Laumontite

Chemical formula: Ca(Si₄Al₂)O₁₂·₄H₂O

Laumontite is a hydrated calcium aluminosilicate from the zeolite group, characterized by its brittleness and tendency to rapidly dehydrate in air.

## Characteristics Laumontite is a mineral from the zeolite group, belonging to the silicate class. It forms prismatic, often elongated crystals with a rhombic cross-section. It also occurs as radial, fibrous, columnar, or granular aggregates. A characteristic feature of laumontite is its instability under low humidity conditions – it quickly loses its crystallization water, becoming brittle, cloudy, and prone to disintegration. For this reason, freshly extracted, vitreous and transparent crystals turn white and crumble into powder after a short time in the air. A variety that has partially lost water is sometimes called leonhardite. ## Physical Properties Laumontite has a Mohs hardness of 3.5-4. It is a relatively brittle mineral. Its luster is vitreous, and on cleavage surfaces, it can be pearly. After dehydration, it becomes dull and earthy. It is transparent to translucent, but specimens exposed to air quickly become cloudy and opaque. The density is approximately 2.25-2.35 g/cm³. ## Colors and Varieties It is most often colorless or white. It can also take on gray, yellowish, pinkish, or brownish hues, usually due to impurities. There are no commercially significant color varieties. The name "leonhardite" refers to a partially dehydrated form of laumontite, not a distinct color variety. ## History and Name The mineral's name comes from the French mineralogist François-Pierre

Properties

Mohs hardness: 3.5-4
Color: Colorless to pink, white, gray, yellowish, brownish, golden brown
Luster: Vitreous
Streak: White
Density: 2.23
Cleavage: on {010}{110}
Fracture: Irregular/Uneven
Transparency: Transparent,Translucent
Crystal system: Monoclinic

Diagnostic features

## Identification Laumontite can be identified by the characteristic, oblique termination of its prismatic crystals (resembling a parallelogram). A key diagnostic feature is its instability – fresh, vitreous crystals quickly become cloudy and brittle in air. It reacts with hydrochloric acid, forming a gel. ## Distinguishing from Similar Minerals It can be confused with other white zeolites, such as scolecite, natrolite, or mesolite, which often form acicular or fibrous aggregates. However, laumontite is distinguished by its crystal habit (thicker, prismatic) and their characteristic oblique termination. Unlike many other zeolites, laumontite is very unstable outside its natural, humid environment. ## Crystal Forms Crystals are typically simple, prismatic, with a rhombic cross-section and a characteristic, obliquely truncated apex. They often form aggregates radiating from a common point, as well as columnar, fibrous, and granular aggregates.

Geological environment

## Genesis Laumontite is a typical low-temperature hydrothermal mineral. It forms as a result of the activity of circulating waters in cavities and fissures of volcanic rocks, such as basalts, andesites, and their tuffs. It also occurs in hydrothermal veins, in metamorphic alteration zones of the zeolite facies, and as a cementing mineral in some sedimentary rocks (arkoses, greywackes). ## Mineral Associations It often co-occurs with other zeolites, such as stilbite, heulandite, chabazite, as well as with calcite, prehnite, epidote, and quartz. In gas vesicles in basalts, it forms characteristic mineral assemblages. ## Localities Significant laumontite specimens come from the Pune and Nashik regions in India, where they occur in vugs within Deccan basalts. Other important localities include Iceland, the Faroe Islands, Nova Scotia in Canada (Bay of Fundy region), Paterson in New Jersey (USA), as well as the Alpine regions in Italy and Switzerland. In Poland, it is found in small quantities in the basalts of Lower Silesia.

Rarity

Not very common

For collectors

## Quality Criteria The most prized specimens by collectors are those with large, well-formed, transparent, and lustrous crystals. Groups of crystals forming attractive, radial aggregates on a rock matrix are particularly sought after. Since the mineral is very unstable, the state of preservation of the specimen is a crucial factor – specimens that have not undergone dehydration (have not whitened or cracked) are much more highly valued. Specimens are often stabilized, which should be clearly indicated by the seller. ## Popular Localities The most famous localities, providing the best quality laumontite specimens, are in India, particularly in quarries around Pune, Nashik, and Mumbai. Specimens from these localities are characterized by large, often colorless or slightly pink crystals forming spectacular druses.

Care and storage

## Cleaning Laumontite specimens should be cleaned with the utmost care. It is best to use a soft brush to remove dust. Avoid water, which can accelerate disintegration processes, especially if the specimen has already been partially dehydrated. If wet cleaning is absolutely necessary, use a minimal amount of distilled water and dry the specimen immediately; however, this is very risky. ## What to Avoid The greatest threat to laumontite is dry air. It causes irreversible loss of structural water, leading to clouding, cracking, and ultimately the disintegration of crystals into a white powder. Avoid all heat sources, direct sunlight, and storage in dry, ventilated rooms. The mineral is sensitive to acids. ## Storage Laumontite requires special storage conditions. To prevent dehydration, specimens should be kept in tightly sealed containers (e.g., plastic boxes with a membrane or zip-lock bags) with a small amount of moisture, such as a piece of paper soaked in distilled water (but not directly touching the specimen) or in a container with water. Some specimens are stabilized by coating their surface with varnish or resin, which prevents water loss.