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Perthite Gemstone: Properties, Meanings, Value & More

Perthite 宝石：属性、含义、价值等

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Perthite is a type of gemstone composed of two or more intergrown feldspar minerals. The term is also used to describe the type of texture characteristic of the mineral.

What colors are perthite? Perthites are usually banded, with the base color of one feldspar and stripes or veins of another feldspar. The stone can be various colors, but it’s often white to brown with pinkish banding. Some exceptional specimens display optical effects like iridescence.

Gemstones that can sometimes be classified as perthites are amazonite, rainbow lattice sunstone, and moonstone. Many perthites are used as decorative stones.

Today, we’ll go over all of perthite’s properties, varieties, history, and prices!

Image credit: Zimbres assumed, CC-BY-SA-3.0

What Is A Perthite Stone?

Perthite is a type of feldspar sometimes used as a semi-precious gemstone. The mineral is sometimes called “anperthite.”

When perthite is adularescent (called moonstone or adularia), it can be used as a June birthstone and astrological Cancer stone. Rainbow lattice sunstone, sometimes considered a perthite variety, is an alternate July and August birthstone.

In terms of industrial perthite uses, a large portion of perthite’s composition is alkali feldspar. Alkali feldspars are great for potassium-argon dating and making glazes for glass and pottery.

Perthite Specifications & Characteristics

Perthite is part of the feldspar mineral group of aluminum tectosilicates containing calcium, sodium, and/or potassium. The main subgroups are plagioclase and alkali.

Plagioclase feldspar minerals range from albite (NaAlSi3O8; sodium endmember) to anorthite (CaAl2Si2O8; calcium endmember).

Alkali feldspars — also called potassium feldspar or K-feldspar — are potassium aluminosilicates that generally range from albite to microcline (KAlSi3O8; potassium endmember). As you can see, albite is technically in both groups.

So, what is the composition of perthite? Perthite is an intergrowth of multiple feldspars. Distinct albite crystals are intergrown with orthoclase or microcline. The general formula of perthite is (K,Na)AlSi3O8.

Orthoclase or microcline is usually the host, while the albite is usually present as exsolved lamellae. “Exsolved” means the two minerals had to “unmix” and separate, creating distinct areas of each mineral. “Lamellae” are thin layers often visible as color banding.

Exsolved lamellae is part of what’s called “perthitic texture.” But what is a perthitic texture?

Pictured above: Thin section sample of microcline with perthitic texture | Image credit: Marguliesgm, CC-BY-SA-4.0

Perthite Texture

Outside of the specific mineral, “perthite” or “perthitic” can describe any type of exsolution in feldspars. On one level, perthitic textures are separated by their sizes (more on that next). On the other hand, perthite can have many textures of its own, like “lamellar” or “patchy” intergrowths.

Here are perthite’s properties listed:

• Mohs hardness: 6-6.5

• Color: White, reddish-brown, gray, colorless, black, pale pink, red, green, blue-green, brown, orange, yellow; Often color-banded

• Crystal structure: Triclinic or monoclinic in parts, depending on feldspars present

• Luster: Vitreous or pearly

• Transparency: Transparent to opaque, usually opaque

• Refractive index: 1.52201.538; Alkali portions will have lower RI, albite & plagioclase will have higher RI

• Density: 2.55-2.69

• Cleavage: Perfect on {001}, good/distinct on {010}

• Fracture: Conchoidal, uneven, or splintery

• Streak: White

• Luminescence: Sometimes fluorescence - patchy blue and green in LW-UV & SW-UV (Zimbabwe specimens)

• Pleochroism: None

• Birefringence: 0.005-0.011

• Dispersion: None

• Optical phenomena: Possibly iridescence, adularescence, aventurescence

Breaking things down further, what are the different types of perthites?

Pictured above: An antiperthitic intergrowth of K-feldspar (the veinlets) in the Na-feldspar | Image credit: Eurico Zimbres, CC-BY-SA-2.5

Types of Perthite

There are various types of perthites which can be separated by categories. The first important distinction is perthite and antiperthite.

So, what is the difference between perthite and antiperthite? It has to do with the “host” feldspar.

Perthites By Predominant Host Feldspar

A “host mineral” is the main, predominant mineral that makes up a rock — in this case, the main feldspar. The lamellae are less dominant and form after the host. This category has three types of perthites:

• True Perthite: Alkali host (orthoclase or microcline) with albite lamellae

• Antiperthite: Plagioclase host (albite and/or oligoclase) with orthoclase lamellae

• Mesoperthite: Nearly equal amounts of alkali and plagioclase feldspars

“True” or “regular” perthite and antiperthite textures can be similar, and these textures can be distinguished by size.

Perthites By Intergrowth Size

There are three types of perthites separated by the size or thickness of their intergrowth:

• Macroperthite: Intergrowth is large and seen with the naked eye or basic hand lens

• Microperthite: Intergrowth is small and only seen with magnification

• Cryptoperthite: Intergrowth is very small (submicroscopic) and only seen with microscope or X-ray diffraction

Perthites By Type of Feldspar

As if the separations so far weren’t enough, some separate perthites based on the specific feldspars present:

• Perthite: Usually microcline with albite or oligoclase

• Orthoclase Perthite: Albite or oligoclase with orthoclase

• Amazonite Perthite: Albite or oligoclase with green microcline (amazonite)

Speaking of amazonite…

Perthite Gemstones

Classifying which gemstones are technically “perthites” can be a bit tricky, given the mineral vs texture definitions. But with a grain of salt, the perthite gemstones are:

• Amazonite: A green microcline variety; Perthite when intergrown with albite or oligoclase

• Moonstone: Though often considered an orthoclase variety with alternating albite layers, moonstone can be a plagioclase or alkali feldspar. In 1988, the Gemological Institute of America (GIA) described moonstone’s composition as having “alternating parallel planes of potassium- and sodium-rich feldspars forming an assemblage called a microperthite.”

• Rainbow Lattice Sunstone: Extremely rare orthoclase variety from Australia with aventurescence caused by iron oxide inclusions and adularescence reportedly caused by “exsolution lamellae (cryptoperthite).”

Phew, that covers all the varieties. Now let’s get into perthite’s history!

Pictured above: Rainbow lattice sunstone

Perthite History

Perthite was first discovered around 1841 in Ontario, Canada. Scottish-Canadian physician and amateur mineralogist Dr. James Wilson made the discovery.

Dr. Wilson is also credited with discovering the minerals wilsonite (named after him) and peristerite (an albite variety).

Since perthite was named after Perth or Perth County (also in Ontario), many publications incorrectly state its discovery happened there. However, Dr. Wilson actually found it in Burgess Township (now Tay Valley Township), which is about 8.5 kilometers (~5 miles) south of Perth.

Dr. Wilson sent his original specimens to a Chemistry professor at the University of Glasgow, Dr. Thomas Thomson, for analysis. Thomson published a description of the new mineral (containing a somewhat botched analysis) and named it “perthite” in 1843, since Wilson lived in Perth.

From 1847 to 1852, Canadian geologist Thomas Sterry Hunt published three papers with more accurate analyses of perthite. Unfortunately, he accidentally listed the location as Bathurst Township in two papers.

The original perthite specimens contained microcline, but it was called “orthoclase” early on, since microcline wasn’t officially described until German mineralogist August Breithaupt did so in 1876. The presence of microcline in perthite was confirmed by American mineralogist James Dwight Dana in 1882.

Pictured above: Amazonite cabochon

Perthite Healing Properties

As a primarily brown and white healing stone, perthite’s meaning reflects the cleansing and grounding properties of white and brown gemstones, respectively. It can be used as a chakra stone for aligning all chakras.

Additionally, perthite crystals are said to promote harmony, new beginnings, and positivity.

Perthite Gemstone Properties

Besides the gemstone varieties — rainbow lattice sunstone, moonstone, and amazonite — most perthites aren’t used as gemstones, so there aren’t standard grading criteria. We’ll still go over how the common value factors apply to perthite, though:

• Color: Since many perthites have somewhat boring white colors, any with brighter colors like green or red may have higher value. The presence of optical effects like adularescence, iridescence, and aventurescence also boosts value.

• Cut: Perthite is rarely to never faceted, unless it’s a gem variety like moonstone or rainbow lattice sunstone. Most perthites are rough (uncut) or cut into cabochons and beads.

• Size: Cut perthites may cost more in larger sizes. The largest single perthite crystal ever documented, found in South Dakota’s Hugo Mine, was 35 ft x 15 ft x almost 6 ft.

Pictured above: Perthite unmixing (the gray irregular spots) in microcline with cross-hatched (or tartan) twinning. Crossed nicols image, magnification 10x (Field of view = 2mm) | Image credit: Strekeisen, CC-BY-SA-4.0

Perthite Formation & Sources

Perthite minerals form when a homogenous, melted mixture of alkali and plagioclase feldspar cools slowly. The cooling causes the feldspars to separate (exsolve).

What type of rock is perthite found in? Perthites are common in granite pegmatites and igneous rocks like dalmation stone and balgarite.

Mining Locations

Cuttable perthite specimens are found in various locations in Canada. Other significant sources include:

• Arizona, USA

• South Dakota, USA

• Zimbabwe

• Norway

Onto prices!

Pictured above: Moonstone cabochon

Perthite Gemstone Price & Value

Most perthite gemstones for sale are rough (uncut) or only polished.

You can find nice peach-colored, palm size, rough perthites for around $10-$15 each. Similar polished specimens are generally $20 to $30.

Attractive specimens from Perth may be slightly higher, around $40 to $50 each.

You can also find lovely perthite necklaces for around $15.

Perthite Care and Maintenance

Luckily, gemstone care for perthite isn’t too difficult. The main considerations are its perfect cleavage, so we recommend protective settings for perthite jewelry.

You can clean perthite with warm water, mild soap, and a soft toothbrush.

Store separately from other gems to avoid scratches.

Is Perthite Perfect For You?

There are lots of feldspars out there — they’re the most common mineral in Earth’s crust, after all. Perthite offers a beautiful mixture of them and a reminder that none of us are just one thing.

Buy perthite and more beautiful gemstones today!