Short answer: feldspar is not one mineral but a group of aluminum silicates that together make up more than half of Earth's crust. Almost all of it falls on two chemical series: the alkali feldspars (potassium to sodium) and the plagioclase feldspars (sodium to calcium). Every member sits near Mohs 6 to 6.5 and breaks along two cleavages close to 90 degrees. Moonstone, labradorite, sunstone and amazonite are simply the members that happen to do something beautiful with light.
Why is feldspar a family and not a single mineral?
Pick up a piece of pink granite and the pink blocks are feldspar. Pick up a gray basalt and the pale specks are feldspar too, but a chemically different one. Geology.com puts feldspars at over 50% of Earth's crust and gives the group a generalized formula of X(Al,Si)4O8, where X is usually potassium, sodium or calcium and rarely barium, rubidium or strontium.
The reason for so many names is substitution. In a cooling melt, sodium and potassium trade places freely in the crystal framework, and so do sodium and calcium (with aluminum swapping for silicon to keep the charges balanced). The result is a continuum, and mineralogists draw it as a triangle with three corners:
- Orthoclase corner: KAlSi3O8, potassium feldspar.
- Albite corner: NaAlSi3O8, sodium feldspar.
- Anorthite corner: CaAl2Si2O8, calcium feldspar.
The left edge of that triangle (potassium to sodium) is the alkali feldspar series. The bottom edge (sodium to calcium) is plagioclase. The space in between, potassium plus calcium, is mostly empty, because those two do not mix well at crustal temperatures.
How are the feldspars divided?
The alkali feldspars
Three minerals share the exact formula KAlSi3O8 and differ only in how ordered their atoms are. Geology.com explains that temperature decides which one grows: sanidine is the high-temperature form, orthoclase the intermediate one, and microcline the low-temperature form. Sanidine and orthoclase are monoclinic; microcline is triclinic. Minerals.net notes that the three are so alike that sometimes it is impossible to tell them apart without x-ray analysis, which is why field geologists just write "potassium feldspar" or "K-spar."
Between pure potassium and pure sodium sits anorthoclase, (Na,K)AlSi3O8. Sanidine is typical of quickly cooled volcanic rocks, orthoclase of granites, and microcline of pegmatites and slowly cooled rocks.
The plagioclase series
Plagioclase is named in steps by how much calcium has replaced sodium. These cut-offs are arbitrary labels on a smooth slope:
| Plagioclase member | Albite (Na) share | Anorthite (Ca) share | Gem connection |
|---|---|---|---|
| Albite | 90 to 100% | 0 to 10% | Peristerite moonstone |
| Oligoclase | 70 to 90% | 10 to 30% | Classic hematite sunstone |
| Andesine | 50 to 70% | 30 to 50% | Red "andesine" controversy |
| Labradorite | 30 to 50% | 50 to 70% | Labradorescence, Oregon sunstone |
| Bytownite | 10 to 30% | 70 to 90% | Rare pale yellow facet gems |
| Anorthite | 0 to 10% | 90 to 100% | Lunar highland rock |
Percentages from Geology.com's plagioclase page. The International Mineralogical Association recognizes only the end members as species; Minerals.net notes that Dana's System of Mineralogy still lists the intermediate names as individual minerals, and the trade uses them constantly.
The rare corners
Beyond these two series are oddities a collector may meet on a label: celsian (a barium feldspar, BaAl2Si2O8), hyalophane (potassium and barium), buddingtonite (an ammonium feldspar found where hydrothermal fluids altered other feldspars) and rubicline, a rubidium feldspar that resembles microcline.
How do you recognize feldspar in a rock or specimen?
Feldspar is the most common mineral most people cannot name. These checks work with a 10x loupe and a steel knife.
The right-angle cleavage test
Turn a broken piece under a lamp. Feldspar breaks along two planes that meet at or close to 90 degrees, and both planes flash at once when the angle is right. The name orthoclase joins the Greek words for "right" and "to cleave," a nod to that square break, according to Webmineral. Quartz, the mineral feldspar is most often mistaken for, has no cleavage at all and breaks with curved, glassy fractures.
The striation test
This is how to separate the two families by eye. On a plagioclase cleavage face, look for fine, perfectly straight parallel lines, like the grooves on an old record. They are the edges of repeated twin layers. Minerals.net notes that potassium feldspars do not show these striations on twinned surfaces, while plagioclase may. Pink blocks with no lines in a granite are almost always potassium feldspar; white or gray grains with lines are plagioclase.
Hardness and weight
Feldspar sits between steel and quartz. Orthoclase is the reference mineral for hardness 6 on the Mohs scale, so a pocketknife will barely mark it while quartz (7) will scratch it. Specific gravity across the group runs about 2.5 to 2.8, and it climbs with calcium: Webmineral lists orthoclase at 2.56, albite at 2.61 to 2.63, and anorthite at 2.72 to 2.75. A labradorite sphere therefore feels a touch heavier than a moonstone sphere of the same size.
Perthite stripes
Some potassium feldspar carries wavy or patchy white ribbons of albite running through it. That intergrowth is called perthite, and Minerals.net describes it as microcline and albite in alternating patterns. Perthite is common in pegmatites and explains the faint white streaks in a lot of amazonite. Moonstone is the same idea at a much finer scale: layers so thin they scatter light instead of showing as stripes.
Which feldspars are gems, and why?
The gem feldspars are a small group of members that host a microscopic structure or inclusion that plays with light. Each has its own guide, so this is the map rather than the detail.
| Gem name | Feldspar underneath | What you see | Guide |
|---|---|---|---|
| Moonstone | Orthoclase plus albite layers (sometimes sanidine or albite alone) | Soft floating glow, adularescence | Moonstone |
| "Rainbow moonstone" | Labradorite | Multicolor glow on a pale body | Labradorite vs moonstone |
| Labradorite, spectrolite | Labradorite | Bright flashes of blue, green, gold | Labradorite |
| Sunstone | Oligoclase, labradorite or orthoclase | Glittering spangles, aventurescence | Sunstone |
| Amazonite | Microcline | Opaque blue-green body color | Amazonite |
Two points on that table surprise buyers. First, GIA states that the stone sold as rainbow moonstone is actually a variety of labradorite rather than orthoclase, and adds that to be called moonstone, a mineral's identity matters less than the beauty of its adularescence. Second, "sunstone" is a trade name defined by sparkle, not chemistry, which is why the sunstone guide spends so much time on which feldspar a given sunstone really is.
Transparent feldspar without any optical effect is also cut, but only for collectors. Geology.com points out that a hardness of 6 to 6.5 plus two perfect cleavages give faceted feldspars durability concerns, and that well-formed crystals are usually worth more as specimens than as cutting rough.
Where does feldspar form and where do collectors find it?
Most feldspar crystallizes straight from magma. Orthoclase dominates granite and rhyolite, plagioclase dominates basalt and gabbro, and both survive into gneiss and schist when those rocks are metamorphosed. The giants grow in pegmatites: Geology.com reports that the largest reported orthoclase crystal, from the Ural Mountains of Russia, was over 30 feet long and weighed about 100 tons.
For specimen collectors the classic localities are pegmatite districts. Minerals.net lists large microcline crystals from the Erongo Mountains of Namibia, the Shigar Valley of Pakistan, and Mount Malosa in Malawi (with black aegirine), and famous twins from Baveno, Italy. Its albite entry singles out bladed clevelandite from Minas Gerais, Brazil, and clear crystals from Amelia Court House, Virginia.
Feldspar also reaches well beyond Earth. Apollo 15 astronauts collected the "Genesis Rock," which Geology.com describes as made up almost entirely of anorthite and about 4 billion years old, and orbital data suggest plagioclase is the most abundant mineral in the crust of Mars.
What happens when feldspar weathers?
Feldspar is less durable than quartz, so it breaks down as rocks erode. Chemical weathering turns orthoclase into kaolinite clay, the raw material of porcelain, and feldspar grains thin out downstream as sand matures. A sandstone with at least 25% feldspar is called arkose, a sign it was deposited close to its granite source.
What is feldspar used for besides gems?
Almost all mined feldspar ends up in glass and ceramics. The USGS explains that in glassmaking feldspar supplies alumina for hardness, durability and resistance to chemical corrosion, and in ceramics it acts as a flux that lowers the temperature at which a ceramic body vitrifies. Geology.com puts United States production at about 600,000 metric tons a year, mainly from quarries in North Carolina, Idaho, California, Virginia, Oklahoma and South Dakota. That industrial scale is why a polished moonstone is inexpensive compared with gems of similar beauty: the family itself is anything but rare.
How should feldspar be handled and stored?
Treat every feldspar as a stone that can split. The same right-angle cleavage that helps identification also means a sharp knock on a hard floor can cleave a sphere or pop a corner off a freeform. Store pieces apart from quartz, which is harder and will scratch polished feldspar surfaces in a shared bag. Room temperature water and a soft cloth are fine for polished pieces; skip ultrasonic cleaners and sudden heat, which can open cleavage cracks. The individual guides cover the quirks of each gem, such as the surface-reaching fractures common in labradorite slabs.
In crystal traditions the family as a whole is rarely discussed; its members carry their own associations, with moonstone linked to the moon and sunstone to the sun.
Choosing feldspar pieces
Decide first whether you want the optical effect or the mineral. For the effect, buy in person or from listings with video, because labradorescence and adularescence switch on only at certain angles and still photos can flatter or hide them. For the mineral, look for a label that names a locality and, ideally, the actual feldspar species. These in-stock pieces cover the main gem members:
- Rainbow labradorite spectrolite specimen, 1.60 lb, Madagascar: a 726 g partly polished plagioclase piece, large enough to see striations and color flash on the same stone.
- Labradorite Crystal Freeform Polished Blue Flash Madagascar: a freeform cut to put the blue flash face-on, a good test of how orientation drives the effect.
- Rainbow Moonstone Sphere: a sphere of the pale labradorite sold as rainbow moonstone; turn it under one light to find the multicolor glow.
- Peach Moonstone Sphere: moonstone with a warm body color; the sheen is softer and more diffuse than labradorite flash.
- Sunstone: look for the glittering inclusion platelets that define the trade name.
- Amazonite: microcline colored blue-green, often with white perthite streaks that confirm it as feldspar rather than dyed stone.
Digital Towns Market
Feldspar gems and specimens in stock
Frequently asked questions
Is feldspar a crystal or a rock?
Feldspar is a mineral group, and its members form true crystals. Rocks such as granite, basalt and anorthosite are made largely of feldspar grains mixed with other minerals. A stone sold as "feldspar" is usually a cleavage chunk or crystal of potassium feldspar or plagioclase, often from a pegmatite.
What is the difference between plagioclase and orthoclase?
Orthoclase is a potassium feldspar; plagioclase is a sodium to calcium series. Both have two cleavages near 90 degrees and a hardness around 6. The practical tell is fine parallel striations on plagioclase cleavage faces, which orthoclase lacks. Plagioclase also runs slightly heavier as calcium rises.
Is moonstone a feldspar?
Yes. Classic moonstone is orthoclase with very thin layers of albite inside, and the layers scatter light into a soft glow. Some moonstone is albite or sanidine, and the stone sold as rainbow moonstone is labradorite, a plagioclase. All of them belong to the feldspar group.
How hard is feldspar?
Most feldspars rate 6 to 6.5 on the Mohs scale, and orthoclase is the reference mineral for hardness 6. That makes feldspar harder than glass in most cases but softer than quartz, so quartz dust and other quartz stones can scratch polished feldspar over time.
Why is feldspar important?
It makes up more than half of Earth's crust, so it shapes the rocks and soils around us. Industrially it is a key ingredient in glass and ceramics, where it adds alumina and lowers firing temperatures. For collectors it supplies four popular gem families.
Sources
- Geology.com, Feldspar: https://geology.com/minerals/feldspar.shtml
- Geology.com, Plagioclase Feldspar: https://geology.com/minerals/plagioclase.shtml
- Geology.com, Orthoclase: https://geology.com/minerals/orthoclase.shtml
- Minerals.net, Microcline: https://www.minerals.net/mineral/microcline.aspx
- Minerals.net, Albite: https://www.minerals.net/mineral/albite.aspx
- Minerals.net, Plagioclase: https://www.minerals.net/mineral/plagioclase.aspx
- Webmineral, Orthoclase Mineral Data: http://webmineral.com/data/Orthoclase.shtml
- Webmineral, Albite Mineral Data: http://webmineral.com/data/Albite.shtml
- Webmineral, Anorthite Mineral Data: http://webmineral.com/data/Anorthite.shtml
- GIA, Moonstone Description: https://www.gia.edu/moonstone-description
- U.S. Geological Survey, Feldspar Statistics and Information: https://www.usgs.gov/centers/national-minerals-information-center/feldspar-statistics-and-information