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Cleavage and Fracture Test identification and geology

Cleavage and fracture describe how a mineral breaks, and together they are among the most powerful diagnostic physical properties in mineralogy. Cleavage is the tendency to break along flat planes of weakness within the crystal structure, corresponding to directions of weakest atomic bonding. Fracture describes the way a mineral breaks when cleavage is absent or when the break does not follow a cleavage plane. Cleavage is described by the number of cleavage directions, the angles between them, and the quality perfect, good, fair, poor . These parameters are directly determined by crystal structure and are therefore consistent for a given mineral species. For example, halite always has three perfect cleavage directions at 90° cubic cleavage , micas have one perfect basal cleavage, and calcite has three perfect rhombohedral cleavage directions at approximately 75° and 105°. The angle between cleavage directions is especially diagnostic: pyroxenes 87° and 93° vs. amphiboles 56° and 124° is a fundamental distinction in silicate mineralogy. Fracture types include conchoidal smooth, curved surfaces like broken glass—quartz, obsidian, olivine , uneven or irregular rough surfaces—most minerals that lack cleavage , hackly jagged, metallic torn surfaces—native metals , splintery fibrous broken surfaces—jade, some pyroxenites , and earthy crumbly, fine-grained surfaces—kaolinite, limonite . Conchoidal fracture in a non-glassy mineral is diagnostic and suggests strong, uniform bonding in all directions. Tips: Cleavage surfaces are flat and reflective; fracture surfaces are irregular. Look for the characteristic 'flash' of light reflecting off parallel cleavage planes as you rotate a specimen. Weathered surfaces can obscure cleavage—break a fresh piece. Cleavage angles are measured between the cleavage planes themselves, not between the surface normals a common student error . For fine-grained rocks, cleavage may only be visible under a binocular microscope. Learning to distinguish amphibole from pyroxene cleavage angles is one of the most important skills in hand-specimen petrography.. Equipment: Hand lens 10× , rock hammer, chisel, safety glasses MANDATORY when breaking specimens , protractor or goniometer for measuring cleavage angles. Optional: binocular microscope for examining cleavage in small specimens. Procedure: 1. Put on safety glasses before breaking any spec

Sources, scope & corrections

These authoritative resources support category-level nomenclature, terminology, identification methods, and safety. When a specimen-specific safety warning appears, the public-health source listed here directly supports that precaution. They are not line-by-line citations for every other property on this entry, and natural specimens vary.

Editorial owner
GeoMiner
Last updated
2026-03-30T15:00:37.048321+00:00
Review status
Published; not independently peer-reviewed
  • How Do Geologists Identify Minerals? — Utah Geological Survey. Practical guidance on diagnostic physical properties and identification tests.
  • Mineral ID Key — Mineralogical Society of America. Detailed test methods, repeatability guidance, and safety cautions.

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Common questions

What is the Cleavage and Fracture Test used for?
Cleavage and fracture describe how a mineral breaks, and together they are among the most powerful diagnostic physical properties in mineralogy. Cleavage is the tendency to break along flat planes of weakness within the crystal structure, corresponding to directions of weakest atomic bonding. Fracture describes the way a mineral breaks when cleavage is absent or when the break does not follow a cleavage plane. Cleavage is described by the number of cleavage directions, the angles between them, and the quality perfect, good, fair, poor . These parameters are directly determined by crystal structure and are therefore consistent for a given mineral species. For example, halite always has three perfect cleavage directions at 90° cubic cleavage , micas have one perfect basal cleavage, and calcite has three perfect rhombohedral cleavage directions at approximately 75° and 105°. The angle between cleavage directions is especially diagnostic: pyroxenes 87° and 93° vs. amphiboles 56° and 124° is a fundamental distinction in silicate mineralogy. Fracture types include conchoidal smooth, curved surfaces like broken glass—quartz, obsidian, olivine , uneven or irregular rough surfaces—most minerals that lack cleavage , hackly jagged, metallic torn surfaces—native metals , splintery fibrous broken surfaces—jade, some pyroxenites , and earthy crumbly, fine-grained surfaces—kaolinite, limonite . Conchoidal fracture in a non-glassy mineral is diagnostic and suggests strong, uniform bonding in all directions.
How do you perform the Cleavage and Fracture Test?
1. Put on safety glasses before breaking any specimen. 2. Examine existing broken surfaces on the specimen with a hand lens. Look for flat, reflective planes that repeat in parallel—these are cleavage surfaces. 3. Note how many distinct directions of cleavage are present. Rotate the specimen and look for additional cleavage sets at different orientations. 4. Estimate or measure the angle between cleavage directions using a protractor. This is especially important for distinguishing amphibole 56°/124° from pyroxene 87°/93° cleavage. 5. Assess cleavage quality: perfect smooth, mirror-like planes that form easily , good mostly smooth planes with minor steps , fair detectable but irregular planes , or poor barely discernible preferred breaking directions . 6. If no cleavage is evident, describe the fracture type: conchoidal, uneven, hackly, splintery, or earthy. 7. For a confirmatory test, carefully break a small fragment with a hammer and chisel and examine the fresh fracture surfaces. 8. Record: number of cleavage directions, angles between them, quality of each, and fracture type on non-cleavage surfaces.
How do you interpret the Cleavage and Fracture Test?
Diagnostic cleavage patterns: 1 perfect direction = micas, graphite, molybdenite basal cleavage ; 2 directions at 90° = feldspars, pyroxenes; 2 directions at 56°/124° = amphiboles; 3 directions at 90° = halite, galena cubic ; 3 directions NOT at 90° = calcite, dolomite rhombohedral at 75°/105° ; 4 directions = fluorite octahedral ; 6 directions = sphalerite dodecahedral . Conchoidal fracture with vitreous luster = quartz, obsidian, olivine, garnet. Hackly fracture = native metals copper, gold, silver . No cleavage + conchoidal fracture + high hardness = garnet.
What equipment is needed for the Cleavage and Fracture Test?
Hand lens 10× , rock hammer, chisel, safety glasses MANDATORY when breaking specimens , protractor or goniometer for measuring cleavage angles. Optional: binocular microscope for examining cleavage in small specimens
What safety precautions apply to the Cleavage and Fracture Test?
The listed safety level is caution.

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