Apophyllite-series

Chemical formula: (K,Na)Ca₄Si₈O₂₀(F,OH)[H₂O]₈

A group of minerals from the silicate class, valued by collectors for their well-formed, vitreous crystals with a characteristic pearly luster on cleavage planes.

## Characteristics Apophyllite is the name of a group of closely related silicate minerals, not a single mineral. It includes fluorapophyllite-(K), fluorapophyllite-(Na), and hydroxyapophyllite-(K). In collecting practice, the name "apophyllite" is used collectively for all members of this group, as distinguishing them without advanced analysis is impossible. Typical specimens consist of well-formed crystals resembling cubes or bipyramids, often growing on a substrate of other minerals, forming spectacular druses. The most characteristic visual feature is a strong, pearly luster on one of the crystal planes, contrasting with the vitreous luster on the other faces. ## Physical Properties Apophyllite crystals are relatively soft, with a Mohs hardness of 4.5 to 5. They are also quite brittle. The density ranges from 2.3 to 2.4 g/cm³. These minerals are characterized by perfect cleavage in one direction, which accounts for the aforementioned pearly luster on fracture planes. Transparency ranges from perfectly transparent to translucent. ## Colors and Varieties Colorless and white apophyllites are most commonly found. They can also exhibit delicate shades of pink, green, yellow, or brown, depending on trace impurities. Specimens with a distinct green color, mainly from India, are particularly prized by collectors. Some crystals show color zoning, where different parts of the crystal have different hues. No formal commercial varieties are distinguished, but specimens are often described by color and location (e.g., "green apophyllite from India"). ## History and Name The name "apophyllite" was given by René Just Haüy in 1806. It comes from the Greek words "apo" (ἀπό), meaning "away from" or "off", and "phyllon" (φύλλον), meaning "leaf". The name refers to the mineral's characteristic property of delaminating and flaking when heated (e.g., in a blowtorch flame), resembling a bursting leaf, which is caused by the loss of water from its crystal structure. ## Uses Apophyllite has no significant industrial uses. However, it is an extremely popular and valued collector's mineral due to its beautiful, well-formed crystals, high luster, and often spectacular associations with other minerals, such as stilbite, heulandite, or calcite.

Properties

Mohs hardness: 4.5-5
Luster: Vitreous, Pearly on cleavage surfaces
Streak: White
Density: 2.3-2.4
Cleavage: Perfect on {001}
Fracture: Uneven
Transparency: Transparent to Translucent
Crystal system: Tetragonal

Diagnostic features

## Identification The key diagnostic feature of apophyllite is the combination of crystal form (often pseudocubic or pyramidal) with a characteristic, strong pearly luster on the cleavage planes (the base of the crystal), while the other faces have a vitreous luster. The test for delamination under heat is destructive but very characteristic. ## Distinguishing from Similar Minerals Apophyllite is sometimes confused with quartz, calcite, or adularia. It differs from quartz by its lower hardness (quartz scratches apophyllite) and perfect cleavage (quartz lacks it). It differs from calcite by its crystal form and lack of reaction with hydrochloric acid. Adularia (a feldspar variety) has similar hardness but usually a different crystal habit and two cleavage directions. ## Crystal Forms Apophyllite crystals crystallize in the tetragonal system. They most often form pseudocubic (seemingly cubic), tabular, or bipyramidal crystals. They frequently occur as druses and crystal brushes, where many small crystals grow on a common rock base.

Geological environment

## Genesis Minerals of the apophyllite group are products of low-temperature hydrothermal processes. They form as secondary minerals, crystallizing from aqueous solutions in cavities and fissures of volcanic rocks, mainly basalts and andesites. They fill gas vesicles (geodes) in lavas, forming spectacular crystal druses. ## Mineral Associations Apophyllite very often co-occurs with minerals from the zeolite group, such as stilbite (a particularly common association), heulandite, scolecite, laumontite, and chabazite. Other common associated minerals include prehnite, pectolite, datolite, calcite, and quartz. ## Localities The most important and well-known apophyllite localities in the world are in India, on the Deccan Plateau (in the states of Maharashtra and Madhya Pradesh), especially in the Pune and Nashik regions. These are the sources of classic, large specimens of green and colorless apophyllites in association with pink stilbites. Other significant localities include Harz in Germany, the Faroe Islands, Iceland, Brazil (Rio Grande do Sul), Canada (Nova Scotia, Quebec), Mexico (Chihuahua), and the USA (New Jersey, Oregon, Colorado).

Rarity

Not very common

For collectors

## Quality Criteria The most highly prized apophyllite specimens are characterized by large, sharply terminated, and undamaged crystals with high transparency and strong luster. Specimens with an intense green color are particularly sought after. The aesthetics of the composition are very important – the attractiveness of a specimen is enhanced by a contrasting, undamaged matrix and a harmonious association with other minerals, such as orange or pink stilbites. Specimens with perfectly formed, single crystals are also highly valued. ## Market Prices Apophyllite prices have a very wide range. Small, single crystals or small fragments of druses can be purchased for a few to a dozen dollars. Medium-sized cabinet specimens (several centimeters) of good quality, from India, typically cost from 100 to 500 dollars. Exceptional, large plates with perfect, green crystals in association with stilbite can reach prices of several thousand dollars on the collector's market. ## Popular Localities By far the most desired specimens by collectors come from quarries in the Pune, Jalgaon, and Nashik regions of India. These localities provide classic specimens of green apophyllite on pink stilbite, which have become an icon of this mineral. Localities in Germany (St. Andreasberg, Harz) and Canada (Mont Saint-Hilaire, Quebec) are also historically significant, yielding specimens with somewhat different morphology.

Care and storage

## Cleaning Apophyllite specimens should be cleaned very carefully. It is best to use a soft brush to remove dust. If necessary, the specimen can be rinsed in distilled water, avoiding prolonged soaking. It should then be thoroughly dried, preferably with compressed air or by leaving it to air dry in a well-ventilated area. ## What to Avoid Apophyllite is sensitive to chemicals, especially acids, which can damage it. It is also sensitive to high temperatures, which cause dehydration and irreversible dulling or disintegration. Ultrasonic cleaners and steam cleaning should be avoided. Prolonged exposure to direct sunlight can cause some colored varieties to fade. ## Storage Due to its relatively low hardness and perfect cleavage, apophyllite is susceptible to scratches and mechanical damage. Specimens should be stored separately in padded boxes or display cases, away from harder minerals. Avoid storage in areas of high humidity or sudden temperature changes.

Sources

IMA Mineral List (CNMNC)