Phillipsite-Ca

Chemical formula: Ca₃(Si₁₀Al₆)O₃₂·12H₂O

Phillipsite-Ca is a hydrated calcium aluminosilicate from the zeolite group, forming characteristic cross-shaped crystal twins.

## Characteristics Phillipsite-Ca is a mineral from the zeolite group, belonging to the phillipsite series. It forms prismatic crystals, which are very often twinned in the form of cross-shaped or stellate intergrowths, which is their most recognizable feature. It also occurs as radial or spherical aggregates. It is a brittle mineral. ## Physical Properties Phillipsite-Ca crystals are transparent and exhibit a vitreous luster. The Mohs hardness is 4-5, and the density is approximately 2.2 g/cm³. This mineral shows distinct cleavage in one plane and indistinct cleavage in another. ## Colors and Varieties Phillipsite-Ca is most often colorless or white. It can also take on pinkish, reddish-white, or pale yellow hues, which is usually caused by the presence of impurities. ## History and Name The name "phillipsite" was given in 1825 by Armand Lévy in honor of the English mineralogist William Phillips. The suffix "-Ca" refers to the dominance of calcium (*calcium*) in its chemical composition. The type locality for phillipsite-Ca is in volcanic tuffs on Oʻahu Island, Hawaii, USA. ## Applications Phillipsite-Ca, like other zeolites, has potential applications as a molecular sieve, catalyst, or in ion exchange processes. It is also valued by mineral collectors for its unique crystal forms.

Properties

Mohs hardness
4-5
Luster
Vitreous
Streak
White
Density
2.2
Cleavage
Distinct on the {010}, indistinct on the {001}
Fracture
Irregular/Uneven
Transparency
Transparent
Crystal system
Monoclinic

Diagnostic features

## Identification The most important diagnostic feature of phillipsite-Ca is its characteristic penetration twinning, forming cross-like or star-like shapes. It is also recognized by its occurrence in gas vesicles in volcanic rocks, low hardness, and vitreous luster. ## Distinguishing from Similar Minerals Phillipsite-Ca can be confused with other zeolites, such as harmotome or chabazite. It is distinguished from harmotome (the barium variety) by the absence of barium in its composition (requires chemical analysis). It differs from chabazite in crystal form – chabazite forms crystals similar to cubes (rhombohedral), while phillipsite-Ca forms twinned prismatic crystals. ## Crystal Forms Crystals have a prismatic, elongated habit. They almost always occur as twins, forming complex groups with pseudo-tetragonal or pseudo-orthorhombic symmetry. Aggregates often take the form of radial rosettes, spherulites, or fibrous masses filling voids in the rock.

Geological environment

## Genesis Phillipsite-Ca is a low-temperature hydrothermal mineral. It primarily forms in vesicles and fissures in volcanic rocks, such as basalts, phonolites, and tuffs. It crystallizes from aqueous solutions in the final stages of lava cooling. It can also form as a result of the alteration of volcanic ash 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 specimens of phillipsite-Ca come from various locations worldwide. Besides the type locality in Hawaii (USA), it occurs, among others, in Aci Castello in Sicily (Italy), County Antrim (Northern Ireland), Iceland, around Vesuvius (Italy), and Richmond in Victoria (Australia).

Rarity

Not very common

For collectors

## Quality Criteria The most valued by collectors are specimens with well-formed, sharp, and distinct cross-shaped or stellate twins. The size of the crystals and their transparency are also important. Specimens where phillipsite-Ca forms aesthetic groups on a contrasting rock matrix are particularly sought after. Purity (lack of damage) and aesthetic composition significantly increase the value of the specimen. ## Popular Localities Classic and highly prized specimens come from basalts in Aci Castello, Sicily, Italy, and from County Antrim, Northern Ireland. Specimens from Vesuvius are also highly regarded by collectors.

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. Allow the specimen to dry completely at room temperature. ## What to Avoid Phillipsite-Ca is sensitive to acids, which can damage it. Avoid sudden temperature changes and exposure to strong chemicals. As a hydrated mineral, it can dehydrate (lose water) in overly dry or hot environments, leading to its destruction. ## Storage Specimens should be stored in stable conditions, away from direct sunlight and heat sources. It is best to keep them in closed boxes or display cases to protect them from dust and mechanical damage. Due to its brittleness, contact with harder minerals should be avoided.

External references

Sources

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