Celadonite
Chemical formula: KMgFe<sup>3+</sup>Si<sub>4</sub>O<sub>10</sub>(OH)<sub>2</sub>
Celadonite is a mica group mineral characterized by its typical green color, occurring as fillings and coatings in volcanic rocks.
Properties
- Mohs hardness
- 2
- Luster
- Earthy
- Streak
- Greenish
- Density
- 2.96 - 3.01
- Cleavage
- Perfect on {001}
- Fracture
- Uneven
- Transparency
- Translucent to opaque
- Crystal system
- Monoclinic
Diagnostic features
## Identification Celadonite is most easily identified by its characteristic green color, earthy or compact form of occurrence, and geological context – most often as vesicle fillings in volcanic rocks. It is soft to the touch and often stains fingers green, similar to graphite. ## Distinguishing from Similar Minerals Celadonite is visually almost identical to glauconite, and distinguishing them without advanced analysis (e.g., X-ray diffraction) is often impossible. Generally, celadonite forms in hydrothermal environments in terrestrial volcanic rocks, while glauconite is a typical authigenic mineral formed in marine sedimentary rocks. It can also be confused with other green secondary minerals, such as chlorites, but celadonite usually has a more bluish or grayish hue. ## Crystal Forms Well-formed crystals are extremely rare. Celadonite forms almost exclusively microcrystalline, earthy, compact, or scaly aggregates. It often occurs in the form of spherical aggregates (spherulites) or as concentric layers filling empty spaces in the rock.
Geological environment
## Genesis Celadonite is a secondary mineral, formed as a result of low-temperature hydrothermal processes or metasomatism. It typically forms during the alteration of volcanic rocks, especially basalts and andesites. It fills gas vesicles (amygdules), fractures, and replaces primary minerals such as pyroxenes, amphiboles, or olivines. ## Mineral Associations Celadonite often co-occurs with minerals from the zeolite group (e.g., heulandite, stilbite), calcite, quartz (including chalcedony and agate), and prehnite. In rocks where it occurs, other secondary minerals such as montmorillonite or other clay minerals may also be found. ## Localities Significant occurrences of celadonite are known from many places around the world. Classic localities include the Deccan Traps in India (around Pune and Mumbai), where it fills amygdules in vast basaltic flows. It also occurs in Cyprus, in the Troodos Mountains, in The Cedars area of Sonoma County, California (USA), in the Faroe Islands, and in basaltic massifs in Brazil (states of Rio Grande do Sul and Santa Catarina).
Rarity
Not very common
For collectors
## Quality Criteria The most prized specimens by collectors are those in which celadonite forms intensely colored, well-defined amygdule fillings in a contrasting host rock, most often white or gray basalt. Specimens where celadonite forms a substrate or inclusions in transparent crystals of other minerals, such as zeolites (heulandite) or quartz, creating spectacular "phantom" growths, are also attractive. Color contrast, aesthetic composition, and the size and quantity of amygdules in a single specimen significantly influence its value. ## Popular Localities The most famous and sought-after celadonite specimens, especially those co-occurring with zeolites, come from India, from quarries around Pune, Nashik, and Mumbai in Maharashtra state. Specimens from Brazil are also highly valued in the collector's market.
Care and storage
## Cleaning Celadonite specimens should be cleaned very carefully. It is best to use compressed air to remove dust. If wet cleaning is necessary, a soft brush moistened with distilled water can be used, avoiding soaking the host rock, which may be porous. Care should be taken as celadonite aggregates are soft and can abrade. ## What to Avoid Avoid ultrasonic and steam cleaners, which can damage delicate aggregates. Also, avoid contact with acids and strong chemicals. Although celadonite is stable in light, prolonged exposure to moisture can be harmful, especially if the mineral occurs on porous host rock. ## Storage Specimens containing celadonite are best stored in enclosed display cases or boxes to protect them from dust and mechanical damage. Due to its low hardness, it should not be stored in contact with harder minerals that could scratch it.