Lawsonite

Key takeaways

Lawsonite belongs to earth science; place it in that map before memorising details.
Learn the definition first, then one example that makes the definition concrete.
Connect Lawsonite to a quantity you can measure, compute or draw — that is where exam questions come from.
Reproduce the core statement of Lawsonite from memory before moving on to harder problems.

Reference excerpt

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 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.

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 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).

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 nα = 1.665, nβ = 1.672–1.676, and nγ = 1.684–1.686, which produces a birefringence of δ = 0.019–0.021 and an optically positive biaxial interference figure.

… excerpt ends here. Continue reading the full article.

Illustrations

Worked examples

Example 1 — a first encounter with Lawsonite

Start with the simplest possible case. Write down what Lawsonite claims or describes in one sentence, then invent the smallest concrete situation in which that sentence is true. In earth science, the smallest case is usually a single object, a single equation or a single measurement. Check that every symbol or term in your sentence has a meaning in that case.

Example 2 — changing one variable

Take the situation from Example 1 and change exactly one quantity: double it, halve it, or set it to zero. Predict what should happen to Lawsonite before you calculate. Comparing your prediction with the result is the fastest way to find out whether you understand the idea or only the words.

Example 3 — an exam-style question

Typical questions about Lawsonite ask you to (a) state it precisely, (b) apply it to given data, and (c) explain a limitation. Practise writing all three answers in under five minutes; the third part is what separates a full-mark answer from an average one.

Applications of Lawsonite

In research: Lawsonite appears in earth science research whenever the underlying quantities have to be modelled precisely. Papers usually cite it as a starting assumption and then explore where it breaks down.
In technology and industry: Engineering practice reuses Lawsonite in design rules, simulations and safety margins. Knowing the idea lets you read a specification sheet and understand why the numbers look the way they do.
In the classroom: Lawsonite is common in secondary-school and first-year university syllabi. It links to neighbouring topics Aluminium minerals, Calcium minerals, Minerals in space group 63, so understanding it makes those chapters shorter.
In everyday life: Look for Lawsonite outside the textbook — in sport, cooking, traffic, electronics or the sky above you. An example you found yourself is remembered far longer than one you were given.

How to study Lawsonite in 20 minutes

Read the reference excerpt below once, without taking notes.
Close the page and write down what Lawsonite means in your own words.
Compare your version with the excerpt and mark what you missed.
Work through the three examples above with pen and paper.
Explain Lawsonite out loud to somebody else — or to Teacher Smith in the lgStudy chat.

Frequently asked questions

What is Lawsonite in simple terms?

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.

Why does Lawsonite matter?

Because it connects several earth science ideas at once: it gives you a definition you can apply, a quantity you can calculate, and a way to check whether a result is plausible.

How should I study Lawsonite?

Read the excerpt, restate it from memory, then work through the examples and applications listed on this page. The five-step study plan above takes about twenty minutes.

What does this page cover?

It gives you a compact reference excerpt plus original lgStudy explanations, examples, applications and study material on Lawsonite.