MIGMATITE

( Fr- migmatite; Ger- Migmatit; Nor- migmatitt; Rus- )

MIGMATITE (See also GNEISS)

A. Migmatite outcrop (width of field - ca. 130 cm) in 30,000 Island district of Georgian Bay, Lake Huron, Ontario, Canada. (© photo by Ed. Bartram)

B. Migmatite. Ptygma ("ptygmatic folds" of some petrologists) (width directly below stamp - 11.4 cm) from type locality in southern Finland. (© photo by C.E. Wegmann)

B*.Migmatite. This postage stamp, one of a three-stamp set that show rocks from famous localities in Finland, was issued in February 1986. Photographs of the other stamps of this set are in the GRANITE entry. (© photo of stamp by Richard Busch, http://mineralstamps.rbnet.net)

C. Migmatite. Ptygma ("ptygmatic folds" of some petrologists) (height - 11.4 cm) from an unknown locality. This fine specimen -- one of my favorite paperweights and "show pieces" -- was sent anonymously to me while I was studying ptgma from several worldwide localities. R.V. Dietrich collection. (© photo by Dick Dietrich)

D. Migmatite paperweight (width - 10.7 cm), exhibiting cross-cutting tourmaline-bearing granitic dikelet, from Little Hammond, St. Lawrence County, New York. R.V. Dietrich collection. (© photo by Dick Dietrich)

DESCRIPTION: Migmatites are macroscopically composite rocks, most of which consist of a dark colored amphibolite or biotite gneiss intimately mixed with a light colored rock of granitic or granodioritic composition. In most of these rocks, the light colored rock appears at one time in the rock's history to have been more mobile than than the darker rock.
    Color - The darker component is typically dark gray to nearly black; the lighter domponent is typically salmon colored or off-white. Both components, however, can be seen to consist of more than one mineral -- see the descriptions of these rocks given in the AMPHIBOLE-RICH ROCKS & GRANITE entries.
    H. (effective) - 6 - 7
    S.G. ranges between the specific gravities of granite (~ 2.65 ) and amphibolite (~2.75 ), depending upon the percentages of these components in any given specimen.
    Light transmission - opaque
    Luster - dull to pearly to subvitreous
    Breakage - irregular.

OTHER NAMES: Migmatites have an extremely complicated nomenclature: Several terms based on appearances and/or hypothesized geneses have been introduced into the geological literature -- for a summary, see Dietrich & Mehnert (1961). In practice, it seems best for anyone other than petrologists working directly with these rockst to call them merely migmatite and to include descriptions of the chief component rocks. Most names given to migmatites used as gemrocks are based on the locality from which they have been recovered plus or minus some adjective, such as flamboid, which directs attention to their flamboyant appearances.

"Rainbow Gneiss" (or "Rainbow Granite") - marketplace name(s) sometimes given the Morton Gneiss (migmatite!) from the Minnesota River Valley.

USES: Large cabochons for such things as belt buckles, bolo ties and brooches; diverse decorative and functional pieces such as paperweights and bookends.

OCCURRENCES: In zones that have undergone so-to-speak ultrametamorphism including mobilization of fluids, in most cases of granitic composition.

NOTEWORTHY LOCALITIES: Migmatites are relatively common in all Precambrian Shield areas -- I have seen migmatites that would be suitable for use as gemrocks in many places; examples are -- sporadically in the Canadian Shield; in some of the phacoliths of the lowlands near Edwardsville and Gouverneur, Saint Lawrence County, New York; in the vicinity of Kristiansand, southern Norway: on several islands in the Baltic Sea, east of Stockholm, Sweden; in the type locality area of southeastern Finland; and also as cobbles and boulders in unconsolidation glacial, alluvial, lacustrine etc. deposits derived from some of these rock areas. And, photographs of rocks from several other areas suggest that their rocks could provide fine specimens for use as gemrocks.

REMARKS: The term migmatite was introduced by the famous Finnish petrologist J.J. Sederholm (1907, p.88 & p.110 of English summary). The designation is based on the Greek word μιγμα (migma - a mixture).

Migmatites from the complex that includes the so-called Morton and Montevideo gneisses of the Minnesota River Valley region are probably the most widely seen migmatites on a worldwide basis. As ashlar, monuments and gravestones, this migmatite (see Figure XX and also Figure 10a on the web site http://www.mgs.md.gov/esic/features/walking/stp10.html ) is used as facing stone for both exterior and interior surfaces, for monuments, and for counter tops (etc.) in such places as banks, libraries and homes. In addition, this rock has found use, albeit not major use, as the rough material from which relatively small carvings, bookends and paperweights have been fashioned. The fact that the attractive patterns of these rocks can be seen, for the most part, only on relatively large surfaces accounts not only for its widespread use as ashlar but also for its general lack of use by carvers and lapidaries. Indeed, its typically large-scale patterns underline the reason, which to me seems well-grounded(!!), why most migmatites have not found wider use by those who fashioin gemstones and ornamental items.

Other than to add that these fascinating rocks are my favorites(!!!...!) -- especially those that include ptygmas -- I only direct attention to the fact that their appearances have been characterized by adjectives such as picturesque, flamboyant, chaotically deformed and the nabisco-like term psychodelicate.

SIMULANTS: None that I have seen or seen described.

REFERENCES: Dietrich 1974; Dietrich and Mehnert, 1961; Mehnert, 1968.

| Top | Home |