Phillipsite-K

Chemical formula: K₆(Si₁₀Al₆)O₃₂·₁₂H₂O

A hydrated potassium aluminosilicate from the zeolite group, forming characteristic cross-shaped twins and spherulitic aggregates.

## Characteristics Phillipsite-K is a mineral from the zeolite group, a hydrated potassium aluminosilicate. It most commonly occurs as well-formed, prismatic crystals, which often form characteristic penetration twins resembling crosses. It also forms radial or spherulitic aggregates, as well as crusts and vesicle fillings in volcanic rocks. Crystals are usually small but can reach several centimeters. ## Physical Properties This mineral is characterized by a hardness of 4-5 on the Mohs scale. It has a vitreous luster and is typically translucent to opaque. It is relatively light, which is typical for zeolites. ## Colors and Varieties Phillipsite-K is most often colorless or white. Impurities can give it yellowish, grayish, or reddish hues. There are no named color varieties or commercial varieties. ## History and Name The name "phillipsite" was given in 1825 by Armand Lévy in honor of the English mineralogist William Phillips (1775-1829). The suffix "-K" was added in 1997 by the International Mineralogical Association (IMA) as part of a revision of zeolite nomenclature to indicate potassium (K) as the dominant extra-framework cation in this particular mineral variety. The type locality for phillipsite (though not necessarily for the potassium-dominant variety) is Aci Castello in Sicily, Italy. ## Applications Phillipsite-K, like other zeolites, has potential applications as a molecular sieve, ion exchanger, and catalyst due to its porous crystal structure. However, it is mainly of scientific and collector's interest.

Properties

Mohs hardness: 4-5
Luster: Vitreous
Streak: White
Density: 0
Transparency: Translucent to opaque
Crystal system: Monoclinic

Diagnostic features

## Identification The most characteristic feature of phillipsite-K is its crystal forms, especially the typical penetration twins forming cross-shaped forms or complex, spherical aggregates with a radial internal structure. Occurrence in amygdaloidal vesicles in volcanic rocks is also a strong diagnostic indicator. ## Distinguishing from Similar Minerals Phillipsite-K can be confused with other zeolites, such as chabazite, harmotome, or wellsite-Ca. Harmotome often forms very similar cross-shaped twins but contains barium as the dominant cation. Chabazite forms crystals with a habit similar to a cube (pseudorhombohedra). Accurate differentiation from other members of the phillipsite group often requires chemical analysis (EDS). ## Crystal Forms Crystals have a prismatic habit, often elongated. They commonly form penetration twins, resulting in characteristic cross-shaped forms. It also occurs as radial and spherulitic aggregates, spherical clusters, and as crusts filling rock cavities.

Geological environment

## Genesis Phillipsite-K is a low-temperature hydrothermal mineral. It forms mainly in vesicles and fractures in volcanic rocks, such as basalts, phonolites, and tephrites, as a result of post-magmatic solutions or circulating waters. It can also crystallize in deep-sea sediments and as a result of the alteration of volcanic tuffs in alkaline saline lakes. ## Mineral Associations It often co-occurs with other zeolites, such as chabazite, analcime, natrolite, as well as with calcite and apophyllite. In volcanic rocks, it is accompanied by rock-forming minerals such as nepheline or leucite. ## Localities Significant specimens of phillipsite-K come from various localities around the world. In Italy, it occurs in the Vesuvius region (Campania) and Capo di Bove (Lazio), which is a type locality. It is also known from basalts in Richmond, Australia (New South Wales), from County Antrim in Northern Ireland, from the Kaiserstuhl area in Germany, and from many places in the Czech Republic (e.g., around Ústí nad Labem).

Rarity

Not very common

For collectors

## Quality Criteria The most valued by collectors are specimens with well-formed, sharp, and distinct cross-shaped twins. Large, spherical aggregates with a silky luster are also highly prized. The aesthetics of the specimen are important – an attractive arrangement of crystals on the rock matrix and the absence of damage. Color contrast with a dark host rock enhances visual appeal. ## Popular Localities Classic and most sought-after specimens come from Italian localities, especially from the Vesuvius region and Rome (Capo di Bove), which have provided historical, well-crystallized specimens. Specimens from Richmond, Australia, and from basalts in Northern Ireland are also valued in specialized collections.

Care and storage

## Cleaning Specimens should be cleaned very carefully, using a soft brush to remove dust. If necessary, distilled water can be used, but prolonged soaking should be avoided. The specimen should be allowed to dry completely at room temperature. ## What to Avoid Avoid contact with acids and other aggressive chemicals that can damage the mineral's structure. Phillipsite-K is sensitive to high temperatures, which can cause dehydration and destruction. It should not be heated or cleaned with ultrasonic cleaners. Protect it from sudden temperature changes. ## Storage Phillipsite-K specimens are brittle and relatively soft, so they should be stored in separate, padded boxes to prevent scratches and mechanical damage. They should be protected from dust and stored in stable humidity conditions.

External references

https://www.mindat.org/min-7228.html