Phillipsite-Na

Chemical formula: Na<sub>6</sub>(Si<sub>10</sub>Al<sub>6</sub>)O<sub>32</sub>·12H<sub>2</sub>O

Phillipsite-Na is a hydrated sodium aluminosilicate from the zeolite group, forming characteristic cross-shaped twins and spherulitic aggregates.

## Characteristics Phillipsite-Na is a mineral from the zeolite group, belonging to the phillipsite series. It forms columnar or tabular crystals, which almost always occur as characteristic penetration twins, resembling crosses. It often also forms radial or spherulitic aggregates, and sometimes crusts and vesicle fillings in volcanic rocks. Crystals are typically small, rarely exceeding a few millimeters in length. ## Physical Properties This mineral is characterized by a hardness in the range of 4-4.5 on the Mohs scale. It is relatively light, with a density of about 2.2 g/cm³. The luster is vitreous, and the mineral can be transparent to translucent. It does not exhibit cleavage, and its fracture is uneven to conchoidal. ## Colors and Varieties Phillipsite-Na is most often colorless or white. Impurities can give it yellowish, grayish, or reddish hues. No named varieties are distinguished, and classification within the phillipsite series is based on the dominant cation (sodium, potassium, calcium). ## History and Name The name "phillipsite" was given in 1825 by Armand Lévy in honor of William Phillips (1775-1829), an English mineralogist and one of the founders of the Geological Society of London. The prefix "-Na" was added in 1997 by the International Mineralogical Association (IMA) as part of a reform of zeolite nomenclature, to indicate sodium as the dominant extra-framework cation in this particular mineral of the series. ## Uses Zeolites, including phillipsite, have potential applications as molecular sieves, catalysts, and in ion exchange processes, for example, for water purification. Phillipsite-Na specimens are primarily of scientific and collector's interest.

Properties

Mohs hardness
4-4.5
Luster
Vitreous
Streak
White
Density
2.2
Cleavage
Good on {010}, poor on {100}
Fracture
Uneven to conchoidal
Transparency
Transparent to translucent
Crystal system
Monoclinic

Diagnostic features

## Identification The most characteristic features of phillipsite-Na are its crystal forms – typical penetration twins, often cross-shaped, and radial aggregates. Its occurrence in amygdaloidal vesicles in volcanic rocks (e.g., basalts) is also a strong diagnostic indicator. It reacts with hydrochloric acid, forming a gel. ## Distinguishing from Similar Minerals It can be confused with other zeolites, such as harmotome (which contains barium and is significantly denser) or chabazite (which forms different crystal habits, often pseudo-cubic). Chemical analysis is often necessary for a definitive distinction from other members of the phillipsite series (phillipsite-K and phillipsite-Ca). ## Crystal Forms Crystals are prismatic, elongated along the a-axis. They almost always occur as penetration twins, forming cross-shaped or double-cross-shaped forms. They often form spherical, radial aggregates (spherulites) or fibrous masses.

Geological environment

## Genesis Phillipsite-Na is a low-temperature mineral. It forms primarily as a result of hydrothermal activity in vesicles and fractures in volcanic rocks, especially in basalts and phonolites. It also forms from the alteration of volcanic tuffs in saline alkaline lake environments and as an authigenic mineral in deep-sea sediments. ## Mineral Associations It often co-occurs with other zeolites, such as chabazite, analcime, natrolite, as well as with calcite and apophyllite. ## Localities Significant localities for phillipsite-Na are found in many places around the world. It occurs in basalts of the Columbia Plateau in Oregon (USA), near Vesuvius and in Sicily (Italy), in volcanic rocks of Auvergne (France), and in basaltic vesicles in County Antrim (Northern Ireland). It is also found in lake sediments, e.g., in Lake Tecopa in California (USA).

Rarity

Not very common

For collectors

## Quality Criteria The most valued by collectors are specimens with well-formed, sharp, and distinct twins, especially those forming aesthetic, cross-shaped forms. The size of the crystals and their placement on the rock matrix are also important. Transparent and colorless specimens are preferred, as are those forming attractive, spherical aggregates. ## Popular Localities Classic specimens come from localities in Italy, particularly from the Vesuvius area and Sicily (e.g., Aci Castello). High-quality spherulitic aggregates are found in basalts in Oregon, USA.

Care and storage

## Cleaning Specimens should be cleaned very carefully, using a soft brush to remove dust. If washing is necessary, distilled water can be used, avoiding prolonged soaking. The specimen should be allowed to dry completely at room temperature. ## What to Avoid Phillipsite-Na is sensitive to acids, which can damage it. Ultrasonic cleaners, chemical agents, and high temperatures should be avoided, as they can cause dehydration and destroy the mineral's structure. As a relatively soft mineral, it is susceptible to scratches. ## Storage Store in stable conditions, away from heat sources and direct sunlight. It is best kept in a closed box or display cabinet to protect delicate crystals from mechanical damage and dust.

Sources

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