ARROJADITE
Arrojadites -KNa4CaMn++4Fe++10Al(PO4)12(OH,F)2 – are very rare phosphates typical of granitic pegmatites. Due to their rarity, they are not considered ore, despite containing 40% P2O5 .
“Arrojadite” is not actually a single mineral; it is a group subdivided into four subgroups, comprising a total of 16 mineral species. The most common is arrojadite-(KFe). The group, redefined in 2006 (Chopin et al., 2006, Am. Min. 91:1260-1270), forms a series with dickinsonite.
Their crystal structures are “fiendishly complex” (Huminicki & Hawthorne, 2002, p. 186), constitute an “enigma” (Chopin et al., 2006, Am. Min. 91:1249-1259), and have undergone a “tumultuous investigative history” (Moore et al. 1981, Am. Min. 66:1034-1049). They are very weakly radioactive.
Due to their olive-green to grass-green colors, arrojadites are frequently confused with other phosphates typical of granitic pegmatites – such as triphylite and lithiophilite – which are far more common.
1. Characteristics
Crystal system: Monoclinic domatic.
Color: Olive green, bottle green, yellowish-green, brown, yellow.
Habit: Arrojadites form crystals up to 15 cm in length.
Cleavage: {100} good, {102} poor, (conflicting information)
Tenacity: No information available.
Twinning: No information available.
Fracture: Irregular to conchoidal.
Mohs Hardness: 5
Parting: No.
Streak: Greenish gray.
Lustre: Vitreous to greasy.
Diaphaneity: Transparent.
Density (g/cm³): 3.56
2. Geology and Deposits
Arrojadites are typical of granitic pegmatites. They are high-temperature (~800°C) primary minerals that occur exclusively in these granitic pegmatites.
3. Mineral Associations
Arrojadites are associated with minerals typical of granitic pegmatites, such as quartz, feldspars (albite, orthoclase, microcline), micas (biotite, muscovite, lepidolite), spodumene, beryl, tourmaline, and cassiterite.
They occur with various other phosphates, such as fluorapatite, triphylite, lithiophilite, graftonite, lazulite, gordonite, wardite, childrenite, fairfieldite, nahpoite, vivianite, kulanite, and whitlockite.
4. Transmitted Light Microscopy
The characteristics presented below are those of arrojadite-(KFe), which is the most common variety.
Refraction indices: nα: 1.664 nβ: 1.670 nγ: 1.675
PLANE POLARIZED LIGHT – PPL
Color / Pleochroism: Pleochroic between yellow-green and grass-green.
Relief: High.
Cleavage: {100} good, {102} distinct.
Habits: No information available.
CROSSED POLARIZED LIGHT – XPL
Birefringence and Interference Colors: birefringence of 0.011: colors ranging from gray-white to straw-yellow of the first order; does not reach orange. Likely masked by the mineral’s strong own colors.
Extinction: Probably oblique.
Elongation sign: No information available.
Twins: No.
Zoning: No.
CONVERGENT LIGHT
Character: B(+).
2V angle: 86º
Alterations: No information available.
May be confused with: other minerals with green pleochroism.
5. Reflected Light Microscopy
Few databases cover arrojadite, and none do so in the desired level of detail.
As these are transparent minerals, reflected-light microscopy is not the appropriate analytical technique for their identification. They likely exhibit dark gray colors and low reflectivity under plane-polarized light (PPL), and widespread internal reflections in olive-green tones under cross-polarized light (XPL).