Molybdophyllite
Chemical formula: Pb²⁺₈Mg₉[Si₁₀O₃₀(OH)₈(CO₃)₃]·H₂O
Molybdophyllite is a rare lead and magnesium mineral, forming soft, platy aggregates with a pearly luster, resembling scales.
Properties
- Mohs hardness
- 3-4
- Color
- Colourless, pale grey, pale yellow, very pale green
- Luster
- Pearly
- Streak
- White
- Density
- 0
- Cleavage
- Parallel to the pseudohexagonal (0001) plane.
- Fracture
- Irregular/Uneven
- Transparency
- Transparent,Translucent
- Crystal system
- Monoclinic
Diagnostic features
## Identification Key diagnostic features of molybdophyllite include its exceptionally low hardness (2 on the Mohs scale), perfect cleavage in one direction, pearly luster, and flexible, but not elastic, lamellae. A characteristic slightly greasy or soapy feel to the touch is also typical. High density, unusual for a mineral of its appearance, is also an important clue. ## Distinguishing from Similar Minerals Molybdophyllite can be confused with talc, pyrophyllite, or micas (e.g., muscovite). It is distinguished from talc and pyrophyllite by its significantly higher density (over 4.7 g/cm³ compared to approximately 2.7-2.8 g/cm³). It differs from micas in that its lamellae are pliable but not elastic (they do not return to their original shape after bending). ## Crystal Forms Crystals are rare, usually tabular or platy with a hexagonal outline. Most often, it occurs in the form of scaly, platy, rosette-like, or radial aggregates.
Geological environment
## Genesis Molybdophyllite is a metamorphic mineral. It forms as a result of contact metamorphism in skarns and thermally altered carbonate rocks (marbles), rich in manganese, lead, and magnesium. ## Mineral Associations It often co-occurs with other minerals typical of skarns and metamorphic deposits, such as calcite, magnetite, serpentine, phlogopite, tephroite, barite, and especially with other rare lead and manganese minerals, such as barysilite, nasonite, or hedyphane. ## Localities The most important and classic localities for this mineral are the skarn deposits in Sweden, primarily Långban and Harstigen in the Värmland region. It is also known from the Franklin Mine in New Jersey, USA, which is famous for its rare minerals. Other confirmed occurrences include the Jakobsberg mine in Sweden and small occurrences in Norway.
Rarity
Rare
For collectors
## Quality Criteria The most valued specimens by collectors are those with well-formed, distinct platy or rosette-like aggregates, ideally contrasting with the rock matrix. The size of the aggregates and the absence of mechanical damage to the delicate lamellae are important. Specimens from classic, historical localities, such as Långban or Franklin, are particularly sought after. ## Popular Localities By far the most desirable specimens come from two world-famous localities: Långban in Sweden and Franklin in New Jersey, USA. These are type localities, from which the best-formed and largest known crystals and aggregates of molybdophyllite originate.
Care and storage
## Cleaning Molybdophyllite specimens are extremely delicate and soft. Mechanical cleaning should be avoided. If necessary, compressed air can be used to remove dust. Contact with water is not recommended. ## What to Avoid Avoid contact with acids and other chemicals that can damage it. Due to its low hardness, it is very susceptible to scratching and mechanical damage. It should be protected from pressure and impacts. As a lead-containing mineral, avoid inhaling dust and wash hands after handling specimens. ## Storage Specimens should be stored in separate, padded boxes to prevent abrasion and damage to the delicate lamellae. It should not be displayed in a location where it might be exposed to vibrations or accidental touching.