Minerals are naturally occurring inorganic solid substances that have a defined chemical composition and a crystalline structure. They exhibit various physical properties that can be used to identify and classify them. Some of the common physical properties of minerals include:
- Hardness: Hardness refers to the ability of a mineral to resist scratching. The Mohs scale of hardness, which ranges from 1 (the softest) to 10 (the hardest), is commonly used to measure the hardness of minerals. For example, talc has a hardness of 1, while diamond, the hardest mineral, has a hardness of 10.
- Color: Color is one of the most noticeable properties of minerals, but it is not always a reliable characteristic for identification. Some minerals may have a distinctive color, while others can occur in various colors due to impurities or other factors.
- Cleavage and Fracture: Cleavage refers to the way a mineral breaks along flat surfaces, whereas fracture refers to the way a mineral breaks along irregular or uneven surfaces. Cleavage is often described in terms of the number of planes and their angles. For example, mica has perfect basal cleavage, meaning it breaks along one plane to produce thin, flat sheets.
- Luster: Luster refers to the way a mineral reflects light. It can be described as metallic, non-metallic, or sub-metallic. Minerals such as gold and silver exhibit a metallic luster, while minerals like quartz and feldspar have a non-metallic luster.
- Streak: Streak refers to the color of a mineral’s powder when it is scraped across an unglazed porcelain plate. It may or may not be the same as the mineral’s external color. For example, hematite, which is commonly red in color, leaves a red streak, while pyrite, which is often yellow or brassy in color, leaves a greenish-black streak.
- Density: Density is the mass per unit volume of a mineral. It can provide information about the composition and chemical structure of a mineral. Different minerals can have significantly different densities due to variations in their chemical composition.
- Crystal form: Crystal form refers to the external shape of a mineral’s crystals. Some minerals have distinctive crystal forms that can aid in their identification. For example, quartz commonly forms hexagonal prisms with pointed terminations, while halite forms cubic crystals.
- Magnetism: Some minerals, such as magnetite, exhibit magnetic properties and are attracted to magnets. This property can be used as a diagnostic test for identifying certain minerals.
- Optical properties: Some minerals exhibit optical properties, such as double refraction or fluorescence, which can be used as diagnostic tests for identification.
- Transparency and opacity: Transparency refers to the ability of a mineral to transmit light, while opacity refers to the inability of a mineral to transmit light. Minerals can be transparent, translucent, or opaque, and this property can provide valuable information for identification. For example, quartz is often transparent, while gypsum is typically translucent.
- Specific gravity: Specific gravity is a measure of the density of a mineral relative to the density of water. It is a useful property for identifying minerals with similar densities. Specific gravity can be determined by comparing the weight of a mineral to the weight of an equal volume of water.
- Tenacity: Tenacity refers to a mineral’s resistance to breaking, bending, or deforming. Minerals can be brittle (break easily), malleable (can be flattened or bent without breaking), sectile (can be cut into thin shavings with a knife), ductile (can be drawn into wires), or flexible (can be bent and then return to their original shape).
- Magnetism: Some minerals exhibit magnetic properties and can be attracted to magnets. Magnetite is a common example of a magnetic mineral.
- Taste and odor: Some minerals have distinct tastes or odors that can aid in their identification. For example, halite (rock salt) has a characteristic salty taste, while sulfur has a distinct odor of rotten eggs.
- Reaction to acid: Some minerals may react with acids, producing effervescence or fizzing. This can be a useful test for identifying minerals such as calcite, which reacts with weak acids like hydrochloric acid.
- Electrical conductivity: Certain minerals can conduct electricity, which can be a helpful property for identification. For example, graphite, a form of carbon, is an excellent conductor of electricity.
- Thermal properties: Minerals may exhibit thermal properties such as melting point, boiling point, and heat resistance, which can be useful for identification or characterization.
- Radioactivity: Some minerals are radioactive and emit radiation, which can be detected using specialized equipment. Uraninite and pitchblende are examples of radioactive minerals.
- Solubility: Solubility refers to the ability of a mineral to dissolve in a liquid, such as water or acid. Some minerals, like halite, are highly soluble in water, while others, like quartz, are insoluble. Solubility can be a useful property for identifying minerals and can be determined by conducting dissolution tests.
- Striations: Striations are parallel lines or grooves on the surface of a mineral, often visible under magnification. They can provide important clues for identifying minerals such as feldspars, which often exhibit characteristic striations on their cleavage surfaces.
- Phosphorescence: Phosphorescence is the ability of a mineral to emit light after being exposed to ultraviolet (UV) radiation. Some minerals, such as fluorite, can exhibit phosphorescence, which can be used as a diagnostic property for identification.
- Piezoelectricity: Piezoelectricity is the ability of a mineral to generate an electric charge when subjected to mechanical pressure or stress. Certain minerals, such as quartz and tourmaline, exhibit piezoelectric properties and can generate electricity under pressure.
- Tectosilicate structure: Tectosilicate structure refers to the arrangement of silicon-oxygen tetrahedra in some minerals, such as quartz and feldspars. This structure can result in unique physical properties, such as high hardness, high melting point, and lack of cleavage, which can aid in identification.
- Twinning: Twinning is the phenomenon where two or more individual crystals of a mineral are intergrown in a symmetrical manner. Twinning can produce distinctive patterns or shapes in minerals and can be used as an identifying characteristic.
- Pseudomorphism: Pseudomorphism is a phenomenon where one mineral replaces another mineral while retaining the original mineral’s shape or structure. This can result in unique physical properties and can be used in identification.
Isotropism is a property exhibited by some minerals, where they show the same physical properties in all directions. In other words, isotropic minerals have physical properties that are uniform, regardless of the direction in which they are observed. This is in contrast to anisotropic minerals, which exhibit different physical properties depending on the direction in which they are observed.
Isotropism is primarily related to the optical properties of minerals, specifically their behavior when interacting with light. Isotropic minerals have a single refractive index, meaning that light travels through them at the same speed in all directions, and they do not exhibit double refraction. As a result, isotropic minerals appear the same when viewed from any direction, and their optical properties, such as color and transparency, are consistent regardless of the orientation of the mineral specimen.
Examples of isotropic minerals include garnet, spinel, and magnetite. These minerals have a cubic crystal structure, which results in isotropic behavior. Other minerals, such as quartz and calcite, are anisotropic because they have a different crystal structure that causes them to exhibit different physical properties in different directions.
The property of isotropism can be determined through various optical tests, such as polarizing microscopy, which involves the use of polarized light to observe the behavior of minerals when interacting with light. Isotropism is an important characteristic used in the identification and classification of minerals, as it can help distinguish isotropic minerals from anisotropic minerals and aid in mineralogical analysis.
In a single crystal, the physical and mechanical properties often differ with orientation. It can be seen from looking at our models of crystalline structure that atoms should be able to slip over one another or distort in relation to one another easier in some directions than others. When the properties of a material vary with different crystallographic orientations, the material is said to be anisotropic.
Alternately, when the properties of a material are the same in all directions, the material is said to be isotropic. For many polycrystalline materials the grain orientations are random before any working (deformation) of the material is done. Therefore, even if the individual grains are anisotropic, the property differences tend to average out and, overall, the material is isotropic. When a material is formed, the grains are usually distorted and elongated in one or more directions which makes the material anisotropic. Material forming will be discussed later but let’s continue discussing crystalline structure at the atomic level.
Physical properties of minerals are directly related to their atomic structure, bonding forces and chemical composition. Bonding forces as electrical forces exist between the atoms and ions are related to the type of elements, and the distance between them in the crystalline structure. Thus, minerals having same chemical composition may show different crystal structure (as a function of changes in P & T or both). So, being crystallized in different Symmetry Systems they exhibit different physical properties, this is called polymorphism. These minerals are said to be polymorphous. They may be Dimorphic, Trimorphic or Polymorphic according to the number of mineral species present in their group.
Cohesion and Elasticity
Cohesion and elasticity are two related concepts that describe the behavior of materials in response to external forces.
Cohesion: Cohesion refers to the internal attraction or bonding between particles within a material, which holds them together. It is the force that allows materials to resist being pulled apart or separated. Cohesion is responsible for the “stickiness” or “stick-together” property of materials. In minerals, cohesion is typically due to the chemical bonds between atoms or ions that make up the mineral’s structure. Minerals with strong cohesion are more resistant to breaking or crumbling.
Elasticity: Elasticity refers to the ability of a material to deform under an applied force and then return to its original shape and size once the force is removed. A material that is elastic can undergo temporary deformation, such as stretching or bending, without permanent damage or change in its structure. Elasticity is related to the strength and flexibility of materials. In minerals, elasticity is typically related to the arrangement and strength of chemical bonds between atoms or ions, as well as the overall structure and arrangement of mineral grains.
Minerals can exhibit a range of cohesive and elastic behaviors, depending on their chemical composition, crystal structure, and other factors. Some minerals may have strong cohesion and high elasticity, making them resistant to breakage and able to deform under stress without permanent damage. Other minerals may have weak cohesion and low elasticity, making them more prone to fracture or deformation. The cohesive and elastic properties of minerals can also be influenced by external factors such as temperature, pressure, and humidity.
The result of cohesion and elasticity in a mineral appears as
Tendency of a crystalline mineral to break in certain directions yielding more or less smooth planar surfaces.These planes of lowest bond energy have minimum value of cohesion. An amorphous body of course has no cleavage. Cleavage planes are usually // to the crystallographic planes. Exceptions: Cal, Flu.
1. Good, distinct, perfect,
2. Fair, indistinct, imperfect,
3. Poor, in traces, difficult.
Being related to the atomic structure of the mineral, cleavage may be in several directions and depending on the force of cohesion some of them may be more developed than the others. So they are classified according to their distinction and smoothness:
Obtained when the mineral is subjected to external force. The mineral breaks along planes of structural weakness. The weakness may result from pressure, twinning or exsolution. Composition planes of twinning and glide planes are usually the direction of easy parting. Parting resembles cleavage. However, unlike cleavage, parting may not be shown by all individuals of the mineral species. Parting is not continuous on crystals.
If the mineral contains no planes of weakness, it will break along random directions called fracture
- Conchoidal: smooth fracture (Qua,glass )
- Fibrous and splintery: sharp pointed fibers (Asbestos, Serpentine),
- Uneven or irregular: rough and irregular surfaces,
- Even: more or less smooth surfaces, may resemble cleavage,
- Hackly: jagged fractures with very sharp edges (Mat).
The resistance that a smooth surtace of a mineral offers to scratching (H) This is an indirect measure of the bond strength in the mineral. Hardness is determined by scratching the mineral with a mineral or substance of known hardness. Moh’s relative scale of hardness exhibited by some common minerals were used to give a numerical result. These minerals are listed below, along with the hardness of some common objects. A series of 10 common minerals were chosen by Austrian mineralogist F. Mohs in 1824 as a scale.
Mohs scale of Hardness
Hardness of other common Objcects
The resistance that a mineral offers to breaking, crushing, bending, cutting, drawing or tearing is its tenacity. It is mineral’s cohesiveness.
- Brittle: A mineral that breaks and powders easily (Sulfides,Carbonates, Silicates and Oxides)
- Malleable: A mineral that can be hammered out without breaking, into thin sheets. They are plastic (Native metals)
- Sectile: A mineral that can be cut with a knife into thin shavings (Native metals)
- Ductile: A mineral that can be drawn into wire (Native metals)
- Flexible: A mineral that bends but retains it bent form. Does not resume its original shape, permanent deformation (Asb, clay minerals, Chl, Tal)
- Elastic: A mineral that after bending springs back and resumes its original position. (Mus).
Specific gravity (SG) or relative density is a unitless number that expresses the ratio between the weight of a substance and the weight of an equal volume of water at 4degree (max ρ).
Density (p) is the weight of a substance per volume= g/cm3. It is different
than SG, and varies from one locality to another (max. at poles, min. at
Diapheneity is amount of light transmitted or absorbed by a solid.Diapheneity generally used strictly for hand specimens also most minerals opaque in hand specimens and transparent in thin sections
Transparent is pass the object behind it seen clearly also size of specimen (thicker specimens may become translucent)
Translucent is light transmitted but object not seen
Opaque is light wholly absorbed
Color is sometimes an extremely diagnostic property of a mineral, for
example olivine and epidote are almost always green in color. But, for some
minerals it is not at all diagnosticbecause minerals can take on a variety of
colors. These minerals are said to be allochromatic.
For example quartz can be clear, white, black, pink, blue, or purple.
Streak is the color of the mineral in powdered form. Streak shows the true color of the mineral. In large solid form, trace minerals can change the color appearance of a mineral by reflecting the light in a certain way. Trace minerals have little influence on the reflection of the small powdery particles of the streak.
The streak of metallic minerals tends to appear dark because the small particles of the streak absorb the light hitting them. Non-metallic particles tend to reflect most of the light so they appear lighter in color or almost white.
Luster is a term used to describe the way light interacts with the surface of a mineral and how it appears in terms of its brightness or shininess. It is one of the basic physical properties of minerals and can provide important clues for identifying minerals. Luster can be observed by examining the reflected light from the surface of a mineral specimen under normal lighting or by using a light source, such as a flashlight, to illuminate the mineral.
There are several common terms used to describe the luster of minerals:
- Metallic: Minerals with a metallic luster have the appearance of polished metal, such as the shine of a fresh steel surface. Examples of minerals with metallic luster include galena, pyrite, and magnetite.
- Submetallic: Minerals with a submetallic luster have a slightly less reflective, duller appearance compared to metallic minerals. They may have a somewhat metallic or dull metallic sheen. Examples include hematite and chalcopyrite.
- Non-metallic: Minerals with a non-metallic luster do not have the reflective, shiny appearance of metallic minerals. Instead, they may have a glassy, vitreous, pearly, silky, greasy, or earthy appearance.
- Glassy/vitreous: Minerals with a glassy or vitreous luster have a shiny, glass-like appearance, similar to the luster of broken glass. Examples include quartz and feldspar.
- Pearly: Minerals with a pearly luster have a reflective, iridescent sheen, resembling the luster of a pearl or the inside of a seashell. Examples include muscovite and talc.
- Silky: Minerals with a silky luster have a fibrous or thread-like appearance, with a sheen resembling silk fibers. Examples include asbestos and gypsum.
- Greasy: Minerals with a greasy luster have a dull, oily appearance and may appear wet or greasy. Examples include nepheline and serpentine.
- Earthy: Minerals with an earthy luster have a dull, powdery appearance, similar to the texture of soil or clay. Examples include kaolinite and limonite.
Luster can be a useful property for identifying minerals, as it provides information about how light interacts with the mineral’s surface. However, it is important to note that luster can sometimes be subjective and can vary depending on the lighting conditions and the quality of the mineral specimen being observed. It is often used in conjunction with other physical properties to accurately identify minerals.
Crystal Form and Habit
Crystal form and habit are two related concepts that describe the external appearance or shape of mineral crystals. They are important characteristics used in mineral identification and can provide valuable information about the internal structure and growth conditions of minerals.
Crystal Form: Crystal form refers to the geometric shape of a mineral crystal, which is determined by the arrangement of atoms or ions in the crystal lattice. Crystal form is a result of the internal structure of the mineral and the conditions under which it formed, including temperature, pressure, and available space for crystal growth. Crystals can exhibit a wide variety of forms, ranging from simple geometric shapes, such as cubes, prisms, and pyramids, to more complex and irregular shapes.
Habit: Habit refers to the characteristic overall shape or external appearance of a group of crystals or an aggregate of minerals. Habit can vary depending on the growth conditions and environment in which the crystals formed. Common mineral habits include:
- Tabular: Crystals that are flat and platy, with a rectangular or tabular shape. Examples include mica and barite.
- Prismatic: Crystals that are long and slender, with a prism-like shape. Examples include quartz and tourmaline.
- Bladed: Crystals that are thin and blade-like in shape, resembling a knife blade. Examples include gypsum and kyanite.
- Acicular: Crystals that are slender and needle-like in shape. Examples include rutile and actinolite.
- Dendritic: Crystals that exhibit a tree-like or fern-like branching pattern. Examples include dendritic quartz and manganese oxide minerals.
- Granular: Crystals that form aggregates or masses of tiny grains or crystals without any distinct shape. Examples include chalcedony and obsidian.
- Botryoidal: Crystals that form rounded, globular or grape-like shapes. Examples include hematite and smithsonite.
- Cubic: Crystals that exhibit a cubic shape with straight edges and right angles, such as halite and pyrite.
- Octahedral: Crystals that exhibit an octahedral shape with eight faces and six vertices, such as fluorite and magnetite.
The crystal form and habit of a mineral can provide important information about its crystallography, symmetry, and growth conditions, which can aid in mineral identification and understanding of mineral properties. However, it’s important to note that crystal form and habit can vary, and some minerals may exhibit multiple habits or forms depending on the specific conditions under which they formed. Therefore, it’s often necessary to consider other physical and chemical properties in conjunction with crystal form and habit for accurate mineral identification.
Magnetism is a physical property exhibited by certain minerals that can attract or repel other magnetic materials, such as iron or steel. It is caused by the alignment of magnetic dipoles within the mineral, which are tiny atomic or molecular magnets that have north and south poles.
There are two main types of magnetism that minerals can exhibit:
- Ferromagnetism: Ferromagnetic minerals are strongly attracted to magnets and can retain their magnetic properties even after the external magnetic field is removed. They can also magnetize other materials. Examples of ferromagnetic minerals include magnetite (Fe3O4) and pyrrhotite (Fe1-xS).
- Paramagnetism: Paramagnetic minerals are weakly attracted to magnets and lose their magnetic properties when the external magnetic field is removed. Examples of paramagnetic minerals include hematite (Fe2O3), chromite (FeCr2O4), and ilmenite (FeTiO3).
In addition to ferromagnetism and paramagnetism, there are other types of magnetism such as antiferromagnetism, where neighboring magnetic dipoles align in opposite directions, and diamagnetism, where minerals are weakly repelled by magnets. However, these types of magnetism are less common in minerals and generally have weaker magnetic effects.
Magnetism can be used as a diagnostic property in identifying certain minerals, as not all minerals are magnetic. For example, if a mineral is strongly attracted to a magnet and retains its magnetism even after the magnet is removed, it may indicate the presence of magnetite. On the other hand, if a mineral is only weakly attracted to a magnet and loses its magnetism when the magnet is removed, it may indicate paramagnetic or diamagnetic properties.
It’s important to note that the presence or absence of magnetism alone is not always sufficient for mineral identification, as other factors such as color, hardness, streak, and other physical and chemical properties should also be considered. Magnetism is just one of the many properties that can be used as a tool in mineral identification and characterization.