PARGASITE
Pargasite – NaCa2(Mg4Al)(Si6Al2)O22(OH)2 – is an inosilicate, an amphibole, with a very restricted occurrence. It has no economic importance beyond its occasional use as a gemstone. Petrologically, it is important because it constitutes the main reservoir of water in the upper mantle.
It is classified in the Amphibole Supergroup, where it gives its name to a specific Group. It may contain various impurities, such as Cr, Ti, K, Mn, F, P, and H2O. It is the Al analog of mangano-pargasite and very close to fluor-canniloite. There is a variety, “carinthine,” which is brown and occurs in eclogites.
1. Characteristics
Crystal system: Monclinic prismatic.
Color: Light brown, brown, brownish-green. Bluish green, dark green, grayish black to black.
Habit: Short prismatic to tabular.
Cleavage: {110} perfect, like all amphiboles.
Tenacity: Brittle.
Twinning: Common, simple, and lamellar.
Fracture: Splintery.
Mohs Hardness: 5- 6
Parting: No.
Streak: Pale gray to brownish green.
Lustre:
Diaphaneity: Transparent.
Density (g/cm³): 3.04 – 3.17
2. Geology and Deposits
Pargasite is characteristic of high-grade metamorphic rocks formed by regional metamorphism, such as marbles and siliceous limestones. In these rocks, the “hornblendes” that should be expected are those of the Pargasite Series and not those of the Tschermakite Group. It is important to remember that the classification of amphiboles is complex and is constantly being updated.
It also occurs in magnesian skarns that have undergone regional metamorphism, in the contact aureole between igneous intrusions and calcareous rocks (limestones, marbles), generally with OH, F, and B metasomatism.
It can be found in andesites and in altered ultramafic rocks.
3. Mineral Associations
It occurs associated with minerals characteristic of high-grade metamorphic rocks: garnet (grossular), corundum (ruby variety), spinel, zoisite, and phlogopite.
It also occurs with calcite, hauyne, anorthite, and gonnardite.
In skarns, it is associated with calcite, garnets, vesuvianite, scapolite, and epidote.
4. Transmitted Light Microscopy
Refraction indices: nα: 1.630 nβ: 1.640 nγ: 1.650
PLANE POLARIZED LIGHT – PPL
Color / Pleochroism: Colorless, very light brown, yellowish or bluish-green with moderate to strong pleochroism, like green hornbendes in general:
α = yellow, yellowish-green;
β = light brown, bluish-green
Relief: Moderate.
Cleavage: {110} perfect, like all amphiboles. In longitudinal sections only one cleavage, in basal sections two cleavages that intersect forming rhombs, with angles of 56 and 124º between them.
Habits: Short prismatic, tabular.
CROSSED POLARIZED LIGHT – XPL
Birefringence and Interference Colors: Maximum birefringence of 0.020, corresponding to colors up to the first order: gray, yellow, orange, red, purple, and blue.
Extinction: Oblique, like all monoclinic amphiboles, with an angle of around 25º.
Elongation sign: Without information, but in green hornblende it is ES(+), including as an important diagnostic characteristic.
Twins: Common, both contact and lamellar, as in green hornblende.
Zoning: Thin, opaque rims can form due to the thermal decomposition of pargasite during magma ascent.
CONVERGENT LIGHT
Character: B(-), B(+) if rich in Mg (2V = 56-90º)
2V angle: 63º
Alterations: No information available, perhaps similar to green hornblende.
May be confused with: Other amphiboles with pleochroism in shades of green, brown, and bluish.
Precise identification of pargasite requires analytical techniques beyond optical microscopy.
5. Reflected Light Microscopy
Reflected light microscopy is clearly not the recommended analytical method for identifying pargasite. However, it is important to prepare a polished slide or section to identify the opaque minerals that occur associated with pargasite.
Sample preparation:
PLANE POLARIZED LIGHT – PPL
Reflection color:
Pleochroism:
Reflectivity:
Bireflectance:
CROSSED POLARIZED LIGHT – XPL
Isotropy / Anisotropy:
Internal reflections:
May be confused with:
General Characteristics: