Types of Feldspar: List of Varieties, Properties & Prices

Feldspars are among the most common minerals on Earth, and many of them are gorgeous crystals. If you’ve ever heard of gems like labradorite, orthoclase, amazonite, or moonstone, you already know some types of feldspar gemstones.

If any of those surprised you, just wait. In this guide, we’ve gathered our knowledge from decades in the gemstone industry to bring you a complete list of the types of feldspar with pictures, their properties, prices, benefits, and more.

Pictured above: Spectrolite cabochon (subtype of labradorite feldspar)

About Feldspar

First off, what is feldspar? “Feldspar” is the term for a group of minerals that are all over Earth — in fact, they make up around 60% of Earth’s surface. Some of these minerals are used as semi-precious gemstones.

Different types of feldspar are important to mineralogists and geologists, as they tell lots of stories about the rocks they formed in. Feldspar is also important to various industries. The main feldspar uses are in making ceramics, glass, abrasives, and fillers for products like plastic or paint. Many of these applications can also use synthetic feldspar instead of just natural feldspar.

If you love commemorative gems, you may know that the feldspar gem moonstone is a June birthstone, a Cancer zodiac stone, and a third wedding anniversary gemstone. The feldspar variety labradorite is also an alternative November birthstone.

Back to the mineral side, let’s explore the science behind these types of feldspar gems.

Feldspar Specifications & Characteristics

All feldspar minerals are aluminum tectosilicates. The two overarching categories of feldspars are plagioclase feldspars and alkali feldspars.

The plagioclase feldspar group is also called the albite and anorthite system. Plagioclase feldspars are all sodium calcium aluminosilicates, but the level of calcium and sodium varies in each, ranging from at least 90 percent sodium and almost no calcium (albite) to at least 90 percent calcium and almost no sodium (anorthoclase).

Alkali feldspars all contain sodium and/or potassium, though they’re usually higher in potassium. Technically, the alkali feldspar group also includes albite, as it’s in the middle of the two groups, along with ferrisanidine (iron potassium silicate) and rubicline (rubidium aluminosilicate).

However, the main subgroup of the alkali family is the K-feldspars or potassium feldspars. Minerals in this subgroup all have the same potassium aluminosilicate formula (with some sodium substituting in sanidine at times) but different crystal systems.

You can think of plagioclase as the sodium-calcium feldspars and alkali as the potassium-sodium feldspars.

Feldspar Properties List

Soon, we’ll go over how the properties among different types of feldspar differ. But first, let’s look at the general properties that all types of feldspar minerals share:

• Mohs hardness: 6-6.5

• Color: Varies by type (see list in next section)

• Crystal structure: Monoclinic (orthoclase & sanidine); Triclinic (microcline, anorthoclase, and all plagioclase feldspars)

• Luster: Vitreous (glassy), sometimes pearly on cleavages

• Transparency: Transparent to opaque

• Refractive index: Overall range: 1.516-1.590; Varies by type (see list in next section)

• Density: Overall range: 2.62-2.76 (plagioclase), 2.54-2.62 (alkali); Varies by type (see list in next section)

• Cleavage: Perfect on {001} & good/distinct on {010} (all types), intersecting at roughly 90 degrees (all but anorthoclase)

• Fracture: Irregular/uneven to conchoidal

• Streak: White

• Luminescence: Varies by type (see list in next section)

• Pleochroism: Rarely in orthoclase (shades of yellow in transparent yellow stones); Weak to none for all other feldspars

• Birefringence: Overall range: 0.003-0.013; Varies among types (see list below)

• Dispersion: Weak to none in all but orthoclase, which is distinct to relatively strong

Pictured above: Rainbow lattice sunstone cabochon (a subtype of orthoclase feldspar)

Types of Potassium (Alkali) Feldspar Gemstones

The first family of feldspar gemstones we’ll examine is the potassium feldspar or alkali feldspar group.

This group includes:

1. Orthoclase

2. Microcline

3. Anorthoclase

4. Sanidine

Let’s look at each type of alkali feldspar more closely, exploring everything from their compositions and colors to their optical effects and subtypes.

Orthoclase

Orthoclase is one of the three major types of feldspar, both in mineralogy and gemology. The name derives from the Ancient Greek orthos, meaning “straight,” and klasis, meaning “fracture,” after its two 90-degree cleavage directions.

The properties for orthoclase are:

• Composition: Potassium aluminum silicate; K(AlSi3O8)

• Crystal structure: Monoclinic

• Birefringence: 0.004-0.005

• Refractive index: 1.52-1.54

• Density: 2.56-2.59

• Colors: Colorless, white, gray, black, yellow, orange, pink, red, green, brown

• Luminescence: Sometimes fluorescence, phosphorescence, and X-ray colors — white, red, pinkish-red, reddish-orange, or orange in SW-UV & white, blue, pink, red, or reddish-orange in LW-UV & violet or white in X-rays

• Optical effects: Sometimes adularescence, aventurescence (schiller)

• Subtypes: Moonstone, Sunstone (including Rainbow Lattice Sunstone), Cat’s Eye Orthoclase, Delawarite

• Other / historical names: Paradoxite, Argillyite, Common Feldspar, Muldan, Murchosonite, Cottaite, Leelite

One important note is that moonstone isn’t solely orthoclase; it’s actually alternating layers of orthoclase and albite, which give the gem its characteristic internal glow.

Also, there are two types of feldspars that can be sunstone: orthoclase sunstone and oligoclase sunstone. Both sunstones display their metallic glittering from inclusions of hematite and/or goethite platelets. But the refractive index in orthoclase sunstones is lower. One of the best types of orthoclase sunstone is the rainbow lattice sunstone from Australia.

Microcline

Pictured above: Amazonite cabochon

Microcline is a common feldspar known for forming some of the biggest crystals of any mineral, though it’s not seen much in the gem sphere. The name comes from the Greek mikron, meaning “little,” and klinein, meaning “to incline,” in reference to microcline’s slightly-less-than-90-degree cleavage intersections.

Here are the microcline properties to know:

• Composition: K(AlSi3O8); Potassium aluminum silicate

• Crystal structure: Triclinic

• Birefringence: 0.007-0.010

• Refractive index: 1.52-1.53

• Density: 2.54-2.57

• Colors: White, gray, grayish-yellow, yellow, tan, orange, pink, bluish-green, green

• Luminescence: Sometimes fluorescence & X-ray colors — yellow-green in LW-UV, cherry-red in SW-UV, green in X-ray

• Optical effects: Sometimes aventurescence

• Subtypes: Amazonite, Chesterlite, Ferruginous Microcline, Hyalophane, and Perthite

• Other / historical names: None

The main type of microcline gemstone is amazonite, a green to bluish microcline variety often found with white splotches. Amazonite can also display aventurescence.

Microcline and orthoclase are very similar, but microcline is a medium-low triclinic polymorph with a highly ordered internal structure while orthoclase is a high-temperature monoclinic polymorph with a partially ordered internal structure.

Anorthoclase

Pictured above: Anorthoclase specimen | Image credit: James St. John, CC-BY-SA-2.0

Next up on the potassium feldspar list is anorthoclase. The name “anorthoclase” derives from the same Ancient Greek as orthoclase, but with the prefix an- meaning “not,” as anorthoclase does not have the right-angle cleavage of orthoclase.

Check out the properties unique to anorthoclase below:

• Composition: (Na,K)AlSi3O8; Sodium or potassium aluminum silicate

• Crystal structure: Triclinic

• Birefringence: 0.008-0.009

• Refractive index: 1.52-1.54

• Density: 2.57-2.60

• Colors: White, colorless, gray, grayish-pink, yellow, green

• Luminescence: Sometimes fluorescence — pink or weak bluish-white in SW-UV, moderate to strong bluish-white in LW-UV

• Optical effects: Sometimes adularescence

• Subtypes: None

• Other / historical names: Analbite, Anortoclasio, Mikroklas, Pseudo-Orthoclase, Pantellarite, Soda-Microcline, Anothose

Adularescence, the optical phenomenon behind moonstone, can occasionally be present in anorthoclase but moonstones are most often a mixture of albite and orthoclase.

Sanidine

Image credit: Rob Lavinsky, iRocks.com – CC-BY-SA-3.0

Sanidine is another type of feldspar not often seen in the gem sphere. This feldspar gets its name from the Greek sanis, meaning “little tablet,” and idos, meaning “to appear,” for sanidine’s commonly tabular, flattened crystal habit.

Let’s take a look at sanidine’s properties:

• Composition: K(AlSi3O8); Potassium aluminum silicate; Note: Can contain 30 to 62% sodium

• Crystal structure: Monoclinic

• Birefringence: 0.003-0.007

• Refractive index: 1.52-1.53

• Density: 2.52-2.62

• Colors: Colorless, white, yellow, brown

• Luminescence: Rarely fluorescence — weak red-orange in LW-UV & SW-UV

• Optical effects: Rarely adularescence

• Subtypes: High Sanidine, Low Sanidine, Barium-Sanidine, Ferrisanidine, Azulicite, Citron Feldspar

• Other / historical names: Glassy Feldspar, Rhyacolite, Riacolite, Gränzerite

The subtypes of sanidine can be grouped into three categories: 1) mineral forms (high vs low sanidine), 2) impurity-based (barium-sanidine & ferrisanidine), and 3) gemstone varieties (azulicite & citron feldspar).

It can be easy to mix up sanidine and orthoclase. The three main differences are their formation temperatures (high for sanidine, lower medium for orthoclase), crystal ordering (disordered for sanidine, ordered for orthoclase), optic angle (higher in orthoclase), and transparency (usually clear for sanidine and cloudy for orthoclase).

Pictured above: Faceted Oregon sunstone pendant (subtype of labradorite)

Types of Plagioclase Feldspar Gemstones

The other major category of feldspar gemstones is the plagioclase feldspar family.

The key feature of plagioclase feldspars compared to alkali feldspars is the presence of striations. Cleavage faces on alkali feldspars are smooth, whereas plagioclase feldspars have etched grooves called striations on at least one of their cleavage planes.

The types of plagioclase feldspar gemstones include:

1. Albite

2. Oligoclase

3. Andesine

4. Labradorite

5. Bytownite

6. Anorthite