Marinellite

Chemical formula: Na₄₂Ca₆Al₃₆Si₃₆O₁₄₄(S₆+O₄)₈Cl₂·₆H₂O

Marinellite is a very rare mineral from the cancrinite group, characterized by hexagonal crystals of a pale blue color.

## Characteristics Marinellite is a complex tectosilicate of sodium, calcium, and aluminum, belonging to the cancrinite group. It forms hexagonal, prismatic crystals, which may be terminated by pyramids. The most commonly encountered specimens are colorless, pale blue, or azure blue. The mineral is characterized by a vitreous luster and is transparent to translucent. Due to its rarity and attractive appearance, it is a prized collector's stone. ## Physical Properties Its Mohs hardness is approximately 5, making it a mineral with moderate scratch resistance. It has a density of 2.45 g/cm³, so it is relatively light. It possesses excellent cleavage in one direction, meaning it can easily break along specific planes under impact. ## Colors and Varieties The color spectrum of marinellite is limited to shades of blue and colorless. The intensity of the color can vary, from very light, almost white tones, to more saturated blue. No commercial names or color varieties are distinguished for it. ## History and Name The mineral was described in 1994 by Paolo Orlandi, Marco Pasero, and Giovanni Vezzalini. Its name commemorates Giorgio Marinelli (1922–1993), a professor of volcanology at the University of Pisa in Italy, for his contributions to the study of Italian volcanoes. The type locality, from which the first described specimens originated, is Pitigliano in Tuscany. ## Applications Marinellite has no industrial applications. Its significance is purely scientific and as a collector's item.

Properties

Mohs hardness: 5
Luster: Vitreous
Streak: White
Density: 2.45
Cleavage: Perfect on {10-10}
Fracture: Conchoidal to uneven
Transparency: Transparent to translucent
Crystal system: Hexagonal

Diagnostic features

## Identification The key diagnostic feature of marinellite is its hexagonal crystal habit, pale blue color, and vitreous luster. Its occurrence environment – volcanic bombs and lavas – is also characteristic. Reaction to ultraviolet light can be helpful, although it can be variable. ## Distinguishing from Similar Minerals Marinellite is sometimes confused with other blue minerals from the sodalite and cancrinite groups, such as afghanite or haüyne. Afghanite often exhibits stronger orange fluorescence under UV light. Haüyne typically forms dodecahedral crystals and has a more intense, azure color. Certain distinction of these minerals often requires advanced analytical methods, such as X-ray powder diffractometry (XRD). ## Crystal Forms Marinellite crystallizes in the form of elongated or short prismatic columns with a hexagonal cross-section. Crystals may be terminated by flat faces or hexagonal pyramid faces. It occurs as single, well-formed crystals or in small, radial aggregates.

Geological environment

## Genesis Marinellite is a mineral of volcanic origin. It forms under high temperature and low pressure conditions inside volcanic bombs (so-called sanidinites) ejected during eruptions. It is a product of contact metasomatism within pyroclastic rocks. ## Mineral Associations This mineral co-occurs with other minerals typical of the sanidinite environment. It is most often accompanied by sanidine, andradite, diopside, haüyne, titanite, and rare borates such as vonsenite. ## Localities Marinellite is an extremely rare mineral, known from only a few localities worldwide. The most important occurrences are the type locality Pitigliano in Tuscany, Italy, and the Eifel volcanic complex in Germany (e.g., in the Emmelberg and Niedermendig areas).

Rarity

Very rare

For collectors

## Quality Criteria The most highly valued marinellite specimens are those with well-formed, sharply terminated crystals of the greatest possible transparency and intense blue color. Crystal size is also important – specimens exceeding a few millimeters are rare. Attractiveness is enhanced by contrasting placement on a light rock matrix, most often sanidine. ## Popular Localities The best specimens in terms of quality and crystal size come from two main regions: the type locality in Pitigliano, Italy, and the Eifel volcanoes in Germany. German specimens are often found in the collector's market and are valued for their good crystal form.

Care and storage

## Cleaning Marinellite specimens should be cleaned very carefully, preferably with a soft brush dry to remove dust. In case of heavier soiling, compressed air can be used or the specimen can be very gently rinsed in distilled water, and then immediately dried. ## What to Avoid The mineral is sensitive to shocks and impacts due to its excellent cleavage. Ultrasonic cleaners, steam cleaning, and contact with acids and strong detergents should be avoided. It should not be subjected to sudden temperature changes. ## Storage It is recommended to store specimens in separate, padded boxes or in display cases, away from harder minerals that could scratch it. It should be protected from dust and potential mechanical damage.

Sources

IMA Mineral List (CNMNC)