Physical properties of minerals are distinct that in most cases can be used for their identification. By examining hand specimens by visual inspection or using a hand lens. Rarely is a mineral identified by a single property. These properties need to be considered together to correctly identify a mineral.
All minerals have certain properties that help geologists identify them. A mineral’s properties are determined by its unique internal crystal structure and chemical composition. Geologists usually determine the properties of minerals using simple tools such as appearance, color, density, hardness, magnetism, luster, and taste. Properties determined by the reflection of light are called optical properties. These include the mineral’s luster, transparency, and color. When the mineral is crushed and the powder is poured, there is a definite color to the powdered sample called the streak color. Minerals with similar internal crystal structures often have similar physical properties. However, this does not mean that any two minerals with the same crystal structure have the same color or density. For example, both corundum and hematite have a hexagonal crystal structure. Yet, corundum is clear in its pure form, and hematite is often red with lighter red streaks. Hematite is also about six times denser than corundum.
Table of Contents
Common Physical Properties of Minerals
1. Crystal Form or Habit
The “form” or habit of a mineral refers to its external shape, often linked to its internal atomic structure. This characteristic can be highly variable, even for the same mineral, and it provides important clues in identifying minerals. Some minerals consistently display the same form, but some minerals may display more than one form. Typical forms include lamellar (thin, separable layers like mica), tabular (slab-like formations), fibrous (thread-like forms as seen in asbestos), and pisolitic (pea-sized spherical grains as in bauxite). Mineral crystal starts the bonding of a few atoms into a three-dimensional geometric pattern. Typically, minerals occur as well-formed crystals and it reflects the crystal structure of the mineral. So, crystal habits is very helpful in mineral identification.
Form alone isn’t conclusive for mineral identification, as different minerals can exhibit similar form or habit or the same mineral may appear in different forms. Therefore, it must be considered along with other physical properties.
List of some common forms and their mineral examples are given below in the table:
| Sl. No. | Form | Description | Mineral Example |
|---|---|---|---|
| 1. | Cubic | Cube shape | Pyrite |
| 2. | Octahedral | Mineral shows shape like octahedrons | Fluorite |
| 3. | Tabular | Tablet or sheet-like | Feldspar, gypsum |
| 4. | Rhombic | Rhombic shape minerals | Dolomite |
| 5. | Bladed | As cluster or independent lath-shaped grains | Kyanite |
| 6. | Pisolitic | Minerals resembling to be made up of small spherical grains | Bauxite |
| 7. | Lamellar | Mineral appears as thin separable layers | Mica |
| 8. | Fibrous | Mineral appears to be made up of fine threads. | Asbestos |
| 9. | Reni form | Kidney shaped | Hemtite |
| 10. | Botryoidal | Smooth bulbous or globular shapes like bunch of grapes | Chalcedony, psilomelane |
| 11. | Acicular | Needle-like; slightly thicker than filiform | Actinolite |
| 12. | Filiform or capillary | Hair-like | |
| 13. | Massive | Aggregate of very small crystals with fine grained appearance | |
| 14. | Drusy | Surface lined with very small, goosebump-like crystals | |
| 15. | Roseiform | Petal-like arrangement of tabular or bladed crystals | |
| 16. | Oolitic | Spherical, concentrically layered, sand sized grain agrregates | Limestone |
| 17. | Amygdaloidal | Spherical to ellipsoidal gas vesicles infilled with secondary minerals | |
| 18. | Dendritic | Tree-like, branching network of crystals | Manganese oxide |
2. Colour
Colour is a primary observation when examining minerals, and it can be consistent or variable. Minerals with a characteristic and constant colour are termed idiochromatic, while those that display various colours due to impurities or structural abnormalities are allochromatic. For example, magnetite and chromite look very similar but can be distinguished through the colour of their streaks when tested. The colour of minerals, especially metallic ones, can provide a useful indication for identification, though it may not always be a reliable indicator due to environmental and structural factors affecting appearance.
Minerals rich in Al, Ca, Na, Mg, and K are often light coloured. Minerals rich in Fe, Ti, Ni, Cr, Co, Cu and Mn are often dark in colour.
Below are the examples of minerals where colour is a usefult criterion of identification:
| Sl. No. | Mineral | Colour |
|---|---|---|
| 1. | Graphite | Shining black |
| 2. | Galena | Dark lead grey |
| 3. | Malachite | Dark Green |
| 4. | Ruby | Red |
| 5. | Olivine | Olive Green |
| 6. | Azurite | Sky blue |
| 7. | Opal | Milky white |
| 8. | Emerald | Green |
| 9. | Pyrite | Brass yellow |
| 10. | Tourmaline | Jet Black |
| 11. | Chlorite | Dark grassy green |
| 12. | Sapphire | Yellow |
| 13. | Hematite | Dark steel grey |
| 14. | Chalcopyrite | Golden yellow |
| 15. | Magnetite | Black |
| 16. | Sulphur | Yellow |
| 17. | Talc | White or pale yellow |
| 18. | Jasper | Red |
| 19. | Limonite | Yellow or browninsh yellow |
| 20. | Calcite | White |
3. Streak
Streak is the colour of a mineral’s powder, obtained by rubbing a mineral specimen on an unglazed white porcelain tile. It is useful for identifying metallic ore minerals. Most of the streak plates have hardness of about 6.5, hence minerals softer than 6 will leave a powder when scratched on the plate.
Streak is useful where it is slightly different than the body colour. The following are few pertinent examples of streak based identification of mineral.
- Magnetite and chromite which are two different and distinct minerals closely resemble one another in form (granular form), colour (black colour), shining (metallic), density (high), diaphaneity (opaque), etc. Thus, it is difficult to distinguish in hand specimens. But when the two are tested for streak, magnetite gives black streak and chromite gives brown streak. In, this way streak provides a valuable clue to distinguish them.
- Graphite resembles molybdenite in most of its physical properties, but is distinguished by its jet black streak, as the streak of molybdenite is greenish black.
- Hematite, psilomelane and chromite in massive forms sometimes resemble one another. Hematite gives an unmistakable cherry red streak. Psilomelane gives a brownish black streak and chromite gives a brown streak. Psilomelane and chromite can be distinguished based on other physical properties too. To identify hematite cherry red streak is particularly useful.
- Pyrite or rather chalcopyrite resembles gold so closely in its yellow colour and metallic lustre that it is called fool’s gold. Since gold occurs in nature in very small quantities it will be difficult to test by other means. Pyrite and gold if tested for streak give dark greenish black and yellow streaks respectively. Thus pyrite can be easily identified by streak and distinguished from gold.
4. Lustre
Lustre refers to how light reflects from a mineral’s surface and is broadly classified as metallic or non-metallic. Metallic lustre is type of shining that appears on the surface of a metal. Galena, pyrite and magnetite are examples of minerals with metallic lusture. Non-metallic types include vitreous (glass-like), pearly, silky, and resinous. Minerals consistently exhibit certain types of lustre, making it a helpful property in their identification. The intensity of lustre depends upon the quantity of reflected light from its surface, the scattering of light from the surface and internally, and the amount of light absorbed by the mineral. Typically, it is more for minerals having higher refractive index. For instance, gemstones like diamonds owe much of their brilliance to high refractive indices that enhance lustre. It will be relevant to note in this context that the original lustre of a mineral would be lost if it undergoes weathering, as seen in minerals like olivine, which loses its vitreous lustre when altered. Following are typical lustre of some common minerals:
| Sl. No. | Lustre | Minerals |
|---|---|---|
| 1. | Metallic lustre | Galena, pyrite, gold, bornite |
| 2. | Submetallic lustre | Hematite, chromite, ilmenite, magnetite, psilomelane |
| 3. | Pearly lustre | Talc, selenite (gypsum), muscovite mica |
| 4. | Silky lustre | Fibrous minerals like asbestos and satinspar |
| 5. | Vitreous lustre | Quartz, calcite. dolomite, feldspar, barytes, etc. |
| 6. | Sub vitreous lustre | Pyroxenes and amphiboles |
| 7. | Greasy lustre | Graphite, serpentine |
| 8. | Resinous lustre | Opal, agate, chalceodony |
| 9. | Earthy or dull lustre | Magnesite, kaolin, chalk, bauxite |
| 10. | Adamantine lustre | Garnet, sphene, zircon, diamond |
5. Fracture
Fracture is the nature of a mineral’s surface after breaking, which can vary widely. Unlike cleavage, fractures occur randomly and are not related to the mineral’s atomic structure. If the broken surface of a mineral is plain and smooth, it is called even fracture. However, if it is rough or irregular, called uneven fracture. Some other types of fractures include hackly (very irregular broken surface like the end of broken stick), and conchoidal (smooth curved surfaces, like glass). While fracture can be a useful clue, it is often less definitive compared to cleavage and other properties in mineral identification. Following is the list of common fractures and minerals displayed them.
| Sl. No. | Type of fracture | Minerals |
|---|---|---|
| 1. | Even fracture | Magnesite, chalk |
| 2. | Uneven fracture | Most of the minerals |
| 3. | Conchoidal fracture | Opal, volcanic glass |
| 4. | Subconchoidal fracture | Agate, flint, jasper, etc. |
| 5. | Hackly fracture | Asbestos, tremolite, kyanite |
6. Cleavage
Cleavage is the tendency of a mineral to break along certain planes, determined by its atomic structure. Minerals with well-developed cleavage planes break predictably along these planes, which appear smooth and shiny. Cleavage is exhibited by only crystalline minerals, though it is not present in all crystalline minerals. Amorphous minerals like limonite, psilomelane, bauxite etc., do not show cleavage. Depending upon the atomic structure, development of cleavage character may differ in different mineral or in the same mineral. Cleavage is categorized by the number of planes, ranging from one (e.g., mica) to six (e.g., sphalerite), or none (e.g., quartz). The quality of cleavage is described as perfect, good, or poor, and it is a reliable property for identifying minerals because it is consistent within the same mineral species.
7. Hardness
The hardness of a mineral is its resistance to being scratched and is closely linked to its atomic structure. It is typically assessed using the Mohs scale, which ranks minerals from talc (1) to diamond (10). Minerals with the same chemical composition can have vastly different hardness levels, as seen with graphite and diamond. Hardness can be influenced by weathering, which weakens the atomic structure, and it is directional in some minerals, such as kyanite, which has different hardness along different axes.
Hardness of a mineral is studied either as absolute hardness or as relative hardness. Absolute hardness means total hardness. It is determined under a microscope with a diamond impregnator. The relative hardness is the comparative hardness. For relative hardness, the mineral is compared with the hardness of a set of ten standard minerals. This standard set of ten reference minerals used to determine (on a comparative basis) the hardness of any unknown mineral is called “Mohs’ scale of hardness“.
8. Specific Gravity (Density)
Specific gravity reflects a mineral’s density and it depends on chemical composition and structure of the minerals. For identification, the “feel” of a mineral’s weight can provide clues, with high-density minerals like magnetite and galena often being ore minerals, while lower-density minerals like gypsum and quartz are more likely rock-forming. Weathering can alter a mineral’s specific gravity by changing its structure or porosity, and this property helps distinguish minerals that are visually similar, such as calcite and barytes.
Specific gravity (SG) is a dimensionless quantity. It is the ration between the density of a material and the density of pure water at standard temperature and pressure (temperature = 3.9 oC, pressure = 1 atmosphere).
9. Transparency (Diaphaneity)
Transparency is the degree to which light passes through a mineral. It is classified as transparent, translucent, or opaque based on the resistance offered by the minerals to pass the light through them. This property can be influenced by thickness, impurities, and weathering. While most rock-forming minerals become transparent in thin sections, ore minerals tend to remain opaque even when very thin. The examination of a mineral’s edges can help differentiate between inherent opacity and the effect of thickness, offering clues to whether the mineral is an ore or rock-forming.
10. Special Properties
Certain minerals exhibit unique physical traits that aid identification. These include talc’s soapy feel, graphite’s ease of marking paper, and galena’s similar soft markings. Some minerals like realgar and arsenopyrite emit distinctive smells when struck or heated. These properties, while uncommon, provide immediate and definitive identification in certain cases.