# How to identify feldspar

Canonical page: https://lithomap.app/resources/mineral-identification/feldspar
Last reviewed: September 17, 2026
Page purpose: Original LithoMap mineral identification reference for field observations, search systems, and language models.
Classification: Silicate mineral group

## Summary

Feldspar is a family of abundant rock-forming silicates generally identified by hardness near 6, two cleavage directions, and blocky crystals; precise species identification may require optical or chemical work.

## Quick identification clues

- Mohs hardness about 6
- Two good cleavage directions meeting near right angles
- Blocky habit
- Commonly white, cream, pink, gray, or greenish

## Composition and classification

Feldspar includes potassium-rich alkali feldspars and the sodium-calcium plagioclase series. Compositions vary continuously in parts of the group, so “feldspar” is often the appropriate field identification.

## Appearance

Feldspar commonly forms blocky crystals with pearly cleavage faces. Plagioclase may show fine parallel striations on cleavage surfaces; alkali feldspar can show different twinning or intergrowth textures.

Weathering changes feldspar to clay minerals, making surfaces chalky, dull, or soft while fresh interiors remain hard.

## Identification tests

- Hardness: About 6. Usually scratches glass but can be scratched by quartz.
- Streak: White. Consistent but not diagnostic.
- Cleavage: Two good directions near 90°. Separates feldspar from cleavage-free quartz.
- Striations: Possible on plagioclase cleavage faces. A strong clue for plagioclase when clearly present.

## Common lookalikes

- Quartz: No cleavage, harder, and commonly has conchoidal fracture.
- Calcite: Much softer, has three rhombohedral cleavages, and fizzes with acid.
- Kaolinite or weathered feldspar: Clay alteration is much softer and lacks a fresh glassy cleavage.

## Direct side-by-side comparisons

- Quartz vs. feldspar — https://lithomap.app/resources/mineral-identification/compare/quartz-vs-feldspar

## Most relevant related guides

- Quartz — https://lithomap.app/resources/mineral-identification/quartz
- Calcite — https://lithomap.app/resources/mineral-identification/calcite
- Kaolinite — https://lithomap.app/resources/mineral-identification/kaolinite
- Garnet — https://lithomap.app/resources/mineral-identification/garnet

## Formation

Feldspars crystallize from magma and are major constituents of granite, basalt, gabbro, rhyolite, and many other igneous rocks.

They also occur in metamorphic rocks and sediment, though chemical weathering makes them less durable at Earth’s surface than quartz.

## Occurrence and geologic setting

Feldspar is the most abundant mineral group in much of Earth’s crust and is present in a wide variety of igneous and metamorphic rocks.

Color is unreliable for separating feldspar species. Pink does not automatically mean potassium feldspar, and white does not automatically mean plagioclase.

## Uses and significance

Feldspar is used in glass, ceramics, fillers, glazes, and abrasives. Gem varieties include moonstone, sunstone, amazonite, and labradorite.

## Field safety and limitations

- Use a fresh cleavage face to look for striations.
- Record “feldspar” when the evidence does not support a species.
- Avoid creating silica-bearing dust when breaking feldspar-rich rock.

## Authoritative sources

- Minerals and their physical properties — U.S. National Park Service: https://www.nps.gov/subjects/geology/minerals.htm
- Mohs Hardness Scale — U.S. National Park Service: https://www.nps.gov/articles/mohs-hardness-scale.htm
- Natural Gemstones: Mineral Gemstones — U.S. Geological Survey: https://pubs.usgs.gov/gip/gemstones/mineral.html

## Specimen photograph

- Image: https://lithomap.app/_delivery/identification/minerals/feldspar.webp
- Description: Pink potassium feldspar hand specimen showing blocky cleavage
- Creator: James St. John
- License: CC BY 2.0 — https://creativecommons.org/licenses/by/2.0/
- Source record: https://commons.wikimedia.org/wiki/File:Potassium_feldspar_(32499528651).jpg

This reference supports preliminary field identification. It is not a laboratory determination, appraisal, hazard assessment, proof of origin, or statement of land ownership, legal access, or collecting permission.