How to identify feldspar
Six steps, in order: cleavage flash, hardness 6 to 6.5, a near-90 degree angle, density under 2.8, striations, context. Plus what you can and cannot call it.
To identify feldspar in a rock or hand specimen: find two flat cleavages that flash together as you turn it, near 90 degrees apart; confirm hardness 6 to 6.5, marked by a hardened file but not a steel point; check density below 2.8. Parallel striations then mean plagioclase; a tartan grid means microcline. Most specimens stop at feldspar.
In short
- Start with the flash, not the colour. Feldspar has two good cleavages, the better one perfect. Turn a fresh surface under a single lamp and flat facets light up together, then go dark together. Quartz, the usual confusion, has cleavage the Handbook calls rarely observable, and a conchoidal fracture.
- Hardness 6 to 6.5, and a cleavage angle near 90 degrees. Of the 176 Handbook sheets in this site's reference set, 7 print a cleavage intersection angle. Two are feldspars at 90 degrees; two are pyroxenes at about 87 and 88; two are amphiboles at 56 and 124; one is kyanite at 79.
- Density parts feldspar from pyroxene cleanly. The six feldspars in the set run 2.54 to 2.76; diopside starts at 3.22 and enstatite at 3.2. On a 20.00 g fragment that is 12.75 g in water at the heaviest feldspar against 13.75 g at the lightest pyroxene, a full gram.
- Striations sort the family. Fine parallel lines on the best cleavage mean polysynthetic albite-law twinning: plagioclase. A crossed grid on (001) means microcline. Wavy exsolution bands, perthite, mean an alkali feldspar with albite inside it.
- The finish state is usually just feldspar, and that is a correct answer. Orthoclase and microcline overlap in hardness and density; labradorite and three other plagioclase names are not on the September 2026 IMA list at all. The ladder of what you may write on the label is in the table below.
| Species | Hardness | Density, measured | Cleavage as recorded | Twinning as recorded | Optical class, 2V |
|---|---|---|---|---|---|
| Microcline | 6 to 6.5 | 2.54 to 2.57 | {001} and {010} perfect, at about 90 degrees | Orthogonal grid on (001); Carlsbad, Baveno, Manebach | Biaxial (-), 66 to 103 degrees |
| Orthoclase | 6 to 6.5 | 2.55 to 2.63 | {001} and {010} perfect | Carlsbad, Baveno, Manebach; no grid | Biaxial (-), 35 to 75 degrees |
| Sanidine | 6 | 2.56 to 2.62 | {001} perfect, {010} distinct | Carlsbad common; Baveno, Manebach rarer | Biaxial (-), 18 to 42 degrees low |
| Albite | 6 to 6.5 | 2.60 to 2.65 | {001} perfect, {010} very good | Polysynthetic striae on {001} or {010} | Biaxial (+) low, 85 to 90 degrees |
| Labradorite | 6 to 6.5 | 2.68 to 2.72 | {001} perfect, {010} less so, at 90 degrees | Albite, Pericline, Carlsbad and others; iridescence on {010} | Biaxial (+), 78 to 87 degrees low |
| Anorthite | 6 to 6.5 | 2.74 to 2.76 | {001} perfect, {010} less so | Commonly polysynthetic, Albite law | Biaxial (-), 78 to 83 degrees |
| Quartz | 7 | 2.65 | Rarely observable, poor | Dauphiné, Brazil, Japan laws; no striae recorded | Uniaxial (+) |
| Diopside | 5.5 to 6.5 | 3.22 to 3.38 | {110} distinct, at about 87 degrees | Not a feldspar test | Not used here |
How to identify feldspar in a rock: the six steps, in order
Day one needs a fresh broken surface, a single lamp, a hand lens and a hardened steel file. A weathered face hides cleavage and reads soft. The steps below run from free to fiddly, and each one either rules out a look-alike or narrows the family.
1. Find the flash. Rotate the specimen under one light. Feldspar's perfect cleavage produces flat, stepped, mirror-like facets that light up together at one angle. Grey glassy grains that never flash and break with curved shell-like surfaces are quartz; the Handbook of Mineralogy records quartz cleavage as rarely observable and its fracture as conchoidal. In a granite this one look usually separates the two main light-coloured minerals.
2. Hardness. Feldspar is 6 to 6.5, sanidine 6. It should not be marked by a steel point at about 5.5, and a hardened file at about 6.5 should mark it. Quartz at 7 resists the file. Test on a fresh face, and read the scratch as described on how to test mineral hardness properly.
3. The angle. Look for a second cleavage at roughly a right angle to the first. Counting across all 176 sheets in this site's reference set, 7 print a cleavage intersection angle at all: microcline at about 90 degrees and labradorite at 90; diopside about 87 and enstatite about 88; tremolite and actinolite 56 and 124; kyanite 79. A 56-and-124 pair is an amphibole, not a feldspar.
4. Density. The six feldspar sheets run from 2.54 to 2.76. The pyroxenes that share the right angle start at 3.2. Worked: a 20.00 g fragment weighs 12.75 g in water at anorthite's 2.76 and 13.75 g at enstatite's 3.2. Below 2.8 is feldspar; 3.2 and above is pyroxene. Quartz at 2.65 sits inside the feldspar range, which is why step 1 comes first. Method on how to measure specific gravity at home.
5. Striations, grid or bands. On the best cleavage face, under the lens, tilted to catch the light: parallel ruled lines are polysynthetic albite-law twinning and mean plagioclase; the albite sheet records polysynthetic striae on {001} or {010}. An orthogonal grid on (001) means microcline. Irregular wavy bands of a second feldspar are perthite, which the microcline sheet records as banded perthitic intergrowths from exsolution of albite. Twin laws in general are on mineral twinning explained.
6. Context. The occurrence fields are strong priors: sanidine is most common in volcanic rocks such as rhyolites and trachytes; microcline and orthoclase in granites, pegmatites and syenites; labradorite in mafic igneous rocks and anorthosites; anorthite the Handbook calls a rare constituent of mafic rocks.
How to identify feldspar in hand specimen: the finish state
A hand determination of feldspar ends on a ladder, and most specimens stop on its first rung. What each result lets you write on the label:
Steps 1 to 4 pass: write feldspar. That is a determination, not a failure.
Parallel striations seen: write plagioclase. Which plagioclase is a composition question. Labradorite's sheet records iridescence especially on {010}, so a striated grey feldspar with a blue flash is commonly called labradorite; but the orthoclase sheet records that it may exhibit opalescence or schiller iridescence and the albite sheet that it may be chatoyant, so a sheen alone is not a determination. The September 2026 IMA-CNMNC Master List lists albite and anorthite and has no entry for labradorite, andesine, oligoclase or bytownite.
Grid seen: write microcline. Perthite seen: write alkali feldspar. No striations and no grid: still feldspar. Absence proves nothing, because lamellae can be too fine for a lens and a Carlsbad twin makes one seam, not a set.
What you cannot call it in hand specimen: orthoclase rather than microcline without a visible grid, since they share hardness 6 to 6.5 and their density ranges overlap; sanidine rather than orthoclase, where the volcanic setting is a strong hint and not a test; or a named plagioclase composition. That is the limit the orthoclase and microcline pages state from the other side, and this method is where it comes from.
How to identify feldspar in thin section or under a microscope
Under the microscope, colour disappears and the numbers in the table's last column take over. Five of the six feldspar sheets record the mineral as colorless in thin section; albite's does not say. What the sheets do record for each is an optical class and a measured 2V: quartz is uniaxial positive; the feldspars are biaxial, with measured 2V running from 15 to 63 degrees in high sanidine to 66 to 103 in microcline.
This page stops there on purpose. Using those properties means a petrographic microscope with crossed polars and a prepared thin section, and teaching that method properly is a course, not a paragraph. We will not compress it into steps we cannot source. What a collector can take from the table is that the tartan grid, too fine for a lens on many specimens, is the thing a thin section shows, and that a stereo microscope on a hand specimen is still a hand-specimen method. Choosing one is on the best microscope for mineral specimens.
The whole identification sequence, for any species, is on how to identify a mineral specimen, and the rest of this section is indexed at the collecting guides. Nothing here is offered for sale; the wanted list is the only commercial route, and twinned feldspar with a named locality is on it.