| Alluaudite | |
| Category: | Phosphate minerals |
| Imasymbol: | Ald[1] |
| Strunz: | 8.AC.10 |
| Dana: | 38.2.3.6 |
| System: | Monoclinic |
| Class: | Prismatic (2/m) (same H-M symbol) |
| Symmetry: | C2/c |
| Unit Cell: | a = 11.03 Å, b = 12.53 Å c = 6.4 Å; β = 97.57°; Z = 4 |
| Color: | Dirty yellow to brownish yellow, grayish green; superficially dull greenish black, brownish black, black, when altered |
| Habit: | Platy to radiating fibrous, nodular, granular, massive |
| Twinning: | Polysynthetic |
| Cleavage: | Distinct/ good on and, good on |
| Mohs: | 5 – 5.5 |
| Streak: | Brownish yellow |
| Diaphaneity: | Translucent |
| Gravity: | 3.4 – 3.5 |
| Opticalprop: | Biaxial (+) |
| Refractive: | nα = 1.782 nβ = 1.802 nγ = 1.835 |
| Birefringence: | δ = 0.053 |
| Pleochroism: | X = pale olive-green, straw-yellow to greenish yellow; Z = pale olive-greenish to brownish yellow |
| 2V: | Measured: 50° to 90°, calculated: 78° |
| References: | [2] |
Alluaudite is a relatively common alkaline manganese iron phosphate mineral with the chemical formula . It occurs as metasomatic replacement in granitic pegmatites and within phosphatic nodules in shales.
It was first described in 1848 for an occurrence in Skellefteå, Västerbotten, Sweden. It was named by Alexis Damour after François Alluaud (II) (1778–1866).[3] [4] The mineral structure was first described in 1955.[5]