| Pargasite | |
| Category: | Inosilicates |
| Formula: | NaCa2(Mg4Al)(Si6Al2)O22(OH)2 |
| Imasymbol: | Prg[1] |
| Strunz: | 9.DE.15 |
| Dana: | 66.1.3a.12 |
| System: | Monoclinic |
| Class: | Prismatic (2/m) (same H-M symbol) |
| Symmetry: | C2/m |
| Color: | Bluish green, grayish black, light brown |
| Habit: | Stout prismatic to tabular |
| Twinning: | Simple and lamellar – common |
| Cleavage: | perfect |
| Fracture: | Splintery |
| Mohs: | 5–6 |
| Luster: | Vitreous |
| Refractive: | nα = 1.630 nβ = 1.640 nγ = 1.650 |
| Opticalprop: | Biaxial (−) |
| Birefringence: | δ = 0.020 max. |
| Gravity: | 3.04–3.17 |
| Diaphaneity: | Translucent, will transmit light on thin edges. |
| References: | [2] [3] [4] [5] [6] |
Pargasite or pargasitic hornblende is a complex inosilicate mineral of the amphibole group with formula NaCa2(Mg4Al)(Si6Al2)O22(OH)2.
It was first described for an occurrence in Pargas, Finland in 1814 and named for the locality.
It occurs in high temperature regional metamorphic rocks and in the skarns within contact aureoles around igneous intrusions. It also occurs in andesite volcanic rocks and altered ultramafic rocks.
Pargasite is the main water-storage site in the uppermost mantle; however, it becomes unstable at depths greater than 90km (60miles). This has significant consequences for the water storage capacity, and the solidus temperature of the lherzolite of the upper mantle.[7]
It is used as a gemstone.[8] [9]