( 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)
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
(© 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.
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
famous Finnish petrologist J.J. Sederholm
(1907, p.88 & p.110 of English summary). The
based on the Greek word μιγμα
- 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
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,
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
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
the nabisco-like term psychodelicate.
SIMULANTS: None that I have seen or seen described.
REFERENCES: Dietrich 1974; Dietrich and Mehnert, 1961; Mehnert, 1968.
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Last update: 12 February 2010
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