PECTOLITE
Pectolite – NaCa2Si3O8(OH) – is a very rare inosilicate that occurs in some very specific paragenesis. The well-known massive blue “larimar” variety, found only in the Dominican Republic, is mined as gemological material.
It is classified in the Wollastonite Group, in the Pectolite-Sérandite Series, constituting the Ca analogue of sérandite. May contain H2O, K, Fe, Li, Sr, Mn, Al and Mg. There is a variety that is rich in Mn.
1. Characteristics
Crystal system: Triclinic pinacoidal.
Color: Colorless, white, can be pale pink, greenish or pale blue.
Habit: Generally radial groups of acicular to fibrous crystals, may form spheres. Columnar, massive.
Cleavage: {100} perfect., {001} perfect.
Tenacity: Brittle.
Twinning: Rare, on {100}.
Fracture: Irregular.
Mohs Hardness: 4.5 – 5
Parting: No.
Streak: White.
Lustre: Sub-vitreous, silky.
Diaphaneity: Transparent.
Density (g/cm³): 2.84 – 2.9
2. Geology and Deposits
Pectolite occurs as a primary mineral in alkaline igneous rocks such as phonolites and nepheline syenites (kaxtorpites and lakarpites).
As a secondary mineral it is found, of hydrothermal origin, in cavities (vesicles) in basaltic flows, diabase dykes and correlated rocks.
It also occurs in metamorphosed mafic-ultramafic rocks, such as serpentinites, peridotites and amphibolites. It is found in skarns (tactites) and other Ca-rich metamorphic rocks.
It forms as one of the alteration products of wollastonite.
3. Mineral Associations
In nepheline syenites it is associated with aegirin, sugilite, garnet (grossular), vesuvianite, diopside, fluorite and pyrite.
In basalt and diabase cavities it is associated with quartz, calcite, zeolites (Ca-heulandite, laumontite, natrolite), prehnite, fluorapophyllite, datolite, hematite, chalcopyrite and pyrite.
4. Transmitted Light Microscopy
Refraction indices: nα: 1.594 – 1.610 nβ: 1.603 – 1.614 nγ: 1.631 – 1.642
PLANE POLARIZED LIGHT – PPL
Color / Pleochroism: Colorless.
Relief: Moderate.
Cleavage: {100} and {001} perfect, which form a grid with a 95º angle between the two cleavages.
Habits: Usually radial groups of acicular crystals elongated parallel to the B axis.
CROSSED POLARIZED LIGHT – XPL
Birefringence and Interference Colors: Maximum birefringence of 0.039, corresponding to bright, intense and colorful colors until the end of 2nd order.
Extinction: Almost parallel (+- 2º) in the longitudinal sections parallel to the “b” axis.
16º in relation to the cleavage planes in the sections parallel to (010).
Elongation sign: ES(+) in sections parallel to the “b” axis.
Twins: Rare, with a twin plane nearly parallel to {100}.
The twin lamellae are parallel to the grain elongation direction.
Zoning: No.
CONVERGENT LIGHT
Character: B(+)
2V angle: 50 – 63º, it can be as low as 35º.
Alterations: pectolite alters to the pink stevensite.
May be confused with: few other minerals, considering paragenesis.
Zeolites (heulandite, laumontite, natrolite), with rare exceptions, show interference colors between gray and white, always well below the end of 1st order.
Wollastonite occurs in other paragenesis and has lower birefringence.
5. Reflected Light Microscopy
Reflected light microscopy is not the recommended analytical method for the identification of pectolite. However, it is important to make a polished thin section or a polished section to identify the opaque minerals that occur associated with pectolite, like pyrite.
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: