Stilbite-Na

Chemical formula: Na₉(Si₂₇Al₉)O₇₂·₂₈H₂O

Stilbite-Na is a hydrated sodium aluminosilicate from the zeolite group, forming characteristic crystal aggregates resembling sheaves or fans.

## Characteristics Stilbite-Na is a mineral from the zeolite group, belonging to the stilbite series. Its name indicates the dominance of sodium (Na) in its chemical composition. It forms tabular crystals, which almost always occur in characteristic, twinned aggregates resembling sheaves of grain, fans, or arcuate forms. Single, well-formed crystals are rare. Crystal surfaces often exhibit a pearly luster, especially on cleavage planes. ## Physical Properties This mineral is characterized by relatively low hardness, ranging from 3.5-4 on the Mohs scale, making it susceptible to scratching. It has perfect cleavage in one direction, which, combined with its brittleness, means specimens should be handled with great care. The luster is usually vitreous, and distinctly pearly on cleavage surfaces. The mineral's density is low, typical for zeolites, and is approximately 2.1-2.2 g/cm³. ## Colors and Varieties Stilbite-Na is most commonly colorless, white, or yellowish. Rarer specimens are pink, orange, reddish, or brown, which is usually the result of impurities of other elements or mineral inclusions. No named commercial or gemological varieties are distinguished. ## History and Name The name "stilbite" comes from the Greek word *stilbein* meaning "to glitter" or "to shine," referring to the mineral's characteristic pearly luster. The prefix "-Na" was added in 1997 by the International Mineralogical Association (IMA) as part of a revision of zeolite group nomenclature, to distinguish it from the calcium-dominant member of the series, stilbite-Ca. ## Applications Stilbite-Na, like other zeolites, has potential applications as a molecular sieve, catalyst, or sorbent in industrial processes. For mineral collectors, it is valued primarily for its unique, aesthetic crystalline forms.

Properties

Mohs hardness: 3.5-4
Color: White, colourless, red, light yellow, light to dark brown, cream
Luster: Vitreous , Pearly
Streak: White
Density: 2.1
Cleavage: Perfect on the {010}
Fracture: Conchoidal
Transparency: Transparent,Translucent
Crystal system: Monoclinic

Diagnostic features

## Identification The most important diagnostic feature of stilbite-Na is its characteristic, sheaf-like or fan-like crystal aggregates, which are the result of twinning. The pearly luster on cleavage planes is also very typical. Low hardness (about 4 on the Mohs scale) and relatively low density aid in identification. ## Distinguishing from Similar Minerals Stilbite-Na is sometimes confused with other zeolites, especially stilbite-Ca and heulandite. Distinguishing it from stilbite-Ca based on visual features is practically impossible and requires chemical analysis (EDS, WDS). Heulandite forms crystals with a more coffin-shaped, rhombic outline and less frequently forms sheaf-like aggregates, and its luster on cleavage surfaces is more strongly pearly. Other zeolites, such as scolecite or mesolite, tend to form acicular or fibrous aggregates. ## Crystal Forms Crystals are tabular, pseudo-rhombic. They almost always occur as twinned, interpenetrating individuals, forming aggregates in the shape of sheaves, fans, spherical clusters, or bow-tie-like structures.

Geological environment

## Genesis Stilbite-Na is a low-temperature hydrothermal mineral. It forms as a result of post-magmatic waters or circulating groundwater altering volcanic rocks. It crystallizes in cavities, gas vesicles, and fissures in rocks such as basalts, andesites, melaphyres, and their tuffs. It can also occur in Alpine-type veins in gneisses and crystalline schists. ## Mineral Associations This mineral often co-occurs with other zeolites, such as stilbite-Ca, heulandite, chabazite, laumontite, apophyllite, as well as with calcite, quartz (including chalcedony and agate), and prehnite. ## Localities Significant localities for stilbite-Na are found in India, in the Pune and Nashik regions (in the Deccan basalt traps), where it occurs in association with other zeolites. It is also known from Iceland (Teigarhorn area), the Faroe Islands, Scotland (Isle of Skye), Italy (Trento area), and the United States (New Jersey, Oregon, Washington).

Rarity

Not very common

For collectors

## Quality Criteria The most valued specimens by collectors are those with well-formed, sharp, and clearly defined sheaf-like or fan-like aggregates. Intense and unusual colors, such as orange or pink, are highly prized, although classic white and colorless specimens with good luster are also sought after. An important criterion is the absence of damage to the delicate crystals and aesthetic placement on the rock matrix, often in contrast with other minerals, e.g., dark basalt. ## Popular Localities Among the most famous localities yielding high-quality collector specimens are India (especially Maharashtra state), Iceland, and the state of New Jersey in the USA. Indian specimens often feature large sizes and perfect forms, frequently in association with apophyllite or heulandite.

Care and storage

## Cleaning Specimens should be cleaned very carefully, using a soft brush to remove dust. For heavier soiling, distilled water can be used, avoiding prolonged soaking. Due to its low hardness, ultrasonic cleaners and mechanical cleaning with hard tools should be avoided. ## What to Avoid The mineral is sensitive to acids, which can damage it. Contact with household and laboratory chemicals should be avoided. As a hydrated aluminosilicate, stilbite-Na can undergo dehydration (loss of water) at elevated temperatures, leading to dulling and destruction of its structure. It should be protected from direct, prolonged exposure to sunlight and heat sources. ## Storage Fragile and delicate stilbite-Na aggregates are best stored in separate, padded boxes to prevent mechanical damage and chipping of crystal edges. It should not be stored in very dry conditions to prevent dehydration.