Arrojadite-(BaNa)
Chemical formula: BaNa<sub>3</sub>(NaCa)Fe<sup>2+</sup><sub>13</sub>Al(PO<sub>4</sub>)<sub>11</sub>(PO<sub>3</sub>OH)(OH)<sub>2</sub>
Arrojadite-(BaNa) is an extremely rare, dark green to black mineral of the arrojadite group, being a complex phosphate.
Description
## Characteristics Arrojadite-(BaNa) is a mineral belonging to the arrojadite supergroup. It occurs as anhedral (irregular) masses and grains, embedded in the host rock. Typical specimens range in color from dark green, through greenish-black, to black. It is characterized by a vitreous luster, locally transitioning to greasy. ## Physical Properties The mineral is translucent. Its density is approximately 3.55 g/cm³. It exhibits good, unidirectional cleavage. Hardness has not been precisely determined for this variety, but for minerals of the arrojadite group, it is typically 4-5 on the Mohs scale. ## Colors and Varieties The basic color range is very narrow and includes shades of dark green and black. Arrojadite-(BaNa) itself is a specific variety within the group, defined by the dominance of barium and sodium in specific positions of the crystal structure. Other color varieties are not known. ## History and Name The mineral was formally described by Marie Louise Lindberg in 1954. The name refers to its belonging to the arrojadite group and to the dominant elements in its chemical composition – barium (Ba) and sodium (Na). The name of the parent group honors the Brazilian geologist Miguel Arrojado Ribeiro de Lisboa (1872-1932). ## Applications Arrojadite-(BaNa) has no industrial applications. Due to its extreme rarity, it is solely an object of scientific interest and is sought after by advanced collectors of systematic minerals.
Diagnostic features
## Identification Visual identification is practically impossible. Dark green or black color, greasy luster, and occurrence in granitic pegmatites are the only clues. Certain identification requires advanced analytical methods, such as X-ray diffraction (XRD) or chemical microanalysis (EDS/WDS), which can confirm the unique chemical composition with a dominance of barium and sodium. ## Distinguishing from Similar Minerals Arrojadite-(BaNa) is impossible to distinguish with the naked eye from other dark phosphate minerals occurring in pegmatites, such as other minerals from the arrojadite group, triphylite, as well as from more common minerals like black tourmaline (schorl) or some amphiboles. Chemical analysis is key. ## Crystal Forms This mineral does not form well-developed crystals. It occurs exclusively as irregular, anhedral masses and grains, usually of small size, embedded in other pegmatite minerals.
Geological environment
## Genesis Arrojadite-(BaNa) is a mineral of magmatic origin. It forms in the late stages of crystallization in complex, phosphate-rich granitic pegmatites. ## Mineral Associations At its type locality, this mineral co-occurs with triphylite, quartz, siderite, and goyazite. It is a product of alterations occurring in primary triphylite. ## Localities The only confirmed occurrence of arrojadite-(BaNa) in the world is its type locality – the Palermo No. 1 mine in North Groton, Grafton County, New Hampshire, USA.
Rarity
Extremely rare
Collector aspects
## Quality Criteria For such a rare mineral, often available only as micromounts, the main criterion for value is the mere presence of the mineral confirmed by analysis. Additional value comes from the richness of the specimen (amount of visible arrojadite) and aesthetic associations with other well-defined minerals from that locality. Shape or size are of secondary importance. ## Popular Localities The only source of specimens is the historic Palermo No. 1 mine in New Hampshire, USA. Material from this locality is highly prized by collectors specializing in pegmatite minerals and systematics.
Care and storage
## Cleaning Specimens should be cleaned very carefully, using a soft brush to remove dust. If necessary, distilled water can be used. Ultrasonic cleaners should be avoided, as they could damage the mineral's structure along cleavage planes. ## What to Avoid The mineral is sensitive to acids and strong chemicals. Avoid sudden temperature changes and prolonged exposure to direct sunlight, although there is no evidence of it fading. ## Storage It is recommended to store specimens in stable conditions, in closed boxes or display cases, to protect them from dust and mechanical damage. Due to its rarity, each specimen should be precisely labeled with its locality.