Amazonite
Amazonite, also known as amazonstone, is a green tectosilicate mineral, a variety of the potassium feldspar called microcline. Its chemical formula is KAlSi3O8, which is polymorphic to orthoclase.
| Amazonite | |
|---|---|
Amazonite from Brazil | |
| General | |
| Category | Tectosilicate |
| Formula (repeating unit) | KAlSi3O8 |
| Crystal system | Triclinic |
| Identification | |
| Color | Green, blue-green |
| Crystal habit | Prismatic |
| Cleavage | Perfect |
| Fracture | Uneven, splintery |
| Tenacity | Brittle |
| Mohs scale hardness | 6.0–6.5 |
| Luster | Vitreous |
| Streak | White |
| Diaphaneity | Translucent, opaque |
| Specific gravity | 2.56–2.58 |
| Refractive index | 1.522–1.530 |
| Birefringence | −0.008 |
| Pleochroism | Absent |
| Dispersion | None |
| Ultraviolet fluorescence | Weak; olive-green |
| Other characteristics | Radioactive 14.05% (K) |
| References | :214–215 |
Its name is taken from that of the Amazon River, from which green stones were formerly obtained, though it is unknown whether those stones were amazonite. Although it has been used for jewellery for well over three thousand years, as attested by archaeological finds in Middle and New Kingdom Egypt and Mesopotamia, no ancient or medieval authority mentions it. It was first described as a distinct mineral only in the 18th century.
Green and greenish-blue varieties of potassium feldspars that are predominantly triclinic are designated as amazonite. It has been described as a "beautiful crystallized variety of a bright verdigris-green" and as possessing a "lively green colour". It is occasionally cut and used as a gemstone.