CHLORASTROLITE

( Fr- chlorastrolite; Ger- Chlorastrolith; Nor- klorastrolitt; Rus- )

CHLORASTROLITE, (Compact, massive variety of pumpellyite -- Ca₂(Mg,Fe)Al₂(SiO₄) (Si₂O₇)(OH)₂·H₂O ).

A. Chlorastrolite pebbles (largest, greatest dimension ~1 cm) from Isle Royale, Michigan. The chiefly pink parts of three of the pebbles are identified as thomsonite. (© photo by Roger Weller, Cochise College)

B. Chlorastrolite (large irregularly shaped polished mass above hand specimen, width - 3.3 cm) from Isle Royale, Michigan. Notice that the three amygdules that are between six and eight o'clock in the hand specimen have hollow centers. R.V. Dietrich collection. (© photo by D.L. Brittain)

C. Chlorastrolite, polished specimen (width - ca. 2.5 cm) from Isle Royale, Michigan. Seaman Museum, Michigan Technological University. (© photo by John Jaszczak)

DESCRIPTION: As noted above, chlorastrolite is a variety of the mineral pumpellyite.
    Color - green and bluish green hues, typically comprising finely radiated or stellate masses
    H. 5½ - 6
    S.G. 3.18-3.23
    Light transmission - subtranslucent to opaque
    Luster - pearly
    Miscellany - individual stellate masses tend to appear chatoyant; some chlorastrolite includes native copper and/or native silver and/or other one or more white, red, pink, green and black minerals, which are listed in the fourth paragraph under the REMARKS subheading.

OTHER NAMES:

Green star stone - name applied, especially in rock shops of Michigan's Upper Peninsula.
Greenstone (e.g., Isle Royal greenstone & Lake Superior greenstone) - misnomer(s) applied for the most part on the Keweenaw Peninsula of Michigan.
Turtle back - name sometimes given because of the segmented pattern that roughly resembles that of turtles' backs -- cf. SEPTARIUM entry. This appearance is especially well exhibited by polished masses of chlorastrolite.
Uigite - "variety of chlorastrolite found on the island of Skye off the coast of Scotland" (Read, 1988).
Zonochlorite - misnomer sometimes given rounded masses of chlorastrolite within or weathered out of amygdaloidal cavities, especially those from the area around Nipigon Bay (on Lake Superior), Ontario, Canada. This application of the term is unfortunate -- among other things, most of the so-named masses are chiefly prehnite that roughly resembles chlorastrolite and is sometimes marketed as chlorastrolite -- see additional remarks in Appendix A.

USES: Relatively small cabochons and freeforms for stickpins, rings, earrings, cufflinks and pendants -- the "relatively small" is noted to emphasize the size limitations related to available "rough." Larger items that featuring chlorastrolite are, however, made; pieces are incorporated into inlays and mosaics, in some cases, along with other gemrocks and/or gem minerals.

OCCURRENCES: As amygdaloids and fracture fillings in basic igneous rocks and as pebbles and granules in loose sediments, such as beach gravels, derived from those rocks.

NOTEWORTHY LOCALITIES: Sporadically along the shore of Lake Superior -- e.g., along the shores of Keweenaw Peninsula and Isle Royale, Michigan; however, collecting chlorastrolite from Isle Royale, a National Park, is prohibited. In addition, several specimens have been collected from mines and mine tailings in Keweenaw County -- e.g., "the Central, Phoenix, Delaware and Central Exploration mines ... from which probably thousands of specimens have been recovered over the years" (George Robinson, personal communication, July 2005).

REMARKS: Chlorastrolite, the name, is based on this gemrock's green color (Greek, chloritis) and the fact that its chatoyant stellate groups resemble asterism (Greek, asterismos) -- i.e., it means "green star stone." "One of the first documentations of Michigan's semi-precious 'greenstone' came from Dr. C. Jackson and Dr. J. Whitney in 1847. The two doctors named the small green stones from Isle Royale 'chlorastrolite'." (www.geocities.com) The less said about the frequently applied misnomer "greenstone," the better (see, for example, Dietrich, 1983).

The fact that some chlorastrolite pebbles and granules are hollow or calcite-filled may lead lapidaries to frustration and losing apparently good material. Although those familiar with chlorastrolite can usually polish only the outer zones of such pebbles (etc.) and thus avoid the hollow or calcite-filled centers, apparent losses still occur. Fortunately, inlays and mosaics provide a use for at least some of pieces that are so-to-speak "spoiled" while fashioning originally planned items such as cabochons. In addition, the lapidary is often faced with choices relating to the ever changing patterns that are exhibited as surfaces of chlorastrolite are abraded -- i.e., questions often arise as to whether the the pattern or the final shape (or size) of the end product is more important.

In the past, chlorastrolite was thought to be an impure variety of prehnite or thomsonite. A brief review of the rather complex history of how chlorastrolite was once named keasargeite and how it finally became recognized as a variety of pumpellyite is given on, for example, the web site www.deq.state.mi.us/documents/ deq-glm-rcim-geology-Greenstone.

The following "mineral inclusions have been observed visually in polished stones...: Datolite - as chalky white masses; Thomsonite - as orange, pink, and white radial aggregates; Prehnite - as pale yellow-green radial aggregates and masses; and Native Copper - as pure metallic blebs ¶ I have also seen calcite, epidote and quartz filling the interiors of chlorastrolite amygdules, but not in polished stones ...¶ Also ... The "thomsonite" noted above is according to the labels that came with the specimens. Most "thomsonites" from the Keweenaw (including some from Grand Marais, Minnesota) that I have checked by microprobe or XRD have proven to be mesolite or a mixture of thomsonite + mesolite, and a few were even natrolite. The ones in the chlorastrolites I have not checked, so they may very well be mesolite or natrolite, too." (George W. Robinson, personal communication, June, 2005)

Chlorastrolite, designated by the misnomer greenstone is the official state gemstone of Michigan. (see www.michigan.gov)

SIMULANT:

Zonochlorite - green banded pebbles found in the Lake Superior region that are wholly or chiefly prehnite that roughly resembles chlorastrolite (Merrill, 1922); see remarks given in Appendix A. - [may require non-macroscopic means].

REFERENCES: Dietrich, 1983.; Huber, 1975.

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