Lawsonite

Lawsonite is a hydrous calcium aluminium sorosilicate mineral with formula CaAl₂Si₂O₇(OH)₂·H₂O. Lawsonite crystallizes in the orthorhombic system in prismatic, often tabular crystals. Crystal twinning is common. It forms transparent to translucent colorless, white, and bluish to pinkish grey glassy to greasy crystals. Refractive indices are nα=1.665, nβ=1.672 - 1.676, and nγ=1.684 - 1.686. It is typically almost colorless in thin section, but some lawsonite is pleochroic from colorless to pale yellow to pale blue, depending on orientation. The mineral has a Mohs hardness of 8 and a specific gravity of 3.09. It has perfect cleavage in two directions and a brittle fracture.

Lawsonite is a metamorphic mineral typical of the glaucophane schist facies. It also occurs as a secondary mineral in altered gabbro and diorite. Associate minerals include epidote, titanite, glaucophane, garnet and quartz. It is an uncommon constituent of eclogite.

It was first described in 1895 for occurrences in the Tiburon peninsula, Marin County, California. It was named for geologist Andrew Lawson (1861-1952) of the University of California.

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Composition

Lawsonite is a metamorphic silicate mineral related chemically and structurally to the epidote group of minerals. It is close to the ideal composition of CaAl2Si2O7(OH)2 . H2O giving it a close chemical composition with anorthite CaAl2Si2O8 (its anhydrous equivalent), yet lawsonite has greater density and a different Al coordination (Comodi et al., 1996). The substantial amount of water bound in lawsonite’s crystal structure is released during its breakdown to denser minerals during prograde metamorphism. This means lawsonite is capable of conveying appreciable water to shallow depths in subducting oceanic lithosphere (Clark et al., 2006). Experimentation on lawsonite to vary its responses at different temperatures and different pressures is among its most studied aspects, for it is these qualities that affect its abilities to carry water down to mantle depths, similar to other OH-containing phases like antigorite, talc, phengite, staurolite, and epidote (Comodi et al., 1996)

Geologic Occurrence

Lawsonite is a very widespread mineral and has attracted considerable interest over the last few years because of its importance as a marker of moderate Pressure (6-12 kilobars) and low Temperature (300 - 400oC) conditions in nature (Clark et al., 2006). This mainly occurs along continental margins (subduction zones) such as those found in: Franciscan Formation in California (Fig. 1), Reed Station, Tiburon Peninsula of Marin County, California, Piedmont metamorphics of Italy, and schists in New Zealand, New Caledonia, China, Japan and from various points in the circum-Pacific orogenic belt. Many times it can simply be found on the surface or in outcrops.

Crystal Structure

Though lawsonite and anorthite have similar compositions, their structures are quite different. While anorthite has a tetrahedral coordination with Al (Al substitutes for Si in feldspars), lawsonite has an octahedral coordination with Al, making it an orthorhombic sorosilicate with a space group of Cmcm which consists of Si2O7 Groups and O, OH, F, and H2O with cations in [4] and/or > [4] coordination. This is much similar to the epidote group which lawsonite is often found in conjunction with, which are also sorosilicates because their structure consists of two connected SiO4 tetrahedra plus connecting cation. The water contained in its structure is made possible by cavities formed by rings of two Al octahedral and two Si2O7 groups, each containing an isolated water molecule and calcium atom. The hydroxyl units are bound to the edge-sharing Al octahedral (Baur, 1978).

Physical Properties

Lawsonite has crystal habits of orthorhombic prismatic, which are crystals shaped like slender prisms, or tubular figures, which are form dimensions that are thin in one direction, both with two perfect cleavages. This crystal is transparent to translucent and varies in color from white to pale blue to colorless with a white streak and a vitreous or greasy luster. It has a relatively low specific gravity of 3.1g/cm3, and a pretty high hardness of 7.5 on Mohs scale of hardness, slightly higher than quartz. Under the microscope, lawsonite can be seen as blue, yellow, or colorless under plan polarized light while the stage is rotated. Lawsonite has three refractive indicies of nα = 1.665 nβ = 1.672 - 1.676 nγ = 1.684 - 1.686, which produces a birefringence of δ = 0.019 - 0.021 and an optically positive biaxial interference figure.

Significance of Lawsonite

So, is this mineral important at all? Does it tell geologist anything other than just sitting around and looking pretty? Well lawsonite most certainly does. One outstanding characteristic of lawsonite is that it is a wonderful index mineral. Index minerals are used in geology to determine the degree of metamorphism a rock has experienced. New minerals are produced through chemical reactions when the protolith experiences changes in temperature and pressure. This new mineral that is produced found in the metamorphosed rock is the index mineral, which indicates the minimum pressure and temperature the protolith must have achieved in order for that mineral to form. As stated above, lawsonite is known to form in high pressure, low temperature conditions, most commonly found in subduction zones such as a cold oceanic slab subducting towards the mantle under continental crust (Comodi et al., 1996). The lower temperature of the oceanic slab attributes to the low temperature conditions. Because this example of subduction is very prevalent, lawsonite crystals have many opportunities to form. Glaucophane, a mineral that forms in conditions much like lawsonite is often found together with lawsonite to form basaltic blueschist-facies (Pawley et al., 1996). These three rocks/minerals whether together or by their selves are great indicators of formation conditions for rocks and minerals around them. Lawsonite has proven to be a very useful and educational mineral. Hopefully more will be discovered through the use and experimentations conducted on this mineral.

References

• Hurlbut, Cornelius S.; Klein, Cornelis, 1985, Manual of Mineralogy, 20th ed., Wiley, ISBN 0-471-80580-7
• Mindat with location data
• Webmineral data
• Comodi P. and Zanazzi P. F. (1996) Effects of temperature and pressure on the structure of lawsonite, Piazza University, Perugia, Italy. American Mineralogist 81, 833-841.
• Baur W. H. (1978) Crystal structure refinement of lawsonite, University of Illinois, Chicago, Illinois. American Mineralogist 63, 311-315.
• Clarke G. L., Powell R., Fitzherbert J. A. (2006) The lawsonite paradox: a comparison of field evidence and mineral equilibria modeling, Australia. J. metamorphis Geol. 24, 715-725.
• Maekawa H., Shozul M., Ishll T., Fryer P., Pearce J. A. (1993) Blueschist metamorphism in an active subduction zone, Japan. Nature 364, 520-523.
• Pawley A. R., Redfern S. A. T., Holland T. J. B. (1996) Volume behavior of hydrous minerals at high pressure and temperature: I. Thermal expansion of lawsonite, zoisite, clinozoisite, and diaspore, U.K. American Mineralogist 81, 335-340.