Hydroxyapophyllite-(K)

Chemical formula: KCa₄Si₈O₂₀(OH,F)·₈H₂O

A silicate from the apophyllite group, forming vitreous, colorless or white crystals, often found in geodes and vugs in volcanic rocks.

## Characteristics Hydroxyapophyllite-(K), with the formula KCa₄Si₈O₂₀(OH,F)·8H₂O, is a hydrated potassium-calcium silicate belonging to the apophyllite group. It is the most common mineral in this group. It forms characteristic, well-developed crystals with a tabular or prismatic habit, often terminated by pyramids, giving them a cube-like appearance. The crystal surfaces typically have a vitreous luster, with the exception of the crystal base (cleavage plane), which exhibits a distinct pearly luster. This mineral is usually colorless or white, less commonly taking on delicate shades of green, pink, or yellow. ## Physical Properties The Mohs hardness of hydroxyapophyllite-(K) is 4.5-5, making it a relatively soft mineral. Its density ranges from 2.3 to 2.4 g/cm³. It is transparent to translucent. The most characteristic physical feature is its perfect cleavage in one plane, which causes the mineral to easily split into thin lamellae with a pearly luster. ## Colors and Varieties The dominant colors are white and colorless. Rarer specimens exhibit pastel shades of greenish, pinkish, or creamy-yellow, which is usually caused by the presence of trace amounts of other elements. In the collector's market, the name "apophyllite" is often used as a general term for all minerals in this group, without precise differentiation into hydroxyapophyllite, fluoroapophyllite, or natroapophyllite. ## History and Name The name "apophyllite" was given by René Just Haüy in 1806 and comes from the Greek words *apo* (ἀπό, "from" or "away") and *phyllon* (φύλλον, "leaf"), referring to its property of exfoliating when heated. The modern name "hydroxyapophyllite-(K)" was sanctioned by the International Mineralogical Association (IMA) to precisely define its chemical composition – the "hydroxy" member indicates the dominance of hydroxyl (OH) groups over fluorine (F), and the "-(K)" suffix indicates the dominance of potassium (K) in the appropriate position in the crystal structure. ## Uses Hydroxyapophyllite-(K) has no significant industrial applications. However, it is a highly valued and popular collector's mineral due to its beautifully formed crystals and attractive luster.

Properties

Mohs hardness: 4.5-5
Luster: Vitreous, Pearly
Streak: White
Density: 2.3-2.4
Cleavage: Perfect on {001}, good on {110}
Fracture: Uneven
Transparency: Transparent to Translucent
Crystal system: Tetragonal

Diagnostic features

## Identification Key diagnostic features are: characteristic crystal habit (pseudo-cubic, tabular), the combination of vitreous luster on the side faces with pearly luster on the crystal base, and perfect cleavage in one plane. Hardness (4.5-5) is lower than quartz. Occurrence in vugs in volcanic rocks in association with zeolites is also a strong indicator. ## Distinguishing from Similar Minerals Hydroxyapophyllite-(K) is sometimes confused with quartz, which is much harder (7 on the Mohs scale) and does not exhibit cleavage. It is distinguished from calcite by its lack of reaction with hydrochloric acid and a different crystal habit. Some zeolites, like analcime, may have a similar appearance, but they crystallize in a different system (isometric) and rarely exhibit such distinct cleavage with a pearly luster. ## Crystal Forms Crystals most often take the form of short, thick prisms or tablets, often with a square cross-section. Combinations of tetragonal prism and tetragonal pyramid are typical, giving the effect of "truncated corners" and making the crystals resemble cubes. It occurs as single crystals, but more often forms crystal druses and linings inside geodes and rock fissures.

Geological environment

## Genesis This is a hydrothermal mineral, crystallizing at low temperatures. It forms as a secondary mineral, filling empty spaces (vugs, fissures, faults) in volcanic rocks such as basalts, andesites, or phonolites. It is less commonly found in hydrothermal veins and in contact metamorphic zones (skarns). ## Mineral Associations It most often co-occurs with a variety of minerals from the zeolite group, especially stilbite, heulandite, scolecite, and laumontite. It is also often accompanied by calcite, prehnite, datolite, babingtonite, and various varieties of chalcedony and quartz. ## Localities The most famous localities in the world, providing the best specimens, are in India, in the Deccan Traps region, particularly in the Pune, Nashik, and Jalgaon districts. Other important localities include the Harz Mountains in Germany, the Faroe Islands, Iceland, as well as numerous localities in the USA (New Jersey, Oregon, Virginia), Canada (Quebec), Brazil (Rio Grande do Sul), and Mexico (Guanajuato).

Rarity

Not very common

For collectors

## Quality Criteria The most highly valued specimens are those with large, fully transparent, well-formed crystals with strong luster and no damage. Compositions where colorless apophyllite crystals contrast with colorful associated minerals, such as orange stilbite, are particularly sought after. Rarer forms, such as greenish crystals or spherical aggregates with a radial internal structure, are also attractive. Clarity (lack of inclusions) and geometric perfection of the crystal significantly increase its value. ## Popular Localities Localities in India (Pune, Jalgaon) are considered classic and provide exemplary specimens. Crystals from there, often in combination with zeolites, set the global standard of quality for this mineral. Specimens from historical localities, e.g., from St. Andreasberg in the Harz Mountains (Germany), are also valued by collectors.

Care and storage

## Cleaning Specimens should be cleaned gently, using a soft brush and distilled water. Lukewarm water with a small amount of mild soap can be used, followed by thorough rinsing. Ultrasonic cleaners should be avoided, as they can cause the crystals to delaminate along cleavage planes. ## What to Avoid The mineral is relatively soft and brittle, so it should be protected from impacts and scratches. It is sensitive to high temperatures – heating leads to the loss of crystallization water, causing it to become opaque and disintegrate. Contact with strong acids and detergents, which can damage the crystal surface, should be avoided. ## Storage It is recommended to store specimens in enclosed display cases or boxes to protect them from dust and mechanical damage. Exposure to direct sunlight and sudden temperature changes, which can lead to dehydration, should be avoided.

Sources

IMA Mineral List (CNMNC)