PUMPELLYITE
Pumpellyite – Ca2(Al,Fe2+,Fe3+,Mg,Mn)Al2[(OH)2|SiO4|Si2O7].H2O – is a relatively rare sorosilicate, indicative of a certain metamorphic grade. It has no economic importance.
“Pumpellyite” is actually not a mineral, but just a generic, informal name referring to the minerals that make up the 11-member Pumpellyite Group. Of these, 5 are “pumpellyite”: pumpellyite-(Al), pumpellyite-(Fe+2), pumpellyite-(Fe+3), pumpellyite-(Mg) and pumpellyite-(Mn+2). Recognition of individual species is not possible either macroscopically or under the petrographic microscope.
The bibliography generally does not differentiate the different members of the Group, always treating these minerals under the collective name of “pumpellyite”.
1. Characteristics
Crystal system: Monoclinic prismatic.
Color: Green, blue-green, brown.
Habit: Lamellar, acicular, tabular, radiated, fibrous
Cleavage: {100} perfect, {001} good.
Tenacity: Brittle.
Twinning: Common on (001) and (100)
Fracture: Irregular.
Mohs Hardness: 6
Parting: No.
Streak: White to gray.
Lustre: Vitreous.
Diaphaneity: Transparent.
Density (g/cm³): 3.18 – 3.23
2. Geology and Deposits
Pumpellyite is an index mineral for low-grade metamorphic rocks, formed under low temperatures and high pressure, of greenschist facies, pumpellyite-prehnite grade. Protoliths can be igneous or sedimentary, but generally protoliths were mafic igneous rocks such as basalts, diabases, gabbros and their pyroclastic equivalents. Hydrothermal alteration processes on these rocks also generate pumpellyite.
It also occurs in glaucophane-schist and marble.
In skarns and associated metamorphic carbonate rocks (such as marbles) pumpellyite can occur but is not common.
May occur detrital and is often confused with epidote.
3. Mineral Associations
In the greenschist facies of metamorphic rocks of the pumpellyite-prehnite zone, it is associated with albite, chlorite, calcite, titanite, sericite, prehnite, datolite, chalcedony, quartz and native copper.
In the greenschist facies of metamorphic rocks, in the pumpellyite-chlorite zone, it is also associated with albite, chlorite, calcite and titanite, accompanied by stilpnomelane, orthoclase, zeolites (laumontite) and actinolite.
In the metamorphic rocks of blueschist facies it is associated with lawsonite, glaucophane, riebeckite, quartz, aegirine, epidote, titanite and chalcopyrite.
In skarns it is associated with calcite, dolomite, tremolite and garnet, among others.
4. Transmitted Light Microscopy
Refraction indices: nα: 1.665 – 1.710 nβ: 1.670 – 1.720 nγ: 1.683 – 1.726
PLANE POLARIZED LIGHT – PPL
Color / Pleochroism: Green, yellow, blue-green, yellow-brown, may be colorless.
Weak to moderate pleochroism, rarely absent:
X = colorless, yellow, greenish yellow or brownish;
Y = colorless, light green, green, blue-green yellow-brown;
Z = colorless, yellow, yellow-brown or red-brown.
Color intensity and pleochroism depend on Fe and Mg contents.
Color zonation is common, with edges (+Fe) darker than the center.
Relief: Moderate to high.
Cleavage: {100} perfect and {001} good.
Habits: Lamellar, acicular, tabular, radiated, fibrous. It forms divergent aggregates, rosettes or spherulites, also in parallel to subparallel bundles. Often in xenomorphic or hypidiomorphic grains.
Intergrowths between pumpellyite and lawsonite may occur in “oakleaf” form, on (010), along with epidote.
CROSSED POLARIZED LIGHT – XPL
Birefringence and Interference Colors: Birefringence of 0.010 to 0.020, but normal interference colors such as 1st order straw yellow to 2nd order blue are rare.
Common are anomalous 1st order interference colors such as berlin blue and leather brown.
Extinction: oblique. If low in Fe: 4º. If rich in Fe: 22º
Elongation sign: ES(+) or ES(-), it is not diagnostic.
Twins: Common in (001) and (100), can be presented with four sectors.
Zoning: Frequently zoned.
CONVERGENT LIGHT
Character: B(+), if rich in Fe, it can be B(-).
2V angle: : 7 – 110º.
If low in Fe: 4º.
If rich in Fe: 85º.
If rich in Cr: up to 150º
Alterations: does not alters, but can react with associated minerals due to metamorphic processes.
May be confused with: diagnostic are high relief, low birefringence, anomalous interference colors, and cleavage. Colorless pumpellyites can be confused with other members of the Epidote Group, requiring other analytical techniques for their identification.
Similar to Epidote Group minerals: epidote (B-), zoisite (parallel extinction) and clinozoisite (less birefringence and higher relief).
Lawsonite has lower birefringence and parallel extinction.
Chlorite has lower relief, lower birefringence, parallel extinction, and different, often anomalous, interference colors.
5. Reflected Light Microscopy
Reflected light microscopy is not the recommended analytical method for the identification of pumpellyite. However, it is important to make a polished thin section or a polished section to identify the opaque minerals that occur associated with pumpellyite.
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: