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The pumpellyite Mineral Group

Pumpellyite on Copper

Pumpellyite is a group of closely related minerals that have a minor of variation of elements among members. Pumpellyite-(Mg) and Pumpellyite-(Fe²⁺) are the most common members, with the other members being rare. In addition to members that have the name designation of Pumpellyite with an added chemical suffix, there are additional extended Pumpellyite group members not discussed in this guide that have their own unique names. Due to the difficulty in distinguishing Pumpellyite members, most specimens are labelled simply as Pumpellyite, without the chemical suffix. However, sometimes a locality is known to produce one type of member, and this can determine the specific form of Pumpellyite.

Pumpellyite was named in 1925 in honor of geologist Raphael Pumpelly (1837-1923), a professor of Mining Science at Harvard University. Pumpelly surveyed the copper region of Michigan, where this mineral was first described.

Chemical Formula

Pumpellyite is a group of several minerals with the following member formulas:
Pumpellyite-(Mg): Ca₂MgAl₂(SiO₄)(Si₂O₇)(OH)₂· (H₂O)

Pumpellyite-(Fe²⁺): Ca₂Fe²⁺Al₂(SiO₄)(Si₂O₇)(OH)₂· (H₂O)

Pumpellyite-(Fe³⁺): Ca₂(Fe³⁺,Mg,Fe₂₊)(Al,Fe³⁺)₂(Si₂O₇)(SiO₄)(OH,O)₂ · H₂O

Pumpellyite-(Mn³⁺): Ca₂(Mn²⁺,Mg)(Al,Mn³⁺,Fe³⁺)₂(Si₂O₇)(SiO₄)(OH)₂ · H₂O

Pumpellyite-(Al): Ca₂(Al,Fe²⁺,Mg)Al₂(Si₂O₇)(SiO₄)(OH,O)₂ · H₂O

Color

Light to dark green, olive green, bluish-green, gray, black

Crystal System

Monoclinic

Properties

Streak
White

Hardness
5.5 - 6

Transparency
Translucent to opaque

Specific Gravity
3.2

Luster
Vitreous, silky, or dull

Cleavage
2,2

Fracture
Uneven

Tenacity
Brittle

Crystal Habits

Individual crystals are rare. Usually in radiating or acicular tufts, fibrous, in small rosettes, as rounded aggregates, and in platy groups of small flattened crystals. Pumpellyite often forms a pseudomorph after other minerals, taking on the shape of the original mineral.

Additional Information

Composition
A complex group of hydrous basic calcium, magnesium, aluminum silicate, with some members containing iron and manganese.

In Group
Silicates; Sorosilicates

Striking Features
Color and crystal habits

Environment
As a secondary mineral in regional metamorphic schists, and in altered basalt and gabbro.

Rock Type
Igneous, Metamorphic

Varieties

Chlorastrolite

- Chatoyant variety of Pumpellyite exibiting chatoyant patterned veins similar to a turtle back, described from the copper-mining regions of the Keweenaw Peninsular of Michigan and Isle Royale in Lake Superior.

Uses

Pumpellyite is usually an uninteresting accessory mineral, though the variety Chlorastrolite can be polished as a collectors mineral, and is occasionally even used as a rare gemstone.

Noteworthy Localities

In Europe, minty green Pumpellyite tufts with Albite come from Obří důl, Krkonoše Mountain, Bohemia, Czech Republic; and acicular green crystal groups from the Los Arenales quarry, Torás, Castellón, Spain. The rare Pumpellyite-(Al) comes from the La Flèche Quarry, Bertrix, Belgium.

In the U.S., the most notable region for this mineral is the Keweenaw Peninsula in Keweenaw Co., Michigan, which is also the type locality. Pumpellyite, including the desirable variety Chlorastrolite, is found in several of the copper mines in this famous mining region. Chlorastrolite is also well-known from Isle Royale in Lake Superior. Other U.S. localities include Porter Creek, near Healdsburg, Sonoma Co., California; the Tiburon Peninsula, Marin Co., California; Paterson and Prospect Park, Passaic Co., New Jersey; and the O&G Quarry, Southbury, New Haven Co., Connecticut.