PERTHITE

( Fr- perthite; Ger- Perthit; Nor- perthitt; Rus- )

PERTHITE

A. Perthite roughs and cabochon (greater axis - 3.5 cm.):  Clockwise from upper right salmon colored specimen, localties are: Limpopo River cliff, Zibmbabwe (formerly Southern Rhodesia);  Pikes Peak area, Colorado;  Bedford area of Westchester County, New York;  and Kragerø area, Telemark County, southern Norway.  R.V. Dietrich collection. (© photo by Dick Dietrich)

B. Perthite beads and cabochons (larger one is the one in "A"), fashioned from perthite from the well-known Amelia Courthouse area, Virginia.  Beads - F. S. Dietrich collection. (© photo by Dick Dietrich)

DESCRIPTION:   Perthite is the name given intimate intermixtures of a potassium feldspar (KAlSi3O8) – either microcline or orthoclase – and the sodium-rich plagioclase feldspar albite (NaAlSi3O8), with potassium feldpar the predominant constituent.  (Similar intermixtures with albite predominant, called antiperthites, are relatively rare and apparently have had no use as gemrock rough.)  There are macroperthites, microperthites and cryptoperthites with the prefix indicating sizes of the components: macro ... -- observable with one's naked eye or using a simple hand-lens;  micro ... -- seen only by using an optical microscope; crypto ... -- only  known to be perthitic on the basis of investigations utilizing procedures such as x-ray diffraction or electron microscopy;  in some cases, however, identity as a crytoperthite may be surmised on the basis of the feldspar's having a moonstone appearance.  The patterns assumed by the two feldspars in perthites are rather diverse;  indeed, several perthites have been described, on the basis of the shapes of their albite constituent, by preceding the term perthite (etc.) with a descriptive adjective – e.g., braid, film, patch, plate, string and vein.
    Colors - nearly colorless, white, flesh- to salmon-pink, diverse shades of green (including bluish green), gray, brown, ...
    H. 6-6½
    S.G. ~ 2.56
    Light transmission -  translucent to subtranslucent
    Luster - pearly 
    Breakage -  two cleavages at or close to 90
°.

OTHER NAMES:

USES: Amazonite has found widespread use as a gemstone (usually cut cabochon), especially for brooches, and also for beads and even buttons.  In addition, it has been fashioned into diverse pieces such as small bowls.  Moonstone, nearly always cut cabochon, has been used widely as a gemstone.   Crowninghshield (1964, p.216) illustrates a "nearly transparent emerald-green amazonite in a pendant ornament, which was presumed to be emerald." 

OCCURRENCES: Nearly all amazonite used as a gemrock occurs as crystals that extend into open spaces, frequently referred to as vugs, in pegmatites or in metasomatized rocks. Most moonstone rough occurs in metamorphic rocks and in gravels derived from those rocks.

NOTEWORTHY LOCALITIES: A relatively large quantity of amazonite used as gemrock has come from pegmatite masses near Miask in the Ilmen range of the southern Ural mountains of Russia;  this locality and the range are of special interest to petrologists and mineralogists because the rock miaskite was named after the place and the mineral ilmenite was named after the range.   Another relatively large quantity has come from the Crystal Peak district, Park and Teller counties, west of Colorado Springs, Colorado.  In addition, workable masses, most of them comprising crystal groups that also include quartz crystals, have come from localities in southeastern Brazil, Canada, India (Kashmir), Madagascar, Mexico, Norway (near Kragerø), and from pegmatite masses near Amelia Court House, Virginia, United States of America.

Much high quality moonstone, including some marketed as black moonstone, has come from Myanmar (formerly Burma) and  the Dumbara district of Sri Lanka (formerly Ceylon);  lesser quantities have come from several localities in Australia, Austria, Brazil, Canada, India, Madagascar, Switzerland, Tanzania (formerly Tanganyika) and from the United States of America -- e.g., from  California, Colorado, North Carolina, and Virginia.

REMARKS: Perthite was named in 1843 for a specimen from the town of Perth, which is west of Smith Falls, Ontario. According to Malcolm Back of the Royal Ontario Museum (personal communication, 2001) "The locality of the original find is believed to be Lot 3, Concession VI, North Burgess Township, Lanark County." Unfortunately, the "type locality" is misidentified in several publications as Perth County, Ontario, which is about 1000 miles ( ~1600 km) to the southwest.  The name amazonite appears to have evolved from the term Amazon stone, which originally was applied to both nephrite and green microcline perthite pebbles worn as amulets by Natives of the Amazon basin of South America.

Many amazonite cabochons exhibit two distinct colors,  bluish green and nearly white.  A few of these stones have been "doctored" by diverse means to make the nearly white zones assume the more desired bluish green color -- e.g., "wax or paraffin treated, using materials ... [that led to] hiding the incipient cleavages..." (Crowningshield, 1963-64). 

Since ancient times, the designation moonstone has been applied to diverse minerals -- e.g., selenite and chalcedony -- and also to milky glass. Even within the last century, the term was applied both to plagioclase feldspars in the albite-oligoclase composition range, which are frequently referred to as peristerite, and to labradorite because both of these feldspars commonly exhibit iridescence (etc.).  Currently, however, most professional gemologists restrict application of the term to the perthitic material described herein.

At temperatures above about 660°C, potassium feldspar (KAlSi3O8) and sodium feldspar (NaAlSi3O8) exist as a so-called solid solution -- i.e., they appear to comprise a homogeneous substance.  Below that temperature, the two feldspars "exsolve," commonly  forming what appear to be intergrowths of the two feldspars -- i.e., one or other of the perthites.  As already mentioned, the components of these "intergrowths" may or may not be discernible macroscopically.  Most petrologists, however, agree that some perthites have been formed by processes other than exsolution. -- Anyone interested in the diverse hypothesized origins of perthites can read about them in mineralogy books such as the one by Berry, Mason, and Dietrich (1983).

The fact that nearly all mineral specimens identified and distributed as microcline by scientific supply houses were amazonite led to a misconception that has so-to-speak haunted petrology professors for decades:  This is true because it led students to believe that microcline is green and potassium feldspars of other colors are orthoclase.  This is not true! -- Although nearly all green potassium feldspar is microcline, much microcline, as well as most orthoclase, is commonly white, flesh- or salmon-pink, etc. -- The one recorded exception to the "green potassium feldspar is microcline" generality that I know of is a green orthoclase from the New Broken Hill Mine, New South Wales, Australia.

In some quarters, moonstone is considered to be an alternative (to pearl) birthstone for the month of June.  According to some accounts, amazonite is the national stone of Brazil;  to the present, however, I have been unable to verify this.

SIMULANT: To date, I have found no record of anything being used to simulate amazonite. Substitutes reported for moonstone, include the following:

California moonstone - see the next listing.

Chalcedony - both milky and bluish white chalcedony -- e.g., that found along some California beaches. - [coarse texture and superior hardness].

***Glass - so-called opalescent glass. - [appearance and inferior hardness].

Quartz - both blue quartz (See Appendix A.), milky quartz and heat-treated amethyst have been so-used. - [superior hardness].

Scapolite - I have been unable to find anything about this substitution;  an aside statement, which I only overheard, led me to include it here. - [inferior hardness].

***Synthetic spinel - a material described as a heat-treated, white spinel. - [superior hardness and specific gravity].

REFERENCE: Smith, 1974.

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