| Allactite | |
| Category: | Arsenate mineral |
| Formula: | Mn7(AsO4)2(OH)8 |
| Imasymbol: | Ala[1] |
| Strunz: | 8.BE.30 |
| Dana: | 41.2.1.1 |
| System: | Monoclinic |
| Class: | Prismatic (2/m) (same H-M symbol) |
| Symmetry: | P21/a |
| Unit Cell: | a = 11.03, b = 12.12 c = 5.51 [Å], β = 114°, Z = 2 |
| Color: | Brown, dark to light purplish red, brownish red, colorless to white |
| Habit: | Elongated prisms, bladed, tabular, rosette-like aggregates |
| Cleavage: | Distinct, |
| Fracture: | Uneven |
| Tenacity: | Brittle |
| Mohs: | 4.5 |
| Luster: | Vitreous, slightly greasy on fracture surfaces |
| Refractive: | nα = 1.755–1.761 nβ = 1.772–1.786 nγ = 1.774–1.787 |
| Opticalprop: | Biaxial (−) |
| 2V: | ~0° |
| Pleochroism: | X = blood-red; Y = pale yellow; Z = sea-green |
| Streak: | Gray to faint brown |
| Gravity: | 3.83 (meas.), 3.94 (calc.) |
| References: | [2] [3] [4] |
Allactite is a rare arsenate mineral of metamorphosed manganese zinc ore deposits. It is found in Sweden and New Jersey, US. Its name originated from Greek αλλάκτειν (allaktein) meaning "to change", referring to the strong pleochroism of the mineral.[3]