Dachiardite-Ca

Chemical formula: Ca<sub>2</sub>(Si<sub>20</sub>Al<sub>4</sub>)O<sub>48</sub>·13H<sub>2</sub>O

Dachiardite-Ca is a rare mineral from the zeolite group, forming colorless or white, acicular crystals in cavities of volcanic rocks.

## Characteristics Dachiardite-Ca is a hydrated calcium aluminosilicate, belonging to the zeolite group. It typically forms fine, acicular or fibrous crystals aggregated into radial clusters, spherical masses, or bundles. Individual crystals are often too small to be seen with the naked eye, and the aggregates resemble tufts of cotton or felt. Less commonly, it occurs as small, tabular crystals with a nearly square cross-section. ## Physical Properties This mineral is characterized by a Mohs hardness ranging from 4 to 4.5. It has a vitreous luster, and sometimes a pearly luster on fracture surfaces. It is transparent to translucent. Its density is approximately 2.14-2.21 g/cm³. ## Colors and Varieties Dachiardite-Ca is most commonly colorless or white. No colored or commercial varieties are distinguished. ## History and Name The name "dachiardite" was given in 1906 by the Italian mineralogist Giovanni D'Achiardi in honor of his father, Antonio D'Achiardi, also a mineralogist, who first studied this mineral. The suffix "-Ca" was added later when its sodium analogue (dachiardite-Na) was discovered, to distinguish members of the series based on the dominant cation.

Properties

Mohs hardness
4-4.5
Luster
Vitreous, Pearly
Streak
White
Density
2.14-2.21
Cleavage
Perfect on {100}, Good on {001}
Fracture
Uneven
Transparency
Transparent to translucent
Crystal system
Monoclinic

Diagnostic features

## Identification A characteristic feature of dachiardite-Ca is its mode of occurrence – radial or spherical aggregates composed of very fine, acicular or fibrous crystals. It usually occurs in vesicles and cavities in volcanic rocks. It reacts with hydrochloric acid. ## Distinguishing from Similar Minerals Dachiardite-Ca can be confused with other acicular zeolites, such as mesolite, natrolite, or scolecite. Differentiation often requires advanced analytical methods, such as X-ray diffraction (XRD). From mordenite, with which it often co-occurs, it differs in crystal form – mordenite forms more "fibrous" or "cotton-like" aggregates.

Geological environment

## Genesis Dachiardite-Ca is a low-temperature hydrothermal mineral. It forms as a result of post-magmatic water activity, filling gas vesicles and fissures in acidic and basic volcanic rocks, such as basalts, andesites, and rhyolites. It is also found in Alpine-type veins in gneisses and schists. ## Mineral Associations It most commonly co-occurs with other zeolites, such as mordenite, clinoptilolite, ferrierite, heulandite, and stilbite. It is also accompanied by quartz (including chalcedony) and calcite. ## Localities Key localities worldwide include the Val di Fassa in Trentino (Italy), where it was discovered; the area around Tanzenberg in Styria (Austria); Niigata Prefecture (Japan); Iceland (e.g., near Teigarhorn); and the states of Oregon, Washington, and Arizona in the USA.

Rarity

Rare

For collectors

## Quality Criteria Most valued by collectors are specimens with well-formed, distinct radial or spherical aggregates, composed of clear, colorless or snow-white crystals. Contrast with the dark host rock (matrix) on which they grew is important. Large, undamaged spherulites (spherical aggregates) command higher prices. Specimens from classic, historic localities, such as those in Italy, are also more sought after. ## Popular Localities Among the most famous localities from which collector specimens originate are Italy (Val di Fassa), Austria (Styria), Iceland, and the USA (especially Oregon).

Care and storage

## Cleaning Specimens should be cleaned only with cold, distilled water and a very soft brush, removing dust from delicate acicular crystals. Ultrasonic cleaners should be avoided, as they can damage fragile aggregates. ## What to Avoid As a zeolite, dachiardite-Ca is sensitive to high temperatures, which can cause water loss from its structure and destroy the mineral. Contact with acids and other chemicals should be avoided. It should not be heated or stored in low humidity conditions, which can lead to its dehydration. ## Storage Dachiardite-Ca specimens are very brittle and susceptible to mechanical damage. They are best stored in closed, padded boxes or display cases, away from dust, direct sunlight, and heat sources.

Sources

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