Kyanite is a blue aluminosilicate mineral found in aluminium-rich metamorphic pegmatites and/or sedimentary rock or the lava zone. Kyanite in metamorphic rocks indicates pressures higher than four kilobars, it is found in quartz. Although stable at lower pressure and low temperature, the activity of water is high enough under such conditions that it is replaced by hydrous aluminosilicates such as muscovite, pyrophyllite, or kaolinite. Kyanite is known as disthene and cyanite. Kyanite is a member of the aluminosilicate series, which includes the polymorph andalusite and the polymorph sillimanite. Kyanite is anisotropic, in that its hardness varies depending on its crystallographic direction. In kyanite, this anisotropism can be considered an identifying characteristic. At temperatures above 1100 °C kyanite decomposes into mullite and vitreous silica via the following reaction: 3 → 3Al2O3·2SiO2 + SiO2; this transformation results in an expansion. Its name comes from the same origin as that of the color cyan, being derived from the Ancient Greek word κύανος.
This is rendered into English as kyanos or kuanos and means "dark blue". Kyanite is used in refractory and ceramic products, including porcelain plumbing and dishware, it is used in electronics, electrical insulators and abrasives. Kyanite has been used as a semiprecious gemstone, which may display cat's eye chatoyancy, though this use is limited by its anisotropism and perfect cleavage. Color varieties include discovered orange kyanite from Tanzania; the orange color is due to inclusion of small amounts of manganese in the structure. Kyanite is one of the index minerals that are used to estimate the temperature and pressure at which a rock undergoes metamorphism. Kyanite's elongated, columnar crystals are a good first indication of the mineral, as well as its color. Associated minerals are useful as well the presence of the polymorphs of staurolite, which occur with kyanite. However, the most useful characteristic in identifying kyanite is its anisotropism. If one suspects a specimen to be kyanite, verifying that it has two distinctly different hardness values on perpendicular axes is a key to identification.
Kyanite occurs in gneiss, schist and quartz veins resulting from high pressure regional metamorphism of principally pelitic rocks. It occurs as detrital grains in sedimentary rocks, it occurs associated with staurolite, sillimanite, hornblende, gedrite and corundum. Kyanite occurs in Manhattan schist, formed under extreme pressure as a result of the two landmasses that formed supercontinent Pangaea. Specific citations General references "Cyanite". Encyclopædia Britannica. 1911
A mineral is, broadly speaking, a solid chemical compound that occurs in pure form. A rock may consist of a single mineral, or may be an aggregate of two or more different minerals, spacially segregated into distinct phases. Compounds that occur only in living beings are excluded, but some minerals are biogenic and/or are organic compounds in the sense of chemistry. Moreover, living beings synthesize inorganic minerals that occur in rocks. In geology and mineralogy, the term "mineral" is reserved for mineral species: crystalline compounds with a well-defined chemical composition and a specific crystal structure. Minerals without a definite crystalline structure, such as opal or obsidian, are more properly called mineraloids. If a chemical compound may occur with different crystal structures, each structure is considered different mineral species. Thus, for example and stishovite are two different minerals consisting of the same compound, silicon dioxide; the International Mineralogical Association is the world's premier standard body for the definition and nomenclature of mineral species.
As of November 2018, the IMA recognizes 5,413 official mineral species. Out of more than 5,500 proposed or traditional ones; the chemical composition of a named mineral species may vary somewhat by the inclusion of small amounts of impurities. Specific varieties of a species sometimes have official names of their own. For example, amethyst is a purple variety of the mineral species quartz; some mineral species can have variable proportions of two or more chemical elements that occupy equivalent positions in the mineral's structure. Sometimes a mineral with variable composition is split into separate species, more or less arbitrarily, forming a mineral group. Besides the essential chemical composition and crystal structure, the description of a mineral species includes its common physical properties such as habit, lustre, colour, tenacity, fracture, specific gravity, fluorescence, radioactivity, as well as its taste or smell and its reaction to acid. Minerals are classified by key chemical constituents.
Silicate minerals comprise 90% of the Earth's crust. Other important mineral groups include the native elements, oxides, carbonates and phosphates. One definition of a mineral encompasses the following criteria: Formed by a natural process. Stable or metastable at room temperature. In the simplest sense, this means. Classical examples of exceptions to this rule include native mercury, which crystallizes at −39 °C, water ice, solid only below 0 °C. Modern advances have included extensive study of liquid crystals, which extensively involve mineralogy. Represented by a chemical formula. Minerals are chemical compounds, as such they can be described by fixed or a variable formula. Many mineral groups and species are composed of a solid solution. For example, the olivine group is described by the variable formula 2SiO4, a solid solution of two end-member species, magnesium-rich forsterite and iron-rich fayalite, which are described by a fixed chemical formula. Mineral species themselves could have a variable composition, such as the sulfide mackinawite, 9S8, a ferrous sulfide, but has a significant nickel impurity, reflected in its formula.
Ordered atomic arrangement. This means crystalline. An ordered atomic arrangement gives rise to a variety of macroscopic physical properties, such as crystal form and cleavage. There have been several recent proposals to classify amorphous substances as minerals; the formal definition of a mineral approved by the IMA in 1995: "A mineral is an element or chemical compound, crystalline and, formed as a result of geological processes." Abiogenic. Biogenic substances are explicitly excluded by the IMA: "Biogenic substances are chemical compounds produced by biological processes without a geological component and are not regarded as minerals. However, if geological processes were involved in the genesis of the compound the product can be accepted as a mineral."The first three general characteristics are less debated than the last two. Mineral classification schemes and their definitions are evolving to match recent advances in mineral science. Recent changes have included the addition of an organic class, in both the new Dana and the Strunz classification schemes.
The organic class includes a rare group of minerals with hydrocarbons. The IMA Commission on New Minerals and Mineral Names adopted in 2009 a hierarchical scheme for the naming and classification of mineral groups and group names and established seven commissions and four working groups to review and classify minerals into an official listing of their published names. According to these new r
Sedimentary rocks are types of rock that are formed by the accumulation or deposition of small particules and subsequent cementation of mineral or organic particles on the floor of oceans or other bodies of water at the Earth's surface. Sedimentation is the collective name for processes; the particles that form a sedimentary rock are called sediment, may be composed of geological detritus or biological detritus. Before being deposited, the geological detritus was formed by weathering and erosion from the source area, transported to the place of deposition by water, ice, mass movement or glaciers, which are called agents of denudation. Biological detritus was formed by bodies and parts of dead aquatic organisms, as well as their fecal mass, suspended in water and piling up on the floor of water bodies. Sedimentation may occur as dissolved minerals precipitate from water solution; the sedimentary rock cover of the continents of the Earth's crust is extensive, but the total contribution of sedimentary rocks is estimated to be only 8% of the total volume of the crust.
Sedimentary rocks are only a thin veneer over a crust consisting of igneous and metamorphic rocks. Sedimentary rocks are deposited in layers as strata; the study of sedimentary rocks and rock strata provides information about the subsurface, useful for civil engineering, for example in the construction of roads, tunnels, canals or other structures. Sedimentary rocks are important sources of natural resources like coal, fossil fuels, drinking water or ores; the study of the sequence of sedimentary rock strata is the main source for an understanding of the Earth's history, including palaeogeography and the history of life. The scientific discipline that studies the properties and origin of sedimentary rocks is called sedimentology. Sedimentology is part of both geology and physical geography and overlaps with other disciplines in the Earth sciences, such as pedology, geomorphology and structural geology. Sedimentary rocks have been found on Mars. Sedimentary rocks can be subdivided into four groups based on the processes responsible for their formation: clastic sedimentary rocks, biochemical sedimentary rocks, chemical sedimentary rocks, a fourth category for "other" sedimentary rocks formed by impacts and other minor processes.
Clastic sedimentary rocks are composed of other rock fragments that were cemented by silicate minerals. Clastic rocks are composed of quartz, rock fragments, clay minerals, mica. Clastic sedimentary rocks, are subdivided according to the dominant particle size. Most geologists use the Udden-Wentworth grain size scale and divide unconsolidated sediment into three fractions: gravel and mud; the classification of clastic sedimentary rocks parallels this scheme. This tripartite subdivision is mirrored by the broad categories of rudites and lutites in older literature; the subdivision of these three broad categories is based on differences in clast shape, grain size or texture. Conglomerates are dominantly composed of rounded gravel, while breccias are composed of dominantly angular gravel. Sandstone classification schemes vary but most geologists have adopted the Dott scheme, which uses the relative abundance of quartz and lithic framework grains and the abundance of a muddy matrix between the larger grains.
Composition of framework grains The relative abundance of sand-sized framework grains determines the first word in a sandstone name. Naming depends on the dominance of the three most abundant components quartz, feldspar, or the lithic fragments that originated from other rocks. All other minerals are considered accessories and not used in the naming of the rock, regardless of abundance. Quartz sandstones have >90% quartz grains Feldspathic sandstones have <90% quartz grains and more feldspar grains than lithic grains Lithic sandstones have <90% quartz grains and more lithic grains than feldspar grainsAbundance of muddy matrix material between sand grains When sand-sized particles are deposited, the space between the grains either remains open or is filled with mud. "Clean" sandstones with open pore space are called arenites. Muddy sandstones with abundant muddy matrix are called wackes. Six sandstone names are possible using the descriptors for grain composition and the amount of matrix. For example, a quartz arenite would be composed of quartz grains and have little or no clayey matrix between the grains, a lithic wacke would have abundant lithic grains and abundant muddy matrix, etc.
Although the Dott classification scheme is used by sedimentologists, common names like greywacke and quartz sandstone are still used by non-specialists and in popular literature. Mudrocks are sedimentary rocks composed of at least 50% silt- and clay-sized particles; these fine-grained particles are transported by turbulent flow in water or air, deposited as the flow calms and the particles settle out of suspension. Most authors presently
The chlorites are a group of phyllosilicate minerals. Chlorites can be described by the following four endmembers based on their chemistry via substitution of the following four elements in the silicate lattice. In addition, zinc and calcium species are known; the great range in composition results in considerable variation in physical, X-ray properties. The range of chemical composition allows chlorite group minerals to exist over a wide range of temperature and pressure conditions. For this reason chlorite minerals are ubiquitous minerals within low and medium temperature metamorphic rocks, some igneous rocks, hydrothermal rocks and buried sediments; the name chlorite is in reference to its color. They do not contain the element chlorine named from the same Greek root; the typical general formula is: 34O102 · 36. This formula emphasizes the structure of the group. Chlorites have a 2:1 sandwich structure, this is referred to as a talc layer. Unlike other 2:1 clay minerals, a chlorite's interlayer space is composed of 6.
This 6 unit is more referred to as the brucite-like layer, due to its closer resemblance to the mineral brucite. Therefore, chlorite's structure appears as follows: -t-o-t-brucite-t-o-t-brucite... That's why they are called 2:1:1 minerals. An older classification divided the chlorites into two subgroups: the orthochlorites and leptochlorites; the terms are used and the ortho prefix is somewhat misleading as the chlorite crystal system is monoclinic and not orthorhombic. Chlorite is found in igneous rocks as an alteration product of mafic minerals such as pyroxene and biotite. In this environment chlorite may be a retrograde metamorphic alteration mineral of existing ferromagnesian minerals, or it may be present as a metasomatism product via addition of Fe, Mg, or other compounds into the rock mass. Chlorite is a common mineral associated with hydrothermal ore deposits and occurs with epidote, sericite and sulfide minerals. Chlorite is a common metamorphic mineral indicative of low-grade metamorphism.
It is the diagnostic species of the zeolite facies and of lower greenschist facies. It occurs in the quartz, sericite, garnet assemblage of pelitic schist. Within ultramafic rocks, metamorphism can produce predominantly clinochlore chlorite in association with talc. Experiments indicate that chlorite can be stable in peridotite of the Earth's mantle above the ocean lithosphere carried down by subduction, chlorite may be present in the mantle volume from which island arc magmas are generated. Chlorite occurs in a variety of locations and forms. For example, chlorite is found in certain parts of Wales in mineral schists. Chlorite is found in large boulders scattered on the ground surface on Ring Mountain in Marin County, California. Clinoclore and chamosite are the most common varieties. Several other sub-varieties have been described. A massive compact variety of clinochlore used as a decorative carving stone is referred to by the trade name seraphinite, it occurs in the Korshunovskoye iron skarn deposit in the Irkutsk Oblast of Eastern Siberia.
Chlorite is so soft. The powder generated by scratching is green, it feels oily. The plates are not elastic like mica. Talc feels soapy between fingers; the powder generated by scratching is white. Mica plates are elastic. Various types of chlorite stone have been used as raw material for carving into sculptures and vessels since prehistoric times. List of minerals Thuringite Hurlbut CS, Klein C. Manual of Mineralogy. New York: Wiley & Sons. ISBN 0471805807. Grove TL, Chatterjee N, Parman SW, et al.. "The influence of H2O on mantle wedge melting". Earth Planet. Sci. Lett. 249: 74–89. Bibcode:2006E&PSL.249...74G. Doi:10.1016/j.epsl.2006.06.043. "The Mineral Chlorite". Amethyst Galleries. 1996. Archived from the original on 25 Nov 2004. Retrieved 22 Mar 2019. "Chlorite Group: Mineral information and localities". Mindat.org. Retrieved 22 Mar 2019. "Chlorite". Maricopa.edu. Archived from the original on 12 Nov 2014. Retrieved 22 Mar 2019.]
Limestone is a carbonate sedimentary rock, composed of the skeletal fragments of marine organisms such as coral and molluscs. Its major materials are the minerals calcite and aragonite, which are different crystal forms of calcium carbonate. A related rock is dolostone, which contains a high percentage of the mineral dolomite, CaMg2. In fact, in old USGS publications, dolostone was referred to as magnesian limestone, a term now reserved for magnesium-deficient dolostones or magnesium-rich limestones. About 10% of sedimentary rocks are limestones; the solubility of limestone in water and weak acid solutions leads to karst landscapes, in which water erodes the limestone over thousands to millions of years. Most cave systems are through limestone bedrock. Limestone has numerous uses: as a building material, an essential component of concrete, as aggregate for the base of roads, as white pigment or filler in products such as toothpaste or paints, as a chemical feedstock for the production of lime, as a soil conditioner, or as a popular decorative addition to rock gardens.
Like most other sedimentary rocks, most limestone is composed of grains. Most grains in limestone are skeletal fragments of marine organisms such as foraminifera; these organisms secrete shells made of aragonite or calcite, leave these shells behind when they die. Other carbonate grains composing limestones are ooids, peloids and extraclasts. Limestone contains variable amounts of silica in the form of chert or siliceous skeletal fragment, varying amounts of clay and sand carried in by rivers; some limestones do not consist of grains, are formed by the chemical precipitation of calcite or aragonite, i.e. travertine. Secondary calcite may be deposited by supersaturated meteoric waters; this produces speleothems, such as stalactites. Another form taken by calcite is oolitic limestone, which can be recognized by its granular appearance; the primary source of the calcite in limestone is most marine organisms. Some of these organisms can construct mounds of rock building upon past generations. Below about 3,000 meters, water pressure and temperature conditions cause the dissolution of calcite to increase nonlinearly, so limestone does not form in deeper waters.
Limestones may form in lacustrine and evaporite depositional environments. Calcite can be dissolved or precipitated by groundwater, depending on several factors, including the water temperature, pH, dissolved ion concentrations. Calcite exhibits an unusual characteristic called retrograde solubility, in which it becomes less soluble in water as the temperature increases. Impurities will cause limestones to exhibit different colors with weathered surfaces. Limestone may be crystalline, granular, or massive, depending on the method of formation. Crystals of calcite, dolomite or barite may line small cavities in the rock; when conditions are right for precipitation, calcite forms mineral coatings that cement the existing rock grains together, or it can fill fractures. Travertine is a banded, compact variety of limestone formed along streams where there are waterfalls and around hot or cold springs. Calcium carbonate is deposited where evaporation of the water leaves a solution supersaturated with the chemical constituents of calcite.
Tufa, a porous or cellular variety of travertine, is found near waterfalls. Coquina is a poorly consolidated limestone composed of pieces of coral or shells. During regional metamorphism that occurs during the mountain building process, limestone recrystallizes into marble. Limestone is a parent material of Mollisol soil group. Two major classification schemes, the Folk and the Dunham, are used for identifying the types of carbonate rocks collectively known as limestone. Robert L. Folk developed a classification system that places primary emphasis on the detailed composition of grains and interstitial material in carbonate rocks. Based on composition, there are three main components: allochems and cement; the Folk system uses two-part names. It is helpful to have a petrographic microscope when using the Folk scheme, because it is easier to determine the components present in each sample; the Dunham scheme focuses on depositional textures. Each name is based upon the texture of the grains. Robert J. Dunham published his system for limestone in 1962.
Dunham divides the rocks into four main groups based on relative proportions of coarser clastic particles. Dunham names are for rock families, his efforts deal with the question of whether or not the grains were in mutual contact, therefore self-supporting, or whether the rock is characterized by the presence of frame builders and algal mats. Unlike the Folk scheme, Dunham deals with the original porosity of the rock; the Dunham scheme is more useful for hand samples because it is based on texture, not the grains in the sample. A revised classification was proposed by Wright, it adds some diagenetic patterns and can be summarized as follows: See: Carbonate platform About 10% of all sedimentary rocks are limestones. Limestone is soluble in acid, therefore forms many erosional landforms; these include limestone pavements, pot holes, cenotes and gorges. Such erosion landscapes are known
The pyroxenes are a group of important rock-forming inosilicate minerals found in many igneous and metamorphic rocks. Pyroxenes have the general formula XY2O6 where X represents calcium, iron or magnesium and more zinc, manganese or lithium and Y represents ions of smaller size, such as chromium, iron, cobalt, scandium, vanadium or iron. Although aluminium substitutes extensively for silicon in silicates such as feldspars and amphiboles, the substitution occurs only to a limited extent in most pyroxenes, they share a common structure consisting of single chains of silica tetrahedra. Pyroxenes that crystallize in the monoclinic system are known as clinopyroxenes and those that cystallize in the orthorhombic system are known as orthopyroxenes; the name pyroxene is derived from the Ancient Greek words for stranger. Pyroxenes were so named because of their presence in volcanic lavas, where they are sometimes seen as crystals embedded in volcanic glass. However, they are early-forming minerals that crystallized before the lava erupted.
The upper mantle of Earth is composed of olivine and pyroxene. Pyroxene and feldspar are the major minerals in gabbro; the chain silicate structure of the pyroxenes offers much flexibility in the incorporation of various cations and the names of the pyroxene minerals are defined by their chemical composition. Pyroxene minerals are named according to the chemical species occupying the X site, the Y site, the tetrahedral T site. Cations in Y site are bound to 6 oxygens in octahedral coordination. Cations in the X site can be coordinated depending on the cation size. Twenty mineral names are recognised by the International Mineralogical Association's Commission on New Minerals and Mineral Names and 105 used names have been discarded. A typical pyroxene has silicon in the tetrahedral site and predominately ions with a charge of +2 in both the X and Y sites, giving the approximate formula XYT2O6; the names of the common calcium–iron–magnesium pyroxenes are defined in the'pyroxene quadrilateral' shown in Figure 2.
The enstatite-ferrosilite series contain up to 5 mol.% calcium and exists in three polymorphs, orthorhombic orthoenstatite and protoenstatite and monoclinic clinoenstatite. Increasing the calcium content prevents the formation of the orthorhombic phases and pigeonite only crystallises in the monoclinic system. There is not complete solid solution in calcium content and Mg-Fe-Ca pyroxenes with calcium contents between about 15 and 25 mol.% are not stable with respect to a pair of exolved crystals. This leads to a miscibility gap between augite compositions. There is an arbitrary separation between the diopside-hedenbergite solid solution; the divide is taken at >45 mol.% Ca. As the calcium ion cannot occupy the Y site, pyroxenes with more than 50 mol.% calcium are not possible. A related mineral wollastonite has the formula of the hypothetical calcium end member but important structural differences mean that it is not grouped with the pyroxenes. Magnesium and iron are by no means the only cations that can occupy the X and Y sites in the pyroxene structure.
A second important series of pyroxene minerals are the sodium-rich pyroxenes, corresponding to nomenclature shown in Figure 3. The inclusion of sodium, which has a charge of +1, into the pyroxene implies the need for a mechanism to make up the "missing" positive charge. In jadeite and aegirine this is added by the inclusion of a +3 cation on the Y site. Sodium pyroxenes with more than 20 mol.% calcium, magnesium or iron components are known as omphacite and aegirine-augite, with 80% or more of these components the pyroxene falls in the quadrilateral shown in Figure 2. Table 1 shows the wide range of other cations that can be accommodated in the pyroxene structure, indicates the sites that they occupy. In assigning ions to sites, the basic rule is to work from left to right in this table, first assigning all silicon to the T site and filling the site with the remaining aluminium and iron. Not all the resulting mechanisms to achieve charge neutrality follow the sodium example above, there are several alternative schemes: Coupled substitutions of 1+ and 3+ ions on the X and Y sites respectively.
For example, Na and Al give the jadeite composition. Coupled substitution of a 1+ ion on the X site and a mixture of equal numbers of 2+ and 4+ ions on the Y site; this leads to e.g. NaFe2+0.5Ti4+0.5Si2O6. The Tschermak substitution where a 3+ ion occupies the Y site and a T site leading to e.g. CaAlAlSiO6. In nature, more than one substitution may be found in the same mineral. Clinopyroxenes Aegirine, NaFe3+Si2O6 Augite, 2O6 Clinoenstatite, MgSiO3 Diopside, CaMgSi2O6 Esseneite, CaFe3+ Hedenbergite, CaFe2+Si2O6 Jadeite, NaSi2O6 Jervisite, Si2O6 Johannsenite, CaMn2+Si2O6 Kanoite, Mn2+Si2O6 Kosmochlor, NaCrSi2O6 Namansilite, NaMn3+Si2O6 Natalyite, NaV3+Si2O6 Omphacite, Si2O6 Petedunnite, CaSi2O6 Pigeonite, Si2O6 Spodumene, LiAl2 Orthopyroxenes Hypersthene, SiO3 Donpeacorite, MgSi2O6 Enstatite, Mg2Si2O6 Ferrosilite, Fe2Si2O6 Nchwaningite, Mn2+2SiO32•(H
Garnets are a group of silicate minerals that have been used since the Bronze Age as gemstones and abrasives. All species of garnets possess similar physical properties and crystal forms, but differ in chemical composition; the different species are pyrope, spessartine, grossular and andradite. The garnets make up two solid solution series: pyrope-almandine-spessartine and uvarovite-grossular-andradite; the word garnet comes from the 14th‑century Middle English word gernet, meaning'dark red'. It is derived from granum; this is a reference to mela granatum or pomum granatum, a plant whose fruits contain abundant and vivid red seed covers, which are similar in shape and color to some garnet crystals. Garnet species are found in many colors including red, yellow, purple, blue, black and colorless, with reddish shades most common. Garnet species' light transmission properties can range from the gemstone-quality transparent specimens to the opaque varieties used for industrial purposes as abrasives.
The mineral's luster is categorized as resinous. Garnets are nesosilicates having the general formula X3Y23; the X site is occupied by divalent cations 2+ and the Y site by trivalent cations 3+ in an octahedral/tetrahedral framework with 4− occupying the tetrahedra. Garnets are most found in the dodecahedral crystal habit, but are commonly found in the trapezohedron habit, they crystallize in the cubic system, having three axes that are all of equal length and perpendicular to each other. Garnets do not show cleavage, so when they fracture under stress, sharp irregular pieces are formed; because the chemical composition of garnet varies, the atomic bonds in some species are stronger than in others. As a result, this mineral group shows a range of hardness on the Mohs scale of about 6.5 to 7.5. The harder species like almandine are used for abrasive purposes. For gem identification purposes, a pick-up response to a strong neodymium magnet separates garnet from all other natural transparent gemstones used in the jewelry trade.
Magnetic susceptibility measurements in conjunction with refractive index can be used to distinguish garnet species and varieties, determine the composition of garnets in terms of percentages of end-member species within an individual gem. Almandine: Fe3Al23 Pyrope: Mg3Al23 Spessartine: Mn3Al23 Almandine, sometimes incorrectly called almandite, is the modern gem known as carbuncle; the term "carbuncle" is derived from burning charcoal. The name Almandine is a corruption of Alabanda, a region in Asia Minor where these stones were cut in ancient times. Chemically, almandine is an iron-aluminium garnet with the formula Fe3Al23. Almandine occurs in metamorphic rocks like mica schists, associated with minerals such as staurolite, kyanite and others. Almandine has nicknames of Oriental garnet, almandine ruby, carbuncle. Pyrope is red in color and chemically an aluminium silicate with the formula Mg3Al23, though the magnesium can be replaced in part by calcium and ferrous iron; the color of pyrope varies from deep red to black.
Pyrope and spessartine gemstones have been recovered from the Sloan diamondiferous kimberlites in Colorado, from the Bishop Conglomerate and in a Tertiary age lamprophyre at Cedar Mountain in Wyoming. A variety of pyrope from Macon County, North Carolina is a violet-red shade and has been called rhodolite, Greek for "rose". In chemical composition it may be considered as an isomorphous mixture of pyrope and almandine, in the proportion of two parts pyrope to one part almandine. Pyrope has tradenames. Another intriguing find is the blue color-changing garnets from Madagascar, a pyrope-spessartine mix; the color of these blue garnets is not like sapphire blue in subdued daylight but more reminiscent of the grayish blues and greenish blues sometimes seen in spinel. However, in white LED light, the color is equal to the best cornflower blue sapphire, or D block tanzanite. Pyrope is an indicator mineral for high-pressure rocks; the garnets from mantle-derived rocks and eclogites contain a pyrope variety.
Spessartine or spessartite is manganese aluminium garnet, Mn3Al23. Its name is derived from Spessart in Bavaria, it occurs most in granite pegmatite and allied rock types and in certain low grade metamorphic phyllites. Spessartine of an orange-yellow is found in Madagascar. Violet-red spessartines are found in rhyolites in Maine. Blue pyrope–spessartine garnets were discovered in the late 1990s in Bekily, Madagascar; this type has been found in parts of the United States, Kenya and Turkey. It changes color from blue-green to purple depending on the color temperature of viewing light, as a result of the high amounts of vanadium. Other varieties of color-changing garnets exist. In daylight, their color ranges fro