ANORTHOSITE

( Fr- anorthosite; Ger- Anothosit; Nor- anorthositt; Rus )

ANORTHOSITE


A. Labradorite grain (size not recorded) from anorthosite mass near Nain, Labrador. (© photo by Jamie Meyer, permission of Geological Survey of Newfoundland and Labrador, www.geosur.gov.nf.ca)

B. Anorthosite (from Madagascar) heart (width  ~ 7 cm). Unconventional lapidarist. (© photo by James Carpenter, www.unconventionallapidarist.com)

C. Labradorite ("Finnish Spectrolite") pendant (height  ~ 5.5 cm). Bernardine Fine Art Jewelry. (© photo by Nancy Bernardine, www.bernardine.com)

DESCRIPTION: Anorthosite is an igneous rock that consists of 90 or more percent of plagioclase feldspar, which is labradorite in a large percentage of the anorthosites used as gemrocks (see additional statements about this rock and its name and about labradorite under the REMARKS subheading). Labradorite properties follow:
    Colors - overall medium to dark gray, commonly exhibiting a bluish sheen or even a play of colors consisting of patches with bluish, greenish, purplish, golden yellow and reddish-bronzy hues;  rarely  white or off-white
    H.
- 6½
    S.G. 2.62-2.82
    Light transmission - translucent to subtranslucent
    Luster
- pearly  to subvitreous
    Breakage - two good cleavages,  irregular in other directions (which, of course, may lead to problems for lapidaries!)
    Miscellany - polysynthetic twinning, which is expressed by numerous parallel lines, is usually discernible on some surfaces. 

OTHER NAMES: Anorthosite is a widely accepted petrographic designation. Several large anorthosite masses around the world have been named on the basis of their locations and are shown on geologic maps and referred to in geologic reports by those binomial names -- e.g., the Marcy Anorthosite of the Adirondack Mountain region of northern New York and the Roseland Anorthosite of Nelson County, Virginia.

USES: Cabochons and freeforms -- especially for relatively large brooches and pendants, but also for smaller pieces of jewelry such as earrings and stickpins and even beads for necklaces and bracelets; most of the smaller stones are fashioned, as just noted for Spectrolite, from individual labradorite grains.  In addition, tumbled pieces have been used in such things as necklaces and  bracelets, and larger pieces of rough have been made into wands, spheres, eggs and hearts (Figure B) and carved/sculpted.

OCCURRENCES:
Anorthosite occurs as relatively large igneous masses or parts of such masses, especially those that are called layered or stratiform masses. The coarser grained phases of some anorthosite masses are sometimes alluded to as pegmatitic, and many relatively large chatoyant grains of labradorite have been recovered from these phases.



D. Anorthosite.  Left. cobbles from beaches on south shore of Lake Erie, near Saybrook (Ashtabula County), Ohio;  Right. polished pebbles and cobbles from the same locality.  (© photos by Isaac Coblentz)

NOTEWORTHY LOCALITIES +: Tabor Island, Nain area of Labrador, Newfoundland Province, Canada;  the county of Ylämaa, southeastern Finland ("Spectrolite");  southwestern Madagascar.  The + locality (Lake Erie beaches in northeastern Ohio) is not really comparable to the other localities for two reasons:  1. It is not a bedrock source;  2. The stones thus far found are not of the high quality usually associated with the anorthosite from the three just noted localities.  The anorthosite beach stones do, however, serve to emphasize the facts that rocks that are well worth fashioning into gemstones etc. are not necessarily attractive in the rough and they may be found where many people might not look for them.  As noted in the caption of Figure D, this anorthosite was found on the south shore of Lake Erie, many kilometers/miles from any recorded bedrock occurrences of anorthosite.  It seems likely that  the cobbles and pebbles represent pieces of rock that were glacially transported from one or more of the anorthosite massifs of the Grenville Province north and northwest of  Montreal, Québec, Canada.  When they became rounded stones is not known;  it could have been prior to or after they were glacially transported into the Lake Erie region.  In any case, Isaac Coblentz (personal communication, January, 2006) notes that the beach pebbles are commonly gray-black when wet, "chalky gray blue" when dry, and exhibit small flashes of labradorescence ("flashes of dark blue, green, light blue, gold, and very rarely red/orange and peach/orange") that is "randomly distributed through the matrix."     

REMARKS: Anorthosite is the accepted name for phaneritic igneous rocks that consist of 90 per cent or more calcic plagioclase feldspar (i.e., labradorite, bytownite or anorthite), 0 - 5 percent quartz and less than 10 percent mafic minerals (e.g., some pyroxene(s)). The name anorthosite comes from anorthose, an old -- now obsolete -- name for triclinic feldspars (which includes all plagioclase feldspars).


Labradorite, as already noted, is the typical  plagioclase of most anorthosite used as a gemrock.  Named after Labrador (now part of Newfoundland and Quebec provinces, Canada),  this plagioclase is recorded as having been discovered in 1770 by a Moravian missionary on the Isle of Paul, Labrador.  This island is in the Labrador Sea, which is part of the north Atlantic, just east of Nain on
mainland Labrador. The quality that makes labradorite anorthosite so attractive, thus accounting for its use as as a gemrock, is an optical phenomenon whereby one sees color changes that involve one or more colors -- commonly blues, greens and/or bronzy reddish hues -- when the mineral is moved beneath a source of light. This appearance, named for the mineral, is widely referred to as labradorescence.  The cover photograph of a labradorite megacryst on the 1998 Journal of Petrology is well worth viewing:  The pictured megacryst (~ 3 cm across), which is from southeastern Wyoming, exhibits spectacular zoning as well as fine labradorescence. 

An atypical anorthosite, also used as a gemrock, comes from a locality recorded only as the Philippines (Johnson and Koivula, 1996). This rock is described as granular, overall white, and containing sporadic green crystals up to 2.0 mm across with its granular material consisting largely of nearly pure anorthite plus minor interstitial oligoclase and crystals of zoned garnet, "primarily uvarovite with varying amounts of andradite in solution."   It is said to roughly resemble jade.

Labradorite is the provincial mineral for Newfoundland and formerly for all of Labrador.  Spectrolite is the national stone of Finland (it "is said to be that colours reflect the nature of Finland: blue is born the glittering of thousands lakes, green is the wuthering of our country wilderness forest. Yellow, red and oranges are as glowing brilliant colors of fall foliage in autumn."(www.finnishspectrolite.com)  Also, in a letter I received from the Governor’s office (dated June 13, 1977), anorthosite was listed as the New York State rock;  however,  I have found no documentation of pertinent enabling legislation for this status.

SIMULANT:

Larvikite -- see LARVIKITE entry.

REFERENCES: No general reference.

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R. V. Dietrich © 2014
Last update: 3 October 2007
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