Lawsonite is a hydrous calcium aluminium sorosilicate mineral with formula CaAl₂Si₂O₇(OH)₂·H₂O. Lawsonite crystallizes in the

orthorhombic In crystallography, the orthorhombic crystal system is one of the 7 crystal systems. Orthorhombic Lattice (group), lattices result from stretching a cubic crystal system, cubic lattice along two of its orthogonal pairs by two different factors, res ...

system in prismatic, often tabular crystals.

Crystal twinning Crystal twinning occurs when two or more adjacent crystals of the same mineral are oriented so that they share some of the same crystal lattice points in a symmetrical manner. The result is an intergrowth of two separate crystals that are tightl ...

is common. It forms transparent to

translucent In the field of optics, transparency (also called pellucidity or diaphaneity) is the physical property of allowing light to pass through the material without appreciable light scattering by particles, scattering of light. On a macroscopic scale ...

colorless, white, pink, and bluish to pinkish grey glassy to greasy crystals.

Refractive indices In optics, the refractive index (or refraction index) of an optical medium is the ratio of the apparent speed of light in the air or vacuum to the speed in the medium. The refractive index determines how much the path of light is bent, or refrac ...

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 The Mohs scale ( ) of mineral hardness is a qualitative ordinal scale, from 1 to 10, characterizing scratch resistance of mineral In geology and mineralogy, a mineral or mineral species is, broadly speaking, a solid substance with a fair ...

of 7.5 and a

specific gravity Relative density, also called specific gravity, is a dimensionless quantity defined as the ratio of the density (mass of a unit volume) of a substance to the density of a given reference material. Specific gravity for solids and liquids is nea ...

of 3.09. It has perfect cleavage in two directions and a brittle fracture. Not to be confused with ''Larsonite'', a fossiliferous jasper mined in Nevada. Lawsonite is a metamorphic mineral typical of the blueschist facies. It also occurs as a secondary mineral in altered

gabbro Gabbro ( ) is a phaneritic (coarse-grained and magnesium- and iron-rich), mafic intrusive igneous rock formed from the slow cooling magma into a holocrystalline mass deep beneath the Earth's surface. Slow-cooling, coarse-grained gabbro is ch ...

and

diorite Diorite ( ) is an intrusive rock, intrusive igneous rock formed by the slow cooling underground of magma (molten rock) that has a moderate content of silica and a relatively low content of alkali metals. It is Intermediate composition, inter ...

. Associate minerals include epidote, titanite, glaucophane,

garnet Garnets () are a group of silicate minerals that have been used since the Bronze Age as gemstones and abrasives. Garnet minerals, while sharing similar physical and crystallographic properties, exhibit a wide range of chemical compositions, de ...

and

quartz Quartz is a hard, crystalline mineral composed of silica (silicon dioxide). The Atom, atoms are linked in a continuous framework of SiO4 silicon–oxygen Tetrahedral molecular geometry, tetrahedra, with each oxygen being shared between two tet ...

. It is an uncommon constituent of eclogite. Its scarcity in eclogite that has been exhumed to the Earth's surface does not reflect its abundance at depth in subduction zones but rather the fact that lawsonite is easily replaced by other minerals. Lawsonite was first described in 1895 for occurrences on Ring Mountain of the Tiburon peninsula,

Marin County, California Marin County ( ) is a County (United States), county located in the northwestern part of the San Francisco Bay Area of the U.S. state of California. As of the 2020 United States census, 2020 census, the population was 262,231. Its county seat a ...

and was named after

geologist A geologist is a scientist who studies the structure, composition, and History of Earth, history of Earth. Geologists incorporate techniques from physics, chemistry, biology, mathematics, and geography to perform research in the Field research, ...

Andrew Lawson (1861–1952) of the

University of California The University of California (UC) is a public university, public Land-grant university, land-grant research university, research university system in the U.S. state of California. Headquartered in Oakland, California, Oakland, the system is co ...

by two of Lawson's graduate students, Charles Palache and Frederick Leslie Ransome.Edson S. Bastin, "Biographical Memoir of Frederick Leslie Ransome, 1868-1935", National Academy of Sciences Biographic Memoirs XXII: 156 and Charles Palache with Frederick Leslie Ransome, "Uber Lawsonit, ein neues Gesteins-bildendes Mineral aus Californien" Zeits. Krist. 24 (1896): 588–592.

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 Anorthite (< ''an'' 'not' + ''ortho'' 'straight') is the

calcium

metamorphism Metamorphism is the transformation of existing Rock (geology), rock (the protolith) to rock with a different mineral composition or Texture (geology), texture. Metamorphism takes place at temperatures in excess of , and often also at elevated ...

. This means lawsonite is capable of conveying appreciable water to great depths in subducting oceanic

lithosphere A lithosphere () is the rigid, outermost rocky shell of a terrestrial planet or natural satellite. On Earth, it is composed of the crust and the lithospheric mantle, the topmost portion of the upper mantle that behaves elastically on time ...

(Clarke 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 Talc, or talcum, is a clay mineral composed of hydrated magnesium silicate, with the chemical formula . Talc in powdered form, often combined with corn starch, is used as baby powder. This mineral is used as a thickening agent and lubricant ...

, phengite, staurolite, and epidote (Comodi et al., 1996).

Geologic occurrence

Lawsonite is a very widespread mineral and has attracted considerable interest because of its importance as a marker of moderate to high pressure (6,000–25,000 bar) and low temperature (300–600 °C) conditions in nature (Clarke et al., 2006). This mainly occurs along continental margins ( subduction zones) such as those found in: the Franciscan Formation in California at Reed Station, Tiburon Peninsula of

;

schist Schist ( ) is a medium-grained metamorphic rock generally derived from fine-grained sedimentary rock, like shale. It shows pronounced ''schistosity'' (named for the rock). This means that the rock is composed of mineral grains easily seen with a l ...

s in

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, and from other points in the circum-Pacific orogenic belt; the

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metamorphic rock Metamorphic rocks arise from the transformation of existing rock to new types of rock in a process called metamorphism. The original rock ( protolith) is subjected to temperatures greater than and, often, elevated pressure of or more, caus ...

s of

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;

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, and

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Crystal structure

Though lawsonite and anorthite have similar compositions, their structures are quite different. While anorthite has a tetrahedral coordination with aluminium (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 and/or > coordination. This is much more 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 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 3.1 g/cm³, and a pretty high hardness of 7.5 on the Mohs scale of hardness, slightly higher than quartz. Under the microscope, lawsonite can be seen as blue, yellow, or colorless under plane polarized light while the stage is rotated. Lawsonite has three refractive indices of ''n''_α = 1.665, ''n''_β = 1.672–1.676, and ''n''_γ = 1.684–1.686, which produces a birefringence of ''δ'' = 0.019–0.021 and an optically positive biaxial interference figure.

Significance of lawsonite

Lawsonite is a significant metamorphic mineral as it can be used as an index mineral for high pressure conditions. Index minerals are used in geology to determine the degree of metamorphism a rock has experienced. New metamorphic minerals form through solid-state cation exchanges following changing pressure and temperature conditions imposed upon the protolith (pre-metamorphosed rock). This new mineral that is produced 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. Lawsonite is known to form in high pressure, low temperature conditions, most commonly found in subduction zones where cold oceanic crust subducts down oceanic trenches into the mantle (Comodi et al., 1996). The initially low temperature of the slab and fluids taken down with it depress isotherms and keep the slab much colder than the surrounding mantle, allowing for these unusual high pressure, low temperature conditions. Glaucophane,

, phengite, and zoisite or other epidote-group minerals are other common minerals in blueschist. Blueschists that form from basaltic parent rocks contain either lawsonite or epidote. The coexistence of glaucophane + lawsonite or epidote is diagnostic of the blueschist facies. Lawsonite also occurs in eclogite, although it is rarely preserved in the geologic record. A common assemblage in lawsonite eclogite is

+ omphacite + lawsonite + phengite +

rutile Rutile is an oxide mineral composed of titanium dioxide (TiO2), the most common natural form of TiO2. Rarer polymorphs of TiO2 are known, including anatase, akaogiite, and brookite. Rutile has one of the highest refractive indices at vis ...

± glaucophane. Lawsonite occurs as inclusions in garnet, as a matrix phase, and in veins, providing a detailed history of subduction and exhumation. Lawsonite most commonly occurs in metabasaltic rocks but also forms in metasedimentary rocks such as metachert and metacarbonate rocks. It also forms in rocks that form by metasomatism during subduction, such as in the contact zone of ultramafic rocks ( serpentinite) and other rocks. In addition to being a major host for water (11.5 wt%) in its crystal structure, lawsonite contains significant amounts of trace elements such as uranium, thorium, lead, strontium, and rare-earth elements relative to other minerals in blueschists and eclogites. Some lawsonite also contains iron, chromium, and titanium. The abundance of these elements typically varies within single crystals. The breakdown of lawsonite has been invoked as one mechanism by which intermediate-depth earthquakes are generated in subduction zones.

References

* Hurlbut, Cornelius S.; Klein, Cornelis, 1985, Manual of Mineralogy, 20th ed., Wiley, * 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. *Whitney, D.L., Fornash, K.F., Kang, P., Ghent, E.D., Martin, L., Okay, A.I., Vitale Brovarone, A. (2020) Lawsonite composition and zoning as tracers of subduction processes: a global review. Lithos 370-371, 105636. {{Authority control Calcium minerals Aluminium minerals Sorosilicates Orthorhombic minerals Minerals in space group 63