Johnbaumite
Chemical formula: Ca<sub>5</sub>(As<sup>5+</sup>O<sub>4</sub>)<sub>3</sub>OH
Johnbaumite is a rare arsenate analog of apatite, forming colorless to white, hexagonal crystals, prized by collectors of rare minerals.
Properties
- Mohs hardness
- 5
- Luster
- Vitreous to resinous
- Streak
- White
- Density
- 3.73
- Cleavage
- Indistinct on {0001}
- Fracture
- Conchoidal to uneven
- Transparency
- Transparent to translucent
- Crystal system
- Hexagonal
Diagnostic features
## Identification The key diagnostic feature of johnbaumite is its hexagonal, prismatic crystal habit, typical of the apatite group. Vitreous luster, color (colorless to white), and occurrence in association with minerals from the Franklin deposit are strong indicators. However, final confirmation requires advanced chemical analysis (EDS) to confirm the dominance of arsenic over phosphorus. ## Distinguishing from Similar Minerals Johnbaumite is visually indistinguishable from other colorless or white minerals of the apatite group, such as hydroxylapatite or fluorapatite, without chemical tests. It can be confused with some forms of aragonite or calcite, but it differs from them by its lack of reaction with hydrochloric acid and higher hardness. ## Crystal Forms Crystals are hexagonal, usually prismatic in habit, elongated along the crystallographic c-axis. They are often terminated by flat pinacoids. It also occurs in the form of granular masses, radial aggregates, or fibrous masses.
Geological environment
## Genesis Johnbaumite is a mineral of metamorphic origin. It forms as a result of contact metamorphism of mineralized zinc, manganese, and iron deposits in carbonate rocks. It occurs in calcitic veins cutting franklinite-willemite ores. ## Mineral Associations This mineral co-occurs with many other rare minerals characteristic of Franklin-type localities. It is most often accompanied by andradite, franklinite, willemite, hydroxylapatite, rhodochrosite, calcite, barite, and other rare arsenates such as allactite or sterlinghillite. ## Localities The most important and classic occurrence of johnbaumite in the world is the Franklin and Sterling Hill deposits in Sussex County, New Jersey, USA. The best and largest crystals originate from there. Its occurrence has also been confirmed in the Långban mine in Sweden, the Kombat deposit in Namibia, and several locations in Japan, but specimens from these localities are much rarer and less impressive.
Rarity
Rare
For collectors
## Quality Criteria The most prized johnbaumite specimens are those with well-formed, sharp, vitreous, and transparent hexagonal crystals. Specimens where single, colorless crystals contrast with a colorful matrix, e.g., with pink rhodochrosite or black franklinite, are highly valued. Crystal size is crucial – specimens with crystals exceeding one centimeter in length are exceptionally rare and sought after. Rich radial aggregates are also attractive. ## Popular Localities Franklin, New Jersey, USA, is by far the most desirable and well-known locality for collectors. Specimens from Franklin are considered the global standard for this mineral.
Care and storage
## Cleaning Johnbaumite 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. It should be dried immediately and thoroughly. ## What to Avoid As an arsenate, it is toxic if ingested or if dust is inhaled – avoid creating dust and wash hands after contact with the mineral. Do not clean it with acids or other aggressive chemicals. It is sensitive to high temperatures. Ultrasonic cleaners should be avoided. ## Storage Store in stable conditions, away from sources of moisture and direct sunlight. It is best kept in a closed display box or cabinet to protect brittle crystals from mechanical damage and dust. Due to its toxicity, it should be stored out of reach of children and pets.