Crystal shape
Fibrous = fiber-like, similar to hair or cloth fibers.
Example: Chrysotile asbestos.
Acicular = needle-shaped.
Example: natrolite (a silicate)
.
Prismatic = prism-shaped, like rods or columns.
Example: black tourmaline in quartz.
Blocky = shaped like a block or brick.
Example: orthoclase
Equant = equal size in all dimensions.
Example: green and pink garnets (left), galena (top right), and limonite after pyrite (bottom right).
Platy = plate-shaped, flat.
Example: muscovite.
Massive = dense, compact mass of interlocking crystals.
Example: magnetite.
Earthy = fine-grained with a dull, dirt-like appearance.
Example: limonite.
Mineral cleavages (planes of easy breaking)
0 cleavages, with no planes of easy breaking, results in the shell-like "concoidal fracture".
Example: obsidian.
1 cleavage, with one plane of easy breaking, results in plate- or sheet-like pieces.
Example: biotite
2 cleavages, with an angle of cleavage intersection about 90°.
Example: orthoclase.
2 cleavages, with an angle of cleavage intersection about 120° and 60°.
Example: amphibole (hornblende).
3 cleavages, intersecting at angles that are not 90°.
Example: calcite.
Crystal faces do not penetrate into the body of the crystal, whereas cleavages, as parallel sets of cracks, can. Cleavages can form steps on the outside of the crystal, each parallel to the cleavage direction.
Example: quartz.
How crystal habit affects luster
Left: earthy hematite is brick red and has non-metallic luster. The crystals are so small that they are partially transparent. Light passing through the grains changes its color to red. The microscopic crystals scatter the light in all directions, giving the sample a dull appearance.
Right: specular hematite is gray and has metallic luster. The crystals are large enough to be opaque. Unbound electrons (partial metallic bonding) cause the crystals to reflect light like a metal. Hematite is actually a semiconductor.
Streak color (color of the mineral powder)
Here are the streak colors of five dark minerals that all appear black, dark gray, or dark brown. Notice how the streak color is more diagnostic than is the bulk mineral color.
| Mineral: | Pyroxene | Hematite | Limonite | Magnetite | Amphibole |
| Streak color: | Colorless | Brick red | Yellow brown | Black | Colorless |
The Mohs hardness scale for minerals
| Mineral | Hardness (H) |
| Talc | 1 |
| Gypsum | 2 |
| Calcite | 3 |
| Fluorite | 4 |
| Apatite | 5 |
| Orthoclase | 6 |
| Quartz | 7 |
| Topaz | 8 |
| Corundum | 9 |
| Diamond | 10 |
How to determine relative hardness of two minerals
Apatite, H = 5, makes a mark on quartz. Is the mark a scratch on the quartz?
No, since the apatite dust rubs off, leaving no scratch.
Topaz, H = 8, also makes a mark on quartz. Is this mark a scratch on the quartz.
Yes, since the dust rubs off, leaving a scratch on the quartz surface. The dust in this case was quartz dust.
Conclusion: quartz is harder than apatite and softer than topaz, so the hardness of quartz is between 5 and 8. In fact, quartz has a hardness of 7 on the Mohs scale.
Gypsum, H = 2, leaves a mark on fluorite. Is this mark a scratch on the fluorite?
No, since the gypsum dust rubs off leaving no mark on the fluorite surface.
Apatite, H = 5, also leaves a mark on the fluorite surface. Is this mark a scratch on the fluorite?
Yes, since, when the fluorite dust is rubbed away, there is a clear scratch on the fluorite.
Conclusion: fluorite is harder than gypsum and softer than apatite, and therefore has a hardness between 2 and 5. In fact, fluorite has a hardness of 4 on the Mohs scale.
The feldspar mineral group: the most abundant minerals in the Earth's crust
All feldspars have similar hardness, structure, streak, and habit. Their most common distinguishing characteristic is color.
Top left: gray labradorite
Bottom left: white albite
Right: pink orthoclase
The plagioclase group feldspars, including albite and labradorite, usually have fine striations on one of the cleavage surfaces. These can be seen by looking closely with a hand lens or microscope and moving the sample to reflect a source of light.
Some labradorite is iridescent, a property best seen when the mineral is wet. The sample will have to be tilted in different directions in bright light to see this. Iridescent colors are most commonly blue and green, but yellow, orange, and red occur too.
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