Pumpellyite is a group of closely related sorosilicate
Silicate minerals
The silicate minerals make up the largest and most important class of rock-forming minerals, constituting approximately 90 percent of the crust of the Earth. They are classified based on the structure of their silicate group...

  • pumpellyite-(Mg): Ca2MgAl2[(OH)2|SiO4|Si2O7]·(H2O)
  • pumpellyite-(Fe2+): Ca2Fe2+Al2[(OH)2|SiO4|Si2O7]·(H2O)
  • pumpellyite-(Fe3+): Ca2(Fe3+,Mg,Fe2+)(Al,Fe3+)2[(OH,O)2|SiO4|Si2O7]·H2O
  • pumpellyite-(Mn2+): Ca2(Mn2+,Mg)(Al,Mn3+,Fe3+)2[(OH)2|SiO4|Si2O7]·(H2O)
  • pumpellyite-(Al): Ca2(Al,Fe2+,Mg)Al2[(OH,O)2|SiO4|Si2O7]·H2O

Pumpellyite crystallizes in the monoclinic-prismatic crystal system
Crystal system
In crystallography, the terms crystal system, crystal family, and lattice system each refer to one of several classes of space groups, lattices, point groups, or crystals...

. It typically occurs as blue-green to olive green fibrous to lamellar masses. It is translucent and glassy with a Mohs hardness of 5.5 and a specific gravity
Specific gravity
Specific gravity is the ratio of the density of a substance to the density of a reference substance. Apparent specific gravity is the ratio of the weight of a volume of the substance to the weight of an equal volume of the reference substance. The reference substance is nearly always water for...

 of 3.2. It has refractive indices
Refractive index
In optics the refractive index or index of refraction of a substance or medium is a measure of the speed of light in that medium. It is expressed as a ratio of the speed of light in vacuum relative to that in the considered medium....

 of nα=1.674–1.748, nβ=1.675–1.754 and nγ=1.688–1.764.

Pumpellyite occurs as amygdaloidal
Amygdaloid, derived from the ancient Greek for almond, can refer to:* the amygdala in the brain.* any almond shape.* a volcanic rock texture in which small volatile cavities or vesicles are filled with secondary minerals....

 and fracture fillings in basalt
Basalt is a common extrusive volcanic rock. It is usually grey to black and fine-grained due to rapid cooling of lava at the surface of a planet. It may be porphyritic containing larger crystals in a fine matrix, or vesicular, or frothy scoria. Unweathered basalt is black or grey...

ic and gabbro
Gabbro refers to a large group of dark, coarse-grained, intrusive mafic igneous rocks chemically equivalent to basalt. The rocks are plutonic, formed when molten magma is trapped beneath the Earth's surface and cools into a crystalline mass....

ic rocks in metamorphic
Metamorphism is the solid-state recrystallization of pre-existing rocks due to changes in physical and chemical conditions, primarily heat, pressure, and the introduction of chemically active fluids. Mineralogical, chemical and crystallographic changes can occur during this process...

A terrane in geology is short-hand term for a tectonostratigraphic terrane, which is a fragment of crustal material formed on, or broken off from, one tectonic plate and accreted or "sutured" to crust lying on another plate...

s. It is an indicator mineral of the prehnite-pumpellyite metamorphic facies
Prehnite-pumpellyite facies
The prehnite-pumpellyite facies is a metamorphic facies typical of subseafloor alteration of the oceanic crust around mid-ocean ridge spreading centres....

. It is associated with chlorite
Chlorite group
The chlorites are a group of phyllosilicate minerals. Chlorites can be described by the following four endmembers based on their chemistry via substitution of the following four elements in the silicate lattice; Mg, Fe, Ni, and Mn....

, epidote
Epidote is a calcium aluminium iron sorosilicate mineral, Ca2Al2O, crystallizing in the monoclinic system. Well-developed crystals are of frequent occurrence: they are commonly prismatic in habit, the direction of elongation being perpendicular to the single plane of symmetry. The faces are often...

, quartz
Quartz is the second-most-abundant mineral in the Earth's continental crust, after feldspar. It is made up of a continuous framework of SiO4 silicon–oxygen tetrahedra, with each oxygen being shared between two tetrahedra, giving an overall formula SiO2. There are many different varieties of quartz,...

, calcite
Calcite is a carbonate mineral and the most stable polymorph of calcium carbonate . The other polymorphs are the minerals aragonite and vaterite. Aragonite will change to calcite at 380-470°C, and vaterite is even less stable.-Properties:...

 and prehnite
Prehnite is a phyllosilicate of calcium and aluminium with the formula: Ca2Al2. Limited Fe3+ substitutes for aluminium in the structure. Prehnite crystallizes in the orthorhombic crystal system, and most oftens forms as stalactitic or botryoidal aggregates, with only just the crests of small...

. It was first described in 1925 for occurrences in the Calumet mine, Houghton Co.
Houghton County, Michigan
-National protected areas:* Keweenaw National Historical Park * Ottawa National Forest -Demographics:As of the census of 2000, there were 36,016 people, 13,793 households, and 8,137 families residing in the county. The population density was 36 people per square mile . There were 17,748 housing...

, Keweenaw Peninisula, Michigan
Michigan is a U.S. state located in the Great Lakes Region of the United States of America. The name Michigan is the French form of the Ojibwa word mishigamaa, meaning "large water" or "large lake"....

 and named for US geologist
A geologist is a scientist who studies the solid and liquid matter that constitutes the Earth as well as the processes and history that has shaped it. Geologists usually engage in studying geology. Geologists, studying more of an applied science than a theoretical one, must approach Geology using...

, Raphael Pumpelly
Raphael Pumpelly
Raphael Pumpelly was an American geologist and explorer.-Early life and ancestors:He was born on September 8, 1837 in Oswego, New York, into a family with deep New England roots that trace back to Thomas Welles , who arrived in Massachusetts in 1635 and was the only man in Connecticut's history to...

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