakite from drill core taken at the Wairakei geothermal field, Taupo Volcanic Zone, New Zealand. The name is for the locality

Crystal structure:
	The framework of wairakite is the same as that of analcime, ANA. Four-membered tetrahedral rings form chains in three dimensions, yielding a complex tetrahedral arrangement. The channel openings are formed by strongly distorted eight-membered rings (aperature 1.6 x 4.2 Å); see the accompanying figure. The channels run parallel to <110> producing six channel directions.
	The near end-member composition is monoclinic, space group I2/a. As found for many analcime crystals, wairakite exhibits fine lamellar twinning which probably formed as a consequence of a cubic to monoclinic phase transformation (Coombs 1955). Liou (1970) showed the existence of a tetragonal disordered phase between 300° and 460°C. The transformation of this phase to ordered (monoclinic) wairakite is very sluggish. Compared with analcime, wairakite has only half the channel cations due to Ca²⁺ for 2Na⁺ substitution.
	These eight Ca ions exhibit an ordered distribution on the sixteen available positions, which also correlates with increased (Si,Al) order (Takeuchi et al. 1979). Monoclinic wairakite has six symmetry independent tetrahedral sites. Four are occupied by Si (gray) and two by Al (green). Ca (red) is six coordinated to four oxygen atoms, associated with two AlO₄ tetrahedra, and to two H₂O molecules (blue). Any Na is accommodated in the M11 and M12 sites (yellow, the Na sites in analcime). Heating of wairakite up to 200°C is accompanied by a significant increase in the unit cell volume. At 145°C, the initial monoclinic phase (I2/a) transforms into a tetragonal one (I4₁/acd). Upon heating above 200°C the dehydration of wairakite begins and is accompanied by continuous contraction with no fundamental changes in the structure while retaining symmetry I4₁/acd (Seryotkin et al. 2003). The pressure dependence of the cell parameters is strongly anisotropic and larger after the transition to the triclinic space group (Ori et al. 2008). The structure distortion of monoclinic wairakite, proceeding via tetrahedral tilting, induces deformations in the 4-, 6-, and 8-membered rings and an increase in the extra-framework Ca coordination number.


Chemical composition:

	Most wairakite occurs either in active hydrothermal systems or in low-grade metamorphic rocks. The compositions of wairakite in both types of occurrence tend to be Ca-rich with some range in Si content. In samples from geothermal systems (red in the accompanying diagram) the Si content is near 32 and Na 0.6 to 1.0 per unit cell. In those from metamorphic rocks the range is wider with most between 31.5 to 32.5 Si per unit cell, and wide ranges in Ca and Na possible. (The black circles in the diagram represent analcime from all occurrences). Like analcime, the H₂O content of wairakite appears to be variable (between 16 and 17 molecules per cell), either through analytical uncertainty or actual compositional variability.

	Diagram for the wairakite-analcime compositional series. Red squares represent wairakite from geothermal wells, and black squares, from metamorphic rocks. Black circles represent analcime.


Identification:
	Even though the morphology of wairakite and analcime are identical, the two minerals are easily distinguished by X-ray powder diffraction.
Occurrences:
	With the exception of a few deep-sea occurrences, wairakite comes from environments that have or had temperatures above 200°C and pressures less than about 50 MPa. Therefore, wairakite is common in active or inactive geothermal systems and in some low-grade metamorphic aureoles, especially where associated with epizonal plutons.
	Metamorphism of volcanic rocks and volcanic sedimentary rocks
		Burial diagenesis and metamorphism of sediment from arc-source terrains, for example the Southland Syncline, New Zealand, or the Green Tuff region of Japan, do not produce wairakite, because the geothermal gradient is too low. However, there many occurrences of wairakite within the Green Tuff region, and these are within aureoles around shallow intrusions, hydrothermal systems, or fossil hydrothermal systems (Utada 1971, Iijima and Utada 1972, Seki 1973, Utada 2001). Wairakite occurs in metamorphosed Miocene volcanic rocks in several areas of the Fossa Magna of central Japan (Seki 1973). The contact metamorphic aureole around the diorite intrusion of the Tanzawa Mountains has been divided into five zones based on stable calcium silicate minerals (Seki et al. 1969). The innermost (V) is amphibolite and the outermost two zones (II and I) are zeolitic. Wairakite occurs in the high temperature part of zone II associated with laumontite and yugawaralite. Two other areas of metamorphosed andesitic volcaniclastic rocks are the Seikoshi gold mine and Matsuaki on the Izu Peninsula, Shizuoka Prefecture. Seki (1973) observed that metamorphic wairakite commonly shows extensive solid solution with analcime. Other areas of contact metamorphism with wairakite include the Karmutsen Volcanics, Vancouver Island, British Columbia. Surdam (1973) and Cho et al. (1986) noted the rare occurrence of wairakite in rocks otherwise characterized by the occurrence of prehnite and pumpellyite. This wairakite also contains a substantial amount of analcime in solid solution. Wise (1959) and Fiske et al. (1963) report the occurrence of wairakite in the Eocene Ohanapecosh Formation in Mount Rainier National Park, Washington, USA, where it formed in response to the intrusion of the Miocene Tatoosh pluton. Donnelly (1962) and Whetten (1965) reported wairakite in meta-spilitic rocks exposed on West Indian islands. Wairakite has been reported from several Deep Sea Drilling Project holes that have penetrated coarse-grained sediment in two trench margins, Leg 126 Holes 787, 792, and 793 in the fore-arc area of the Iu-Bonin arc (Marsagalia and Tazaki 1992), and Leg 135 Hole 841, Tonga Trench (Vitali et al. 1995). Most occurrences are from the Leg 126 sites, from which the identifications were made with on-board X-ray powder diffraction methods. Independent methods of identification, such as microprobe analysis have not been made from any of the core samples. In the Tonga core wairakite is associated with analcime and may be related to intrusion of sills, but there is no evidence of heating above 200°C. Also in Leg 126 in the Izu-Bonin fore-arc sites, wairakite is associated with smectite, phillipsite, calcite, and analcime. Thermodynamic modelling of equilibrium between observed phases and compositions of pore fluids does not account for the stability of wairakite (Egeberg 1992). The metamorphosed Horokanai Ophiolite was tectonically emplaced in the Kamuikotan Zone, Hokkaido, Japan. Prograde metamorphism has produced four mineral-facies zones, ranging from zeolite to granulite facies (Ishizuka 1985). The zeolite zone, affecting mostly pillow lavas, is divided into three subzones with the key minerals, chabazite, laumontite, and wairakite, respectively. Assemblages of the wairakite subzone are chlorite+wairakite+analcime+thomsonite, chlorite+wairakite+thomsonite+albite, chlorite+wairakite+pumpellyite+albite, and chlorite+wairakite+albite+quartz. The next higher zone typically contains actinolite, pumpellyite, and chlorite. Ishizuka (1985) interprets the assemblages originating through low-pressure, ocean-floor metamorphism.
	Active hydrothermal systems
		Wairakite was first discovered in rock ejected by steam from drill holes at the Wairakei geothermal field in the Taupo Volcanic Zone, New Zealand (Steiner 1955, 1977). The conditions of its occurrence in active hydrothermal systems are the best guide to interpretation of old (fossil) areas of hydrothermal alteration. At Wairakei wairakite occurs at depths between 260 and 1140 m (hydrostatic pressures of 1.5 to 27 MPa) and at temperatures from about 200° to 250°C (Steiner 1955, 1977). Elsewhere in the Taupo Volcanic Zone, wairakite is rare in the Ohaki-Broadlands geothermal field, but where it does occur, the temperatures are from 232° to 276°C, and at the Tauhara field, from 222° to 260°C (Browne and Ellis 1970). It mainly occurs mostly in amygdales and veins cutting altered volcanic rocks, and replaces the matrix of tuffaceous rocks. It has also been identified in drill cores from the Kawerau (Milicich et al. 2017), Te Kopia and Waiotapu geothermal fields (Steiner, 1977; Browne, 1978). Milicich et al. (2017) identified an early alteration event of veins of prehnite ± epidote ± calcite rimmed by wairakite in the basement greywacke rocks. Minor wairakite also occurs in the deeper part of a later illite alteration zone. Wairakite occurs in all major geothermal fields of Japan. Miocene to Pleistocene, andesiteic to dacitic volcanic rocks have been altered by on-going hydrothermal activity at the Onikobe geothermal area northeastern Japan. With depth the zeolite sequence progresses from mordenite to laumontite to yugawaralite and to wairakite, which is followed by prehnite and epidote (Seki et al. 1983, Liou et al. 1985). The wairakite zone ranges in temperature from 100 to 140°C over a depth interval of 160 to 600 m. At other geothermal areas in Japan wairakite at temperatures from 160 to 200°C and depths from 290 to 340 m at the Otake geothermal field, Kyushu, and between 170 to 180° at depths, of 230 to 380 m at the Oshirakawa geothermal field, Ishikawa Prefecture (Seki 1966). At the Cerro Prieto geothermal field in northern Baja California, Mexico, wairakite occurs at temperatures from 200 to 300°C and at depths from 1500 to 2000 m (Schiffman et al. 1984). Stakes and Schiffman (1999) describe the hydrothermal effects of sills injected into sediment of the Middle Valley northern Juan De Fuca Ridge. The drill hole at Site 857 of the Ocean Drilling Program penetrated sills between 460 and 940 m below the seafloor. The sill margins and adjacent sediment are altered to Mg-chlorite, and are cross-cut by veins filled with quartz, chlorite, sulfides, epidote, and wairakite. Fluid inclusions give pressure corrected temperatures between 245 and 270°C, which is approximately the temperature of water being vented at the seafloor near the drill site.
	Fossil hydrothermal areas
		Few fossil geothermal areas have been recognized, but the three that have been described in Japan and another in Chile suggest more will be found. Seki (1973) reviews the alteration and minerals, including wairakite at the Kawaji damsite in central Japan; the Hanawa mining district, Aomori Prefecture, northernmost Honshu; and the Miocene pyroclastic rocks forming the base of Yugawara Volcano, Izu Peninsula. Wairakite+quartz+epidote, wairakite+laumontite+quartz, and wairakite+sodic plagioclase+epidote+quartz assemblages are typical. Vergara et al. (1993) describe the mineralogy of a hydrothermal aureole within burial metamorphosed Upper Cretaceous and Tertiary volcanic sequences in the central Chilean Andes. Folded mafic lavas and intermediate tuff and ignimbrite beds have been altered to heulandite, laumontite, and prehnite-pumpellyite assemblages. Superimposed on these assemblages, a later alteration with zones containing yugawaralite, wairakite, and epidote indicate a local hydrothermal system, possibly related to a nearby Neogene caldera. Wairakite was recorded in a chlorite-epidote alteration zone in dacite breccias adjacent to porphyry copper-bearing dacite porphyry stock of Miocene age in the Thames district, New Zealand (Brathwaite et al. 2001)


References:
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