Stilbite-Ca

Chemical formula: NaCa₄(Si₂₇Al₉)O₇₂·₂₈H₂O

Stilbite-Ca is a common mineral of the zeolite group, forming characteristic crystal aggregates in the shape of sheaves or bow-ties.

## Characteristics Stilbite-Ca is a hydrated sodium and calcium aluminosilicate, belonging to the zeolite group. Its most recognizable feature is the forms of aggregates it creates. Although the crystals themselves are tabular, they almost always occur in intergrown aggregates resembling the shape of wheat sheaves, fans, or bow-ties. These characteristic forms result from a specific type of twinning. Individual crystals within an aggregate often have slightly curved faces, which gives the entire aggregate a soft, rounded appearance. ## Physical Properties This mineral is relatively soft, with a Mohs hardness of 3.5-4. It is also quite light, with a density of about 2.1-2.2 g/cm³. The luster is predominantly vitreous, but on perfect cleavage surfaces, it becomes distinctly pearly, which is one of its distinguishing features. It is transparent to translucent. ## Colors and Varieties Stilbite-Ca most commonly occurs in white, cream, salmon-pink, or colorless. Yellowish, orange, reddish-brown, or gray specimens are less common. The name "stilbite" was formerly used for the entire series, but in 1997, the International Mineralogical Association (IMA) divided it into Stilbite-Ca (with dominant calcium) and the much rarer Stilbite-Na (with dominant sodium). Most collectible specimens labeled as "stilbite" are in fact Stilbite-Ca. ## History and Name The mineral's name comes from the Greek word *stilbein*, meaning "to shine" or "to glitter," referring to its characteristic pearly luster. The mineral was first described by the French mineralogist René Just Haüy in 1796. ## Uses Stilbite-Ca is primarily a valued and popular collector's mineral. Due to its properties typical of zeolites (ion exchange capacity and reversible dehydration), it has potential industrial applications as a molecular sieve, but it is not exploited on a large scale for this purpose.

Properties

Mohs hardness: 3.5-4
Luster: Vitreous, Pearly on cleavage surfaces
Streak: White
Density: 2.1-2.2
Cleavage: Perfect on {010}
Fracture: Uneven
Transparency: Transparent to Translucent
Crystal system: Monoclinic

Diagnostic features

## Identification The most important diagnostic feature of Stilbite-Ca are its unique aggregates in the form of sheaves, fans, or "bow-ties." The combination of this form with a pearly luster on cleavage surfaces is almost unambiguous. It reacts with hydrochloric acid, effervescing and decomposing. ## Distinguishing from Similar Minerals Stilbite-Ca is sometimes confused with other zeolites, mainly heulandite. However, heulandite forms crystals of a different shape (often described as "coffin-shaped"), has a stronger pearly luster on one main cleavage plane, and rarely forms sheaf-like aggregates. Laumontite, another similar zeolite, tends to dehydrate in dry air, becoming brittle and dull, which is not observed in stable Stilbite-Ca. ## Crystal Forms Crystals are monoclinic, tabular in habit, flattened parallel to the cleavage plane. They almost always occur in characteristic, cross-shaped twins that build larger, radial or sheaf-like aggregates. They can also form spherical and spherulitic aggregates.

Geological environment

## Genesis Stilbite-Ca is a typical low-temperature hydrothermal mineral. It forms mainly as fillings of gas vesicles (amygdules) and fractures in basic volcanic rocks, such as basalts and andesites. It also occurs in hydrothermal veins and as a product of alteration in some low-grade metamorphic rocks. ## Mineral Associations It most commonly co-occurs with other zeolites, such as heulandite, apophyllite, chabazite, laumontite, and scolecite. It is often accompanied by calcite, quartz (including chalcedony and agate), and prehnite. ## Localities It is a widely distributed mineral worldwide. Classic, historical localities include Berufjörður in Iceland and the Faroe Islands. The most famous modern localities, providing thousands of specimens to the collector's market, are in India, in the Pune and Nashik regions (Maharashtra state), in the area of the Deccan Traps. Significant specimens also come from the USA (New Jersey, Oregon), Canada (Nova Scotia), and Brazil (Rio Grande do Sul).

Rarity

Common

For collectors

## Quality Criteria The most highly prized Stilbite-Ca specimens are characterized by large, well-formed, and sharply defined sheaf-like aggregates. An intense, salmon-pink color is particularly sought after, as are pure white or colorless specimens. The aesthetics of the composition play a significant role – specimens where stilbite sheaves contrast in color and form with associated minerals, such as green apophyllite, blue chalcedony, or white scolecite spheres, are attractive. An intact, aesthetic rock matrix is also important. ## Popular Localities By far the most valued specimens by collectors come from quarries in the Pune (formerly Poona) and Nashik regions of India. It is from there that the largest and most beautiful salmon-colored "sheaves" originate, often in spectacular associations with other zeolites.

Care and storage

## Cleaning Stilbite-Ca specimens should be cleaned very carefully due to their softness and perfect cleavage. It is best to use a soft brush and distilled water to remove dust and loose dirt. Ultrasonic cleaners should be avoided, as they can cause delicate aggregates to break apart. ## What to Avoid The mineral is sensitive to acids. As a hydrated mineral, it can be damaged (dehydrated) by high temperatures, leading to a loss of luster and transparency. It should be protected from impacts and scratching by harder minerals. ## Storage It is recommended to store specimens in separate, padded boxes or in display cases, away from dust and the possibility of contact with other minerals. It does not require special protection from sunlight.

External references

https://en.wikipedia.org/wiki/https%3A%2F%2Fen.wikipedia.org%2Fwiki%2FStilbite