Lawsonite

Chemical formula: CaAl₂Si₂O₇(OH)₂·H₂O

Lawsonite is a calcium-aluminum silicate, a key indicator mineral for blueschist facies metamorphic rocks, forming characteristic tabular or columnar crystals.

## Characteristics Lawsonite is a hydrated calcium-aluminum silicate, valued in geology as an indicator mineral for metamorphic processes occurring under high pressure and relatively low temperature. It forms colorless, whitish, grayish, or pale blue crystals with a tabular, columnar, or prismatic habit. It often occurs as well-formed crystals embedded in rock, which can reach several centimeters in length. It is a brittle mineral with a vitreous or greasy luster. ## Physical Properties Lawsonite is characterized by a relatively high hardness, ranging from 7.5 to 8 on the Mohs scale, which is unusual for many silicates. Its density ranges from 3.05 to 3.1 g/cm³. It exhibits perfect cleavage in two directions, which, combined with its hardness, is an important diagnostic feature. It is transparent to translucent. ## Colors and Varieties It is usually colorless, white, or gray. Rarer specimens are pale blue, greenish, or pinkish. No named color varieties or commercial varieties are distinguished. ## History and Name The mineral was first described in 1895 by Frederick Leslie Ransome and Charles Palache. The name "lawsonite" was given in honor of Andrew Cowper Lawson (1861–1952), a Scottish-American geologist and professor at the University of California, Berkeley, who first provided samples of the mineral for study. The specimens came from the Tiburon Peninsula in Marin County, California, USA. ## Uses Lawsonite has no industrial application. Its significance is purely scientific – as an important indicator of high-pressure metamorphic conditions – and as a collector's item. Specimens, especially well-formed crystals embedded in a contrasting rock matrix (e.g., in glaucophane), are sought after by petrological mineral collectors.

Properties

Mohs hardness: 7.5
Color: colorless, white, grayish blue, pale blue, pink
Luster: Vitreous to greasy
Streak: white
Density: 3.1
Cleavage: [001] perfect [110] poor
Fracture: Irregular/Uneven
Transparency: Translucent
Crystal system: Orthorhombic

Diagnostic features

## Identification Lawsonite is most easily recognized by its characteristic tabular or prismatic crystals, often with a rhombic cross-section. Key features include high hardness (7.5-8 on the Mohs scale), vitreous luster, and perfect cleavage in two directions. Its occurrence in blueschist facies metamorphic rocks, such as glaucophanites, is a strong diagnostic indicator. ## Distinguishing from Similar Minerals Lawsonite is sometimes confused with some feldspars (e.g., anorthite) or epidote, with which it can co-occur. It is distinguished from feldspars by its significantly higher hardness and density. Epidote typically has a characteristic pistacio-green color, while lawsonite is most often colorless or whitish. Under a petrographic microscope, lawsonite exhibits characteristic high relief and low interference colors. ## Crystal Forms Lawsonite crystals are typically tabular, flattened along one of the axes, or short-prismatic and prismatic. They often form twins. They occur as single, well-formed crystals (idioblasts) embedded in a fine-grained rock mass or in the form of granular aggregates.

Geological environment

## Genesis Lawsonite is a mineral typical of rocks that have undergone metamorphism under high-pressure and relatively low-temperature (HP/LT) conditions. It is a characteristic mineral of the blueschist facies and eclogite facies. It forms as a result of the transformation of aluminum- and calcium-rich sedimentary rocks (e.g., marls, greywackes) and igneous rocks (e.g., basalts, gabbros) in subduction zones, where tectonic plates plunge into the Earth's mantle. ## Mineral Associations This mineral often co-occurs with other high-pressure minerals. Its typical associates include glaucophane, jadeite, omphacite, pumpellyite, epidote, garnet (almandine-pyrope), actinolite, chlorite, and quartz. The presence of lawsonite in a mineral assemblage allows geologists to precisely determine the pressure and temperature conditions to which a given rock was subjected. ## Localities The most important and classic lawsonite localities are in California (USA), especially on the Tiburon Peninsula in Marin County (type locality) and in Mendocino County. Beautiful specimens also come from the Western Alps, particularly from Piedmont in Italy. Other known occurrences include New Caledonia, Corsica (France), Syros Island (Greece), Guatemala, and Japan.

Rarity

Not very common

For collectors

## Quality Criteria Most prized by collectors are specimens with large, sharply terminated, transparent lawsonite crystals embedded in a contrasting rock matrix. Particularly sought after are crystals set on blue glaucophane or greenish omphacite, forming aesthetic compositions. High purity and absence of fractures in the crystals, as well as their size, are highly valued – specimens with crystals exceeding 2-3 cm are rare and valuable. A pale blue coloration also enhances the attractiveness of a specimen. ## Popular Localities Classic and most desirable specimens come from historical localities in California, USA, especially from the Reed Station area in Marin County. These specimens, often with sharp, white crystals against a dark rock background, are considered a world standard for this mineral. High-quality lawsonites, often in association with jadeite and glaucophane, also come from the Monviso valley in Piedmont, Italy.

Care and storage

## Cleaning Lawsonite specimens should be cleaned very carefully, using a soft brush and distilled water. Due to its perfect cleavage, ultrasonic cleaners and high-pressure mechanical cleaning should be avoided, as they could cause crystals to fracture along cleavage planes. ## What to Avoid Avoid contact with acids and strong chemicals. Lawsonite is stable under normal conditions, but prolonged exposure to direct sunlight is not recommended. It should not be heated, as it contains water in its structure, which it may lose at elevated temperatures. ## Storage Specimens are best stored in separate, padded boxes or display cases to prevent scratching by harder minerals or damage to more delicate specimens. They should be protected from dust and sudden temperature changes.