Mineral Mastery

Minerals: A Detailed Explanation

• A mineral is a naturally occurring solid substance with a specific crystal structure and a well-defined chemical composition.

• Minerals exclude compounds that occur only in living organisms, but living organisms often synthesize inorganic minerals that occur in rocks.

• Rocks may consist of one mineral or an aggregate of two or more different types of minerals.

• Some natural solid substances without a definite crystalline structure are called mineraloids.

• The International Mineralogical Association (IMA) is the standard body for the definition and nomenclature of mineral species. As of March 2023, the IMA recognizes 5,914 official mineral species.

• Minerals may have variable proportions of two or more chemical elements that occupy equivalent positions in its structure, forming a mineral group.

• Minerals are classified by key chemical constituents, with silicate minerals comprising approximately 90% of the Earth's crust.

• Biogenic minerals are a topic of contention among geologists and mineralogists, with some including them in the mineral kingdom and others excluding them.

• Rocks are an aggregate of one or more minerals or mineraloids, with some rocks composed primarily of one mineral.

• Ores are minerals that have a high concentration of a certain element, typically a metal.

• Gems are minerals with ornamental value, distinguished from non-gems by their beauty, durability, and rarity.

• Chemical composition and coordination polyhedra explain the common feature of minerals having variable compositions and substitutions.Minerals: Physical Properties and Classification

• Aluminosilicates kyanite, andalusite, and sillimanite share the formula Al2SiO5, but differ by the coordination number of the Al3+; they transition from one another as a response to changes in pressure and temperature.

• Minerals of most elements are substituted into common rock-forming minerals, and distinctive minerals of most elements are quite rare, being found only where these elements have been concentrated.

• Changes in temperature, pressure, and composition alter the mineralogy of a rock sample, and changes in thermodynamic conditions make it favorable for mineral assemblages to react with each other to produce new minerals.

• Physical properties applied for mineral classification include crystal structure and habit, hardness, lustre, diaphaneity, colour, streak, cleavage and fracture, and specific gravity.

• Minerals are typically described by their symmetry content, and crystals are restricted to 32 point groups, which differ by their symmetry. The six crystal families are cubic, tetragonal, orthorhombic, hexagonal, monoclinic, and triclinic.

• Polymorphism extends beyond pure symmetry content. For example, the aluminosilicates are a group of three minerals – kyanite, andalusite, and sillimanite – which share the chemical formula Al2SiO5 but have different structures.

• Twinning is the intergrowth of two or more crystals of a single mineral species. There are several types of twins, including contact twins, reticulated twins, geniculated twins, penetration twins, cyclic twins, and polysynthetic twins.

• Crystal habit refers to the overall shape of a crystal, and quality of crystal faces is diagnostic of some minerals.

• Hardness of a mineral defines how much it can resist scratching, and a mineral's hardness is not necessarily constant for all sides.

• Lustre indicates how light reflects from the mineral's surface, with regards to its quality and intensity, and non-metallic lustres include adamantine, vitreous, pearly, resinous, and silky.

• The diaphaneity of a mineral describes the ability of light to pass through it, and the diaphaneity of a mineral depends on the thickness of the sample.

• The colour of a mineral is non-diagnostic and is caused by electromagnetic radiation interacting with electrons, and minerals can have various other distinctive optical properties, such as play of colours, asterism, chatoyancy, iridescence, tarnish, and pleochroism.

• The streak of a mineral refers to the colour of a mineral in powdered form, and the most common way of testing this property is done with a streak plate. Cleavage, parting, fracture, and tenacity are other physical properties used for mineral classification.Properties and Classification of Minerals

• Minerals have a characteristic atomic arrangement, and weakness in this structure causes planes of weakness leading to cleavage.

• Cleavage can be described based on how cleanly and easily the mineral breaks, and there are different types of cleavage depending on the number of directions in which the mineral can break.

• Parting is a similar phenomenon to cleavage but is caused by structural defects in the mineral instead of systematic weakness.

• Fracture occurs when a mineral is broken in a direction that does not correspond to a plane of cleavage, and there are different types of uneven fracture.

• Tenacity describes how resistant a mineral is to breaking, and minerals can be described as brittle, ductile, malleable, sectile, flexible, or elastic.

• Specific gravity numerically describes the density of a mineral and can be used to diagnose minerals, with high specific gravity being a diagnostic property.

• Dropping dilute acid onto a mineral aids in distinguishing carbonates from other mineral classes.

• Magnetism, taste, smell, and radioactivity are other properties that can be used to diagnose certain minerals.

• Minerals are classified by variety, species, series, and group, with the basic level of definition being that of mineral species distinguished from others by unique chemical and physical properties.

• Silicates are the most important class of minerals in terms of rock formation and diversity, with most rocks composed of greater than 95% silicate minerals.

• The base unit of a silicate mineral is the [SiO4]4− tetrahedron, and different combinations of elements are required to balance out the resultant negative charge.

• Non-silicate minerals include native elements, sulfides, halides, oxides and hydroxides, carbonates and nitrates, borates, sulfates, phosphates, and organic compounds, and they have great economic importance.Overview of the Silicate Mineral Subclasses

• Silicate minerals are composed of silicon and oxygen, with other cations and anions present in varying amounts.

• Tectosilicates have the highest degree of polymerization, with all corners of a tetrahedra shared and a silicon:oxygen ratio of 1:2; examples include quartz, feldspars, feldspathoids, and zeolites.

• Phyllosilicates consist of sheets of polymerized tetrahedra with a characteristic silicon:oxygen ratio of 2:5; examples include mica, chlorite, and the kaolinite-serpentine groups.

• Inosilicates consist of tetrahedra repeatedly bonded in chains, with single-chain silicates being most commonly pyroxenes and double-chain silicates being amphiboles.

• Cyclosilicates, or ring silicates, have a ratio of silicon to oxygen of 1:3, with six-member rings being most common; examples include the tourmaline group and beryl.

• Sorosilicates, also termed disilicates, have tetrahedron-tetrahedron bonding at one oxygen, which results in a 2:7 ratio of silicon to oxygen; examples include epidote, lawsonite, and vesuvianite.

• Orthosilicates consist of isolated tetrahedra that are charge-balanced by other cations, with a silicon:oxygen ratio of 1:4; examples include the aluminosilicates, the olivine group, and the garnet group.

• Quartz is the most abundant mineral species on Earth, making up 12% of the crust, and has several polymorphs.

• Feldspars are the most abundant group in the Earth's crust, with 22 mineral species, and are subdivided into alkali and plagioclase subgroups.

• Feldspathoids are structurally similar to feldspar but form in Si-deficient conditions; nepheline is a common example.

• Micas incorporate aluminum into the tetrahedral sheets, have a greater hardness than other phyllosilicates, and are used in electronics, construction, and cosmetics.

• Asbestos minerals, including amphibole asbestos, are carcinogenic and cause various illnesses, but have applications in construction materials due to their strength and heat resistance.

• Tourmalines have a complex chemistry, with the T site usually being Si4+ but substitutable by Al3+ or B3+, and can be subgrouped by the occupancy of the X site and the chemistry of the W site.

• Garnets have a general formula of X3Y2(SiO4)3, with six ideal endmembers split into two groups: pyralspite garnets have Al3+ in the Y position, and ugrandite garnets have Ca2+ in the X position.