Lawsonite
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| Lawsonite | |
|---|---|
 Sample from the type locality in California with two elongated, lustrous and translucent crystals of pastel pink, lawsonite in mica schist. Size: 6.1 cm × 3.2 cm × 2.5 cm (2.4 in × 1.3 in × 1.0 in) | |
| General | |
| Category | Sorosilicate |
| Formula | CaAl2Si2O7(OH)2·H2O |
| IMA symbol | Lws[1] |
| Strunz classification | 9.BE.05 |
| Crystal system | Orthorhombic |
| Crystal class | Dipyramidal (mmm) H-M symbol: (2/m 2/m 2/m) |
| Space group | Cmcm |
| Unit cell | a = 5.847, b = 8.79, c = 13.128 [Å]; Z = 4 |
| Identification | |
| Color | Colorless, white, pale blue to grayish blue |
| Crystal habit | Commonly prismatic, tabular; also granular, massive |
| Twinning | Common on {101} lamellar |
| Cleavage | Perfect on {100} and {010}, imperfect on {101} |
| Tenacity | Brittle |
| Mohs scale hardness | 7.5 |
| Luster | Vitreous, greasy |
| Streak | White |
| Diaphaneity | Translucent |
| Specific gravity | 3.05–3.12 |
| Optical properties | Biaxial (+) |
| Refractive index | nα = 1.665 nβ = 1.672–1.676 nγ = 1.684–1.686 |
| Birefringence | δ = 0.019–0.021 |
| Pleochroism | Weak; X = blue, pale brownish yellow; Y = deep bluish green, yellowish green; Z = colorless, yellowish |
| 2V angle | Measured: 84°–85° |
| Dispersion | Strong, r > v |
| References | [2][3][4] |
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, pink, 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 7.5 and a specific gravity 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 and diorite. Associate minerals include epidote, titanite, glaucophane, garnet and quartz. 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[5] for occurrences on Ring Mountain of the Tiburon peninsula, Marin County, California and was named after geologist Andrew Lawson (1861–1952) of the University of California by two of Lawson's graduate students, Charles Palache and Frederick Leslie Ransome.[6]
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 great depths in subducting oceanic lithosphere (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, 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 Marin County, California; schists in New Zealand, New Caledonia, and from other points in the circum-Pacific orogenic belt; the Piedmont metamorphic rocks of Italy; China, Japan, Greece, and Turkey.
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 [4] and/or >[4] 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).
