# Volcanoes/Volcanic minerals

Volcanic minerals are the ones expected to be found in volcanic rocks.

## Allotriomorphs

Def. a "mineral that did not develop its otherwise typical external crystal form because of late crystallization between earlier formed crystals, typical of matrix minerals in rapidly crystallizing volcanic lavas and shallow igneous intrusions" is called an allotriomorph, or xenomorph.

## Felsic minerals

Def. a mineral enriched "in the lighter elements such as silicon, oxygen, aluminium, sodium, and potassium" is called a felsic mineral.
These are muscovite, orthoclase, and the sodium-rich plagioclase feldspars and quartz.

## Metasilicates

Def. the "oxyanion of silicon SiO$_3$$^{2-}$ or any salt or mineral containing this ion" is called a metasilicate.
"This hydrated sodium-manganese silicate [raite] extends the already wide range of manganese crystal chemistry (3), which includes various complex oxides in ore deposits and nodules from the sea floor and certain farming areas, the pyroxmangite analog of the lunar volcanic metasilicate pyroxferroite, the Mn analog yofortierite of the clay mineral palygorskite, and the unnamed Mn analog of sepiolite."
The pyroxferroite crystals in the image on the right are 0.6 x 1.1 x 0.7 cm in dimensions.

## Orthosilicates

Def.
* "any salt or ester of orthosilicic acid, (M$^+$)$_4$SiO$_4$$^{4−}$ or Si(OR)$_4$" or
* "any silicate mineral, such as garnet or olivine, in which the SiO$_4$ tetrahedra do not share oxygen atoms with each other"
is called an orthosilicate.

## Olivines

Def. "[a]ny of a group of olive green magnesium-iron" orthosilicates "that crystallize in the orthorhombic system" is called an olivine.
At right is a visual close up of green sand which is actually olivine crystals that have been eroded from lava rocks. Some olivine crystals are still inside the lava rock.
"Forsterite (Mg$_2$SiO$_4$) is the magnesium rich end-member of the olivine solid solution series."
"Forsterite is associated with igneous and metamorphic rocks and has also been found in meteorites. In 2005 it was also found in cometary dust returned by the Stardust probe. In 2011 it was observed as tiny crystals in the dusty clouds of gas around a forming star."
"Two polymorphs of forsterite are known: wadsleyite (also orthorhombic) and ringwoodite (isometric). Both are mainly known from meteorites."

## Garnets

"Abundances of rare earth elements, Hf, Sc, Co, Cr and Th in garnet megacrysts and their volcanic hosts or matrices are used to estimate garnet/liquid partition coefficients for these elements."
"The wide variation in garnet/liquid partition coefficients from kimberlites to rhyolites cannot be explained as an effect of temperature and we conclude that a major factor is the composition of the melt from which the garnet crystallized."
"Garnet = Mineral Group of general formula: A$_3$B$_2$(SiO$_4$)$_3$, A = Ca, Mg, Fe$^{2+}$, Mn$^{2+}$; B = Al, Fe$^{3+}$, Cr, V, Zr, Ti."
Def. a "hard transparent mineral that is often used as gemstones and abrasives" is called a garnet.

## Nesosilicates

Def. "any simple silicate mineral in which the SiO$_4$ tetrahedra are isolated and have metal ions as neighbours" is called a neosilicate.
Def. a "type of silicate crystal structure characterized by the linking of SiO$_4$ tetrahedra through other cations rather than the sharing of oxygens among SiO$_4$ tetrahedra" is called a nesosilicate.

## Almandines

"Almandine garnet-bearing andesites and dacites occur frequently in the Neogene calc-alkaline volcanic series of the northern Pannonian Basin (Hungary and Slovakia)."
"Coexisting phenocrysts of primary garnets include Ca-rich plagioclase, hornblende (magnesiohastingsite to tschermakite) and/or biotite. The primary garnets have high CaO (>4 wt %) and low MnO contents (<3 wt %). They have strongly light rare earth element depleted patterns and are enriched in heavy rare earth elements."
Almandine (or almandite) has the chemical formula Fe$^{2+}$$_3$Al$_2$Si$_3$O$_{12}$.

## Andalusites

Although andalusite is a metamorphic mineral, it also occurs as in the specimen on the right in volcanic rocks.
Def. an "aluminium nesosilicate mineral, Al$_2$SiO$_5$" is called an andalusite.

## Kyanites

"Volcanoes of the Eifel district have sampled the total crust. [...] Types with chlorite + clinozoisite + oligoclase, with chlorite + garnet + biotite + clinozoisite + oligoclase lead to types with garnet + clinozoisite within garnet but none outside, kyanite + staurolite + almandite + oligoclase."

## Sillimanites

"Ash-flow tuffs of Neogene age exposed over 2,500 km$^2$ in the Macusani region of southeastern Peru are the volcanic equivalent of S-type granites. The strongly peraluminous tuffs contain phenocrysts of andalusite, sillimanite, and muscovite and have high $^{87}$Sr/$^{86}$Sr$_i$ (0.7258 and 0.7226) and δ$^{18}$O (+11‰)."
Sillimanite is a nesosilicate.

## Staurolites

"Staurolite from the Dry River South volcanic-hosted massive sulfide deposit in northern Queensland, Australia is enriched in ZnO (2.5–6.8%) only within the massive sulfide lens or in highly pyritic biotite-chlorite schist just below the massive sulfide lens."
"Staurolite grains from the Dry River South and other massive sulfide lenses have low TiO$_2$ concentrations (mainly <0.4%) relative to staurolite grains from metasediments and alteration zones (mainly >0.4%). The low concentration of TiO$_2$ in staurolite from the massive sulfide lens results from the low initial Ti concentration in exhalative ores. High ZnO and low TiO$_2$ values are indicative of staurolite associated with Zn-rich massive sulfide."

## Sorosilicates

Def. any group of silicates that have structurally isolated double tetrahedra (the dimeric anion Si$_2$O$_7$$^{6-}$) is called a sorosilicate.

## Epidotes

Epidote is another usually metamorphic mineral that can occur in volcanic rocks.
Def. any "of a class of mixed calcium iron aluminium sorosilicates found in metamorphic rocks" is called an epidote.

## Clinozoisites

"Epidote and clinozoisite are widely distributed in the andesitic lavas and tufts of the Borrowdale Volcanic Series. Four main types of occurrence may be distinguished, namely quartz-epidote vein fillings; 'Shap type' veins, bordered by pink reaction zones rich in SiO$_2$, Na$_2$O, and K$_2$O; autometasomatized lavas and tufts; and tufts subjected to alkali metasomatism. Epidote alone formed in the quartz-epidote veins; clinozoisite is found with epidote in the other types of occurrence."
Def. a sorosilicate, basic calcium aluminosilicate "mineral found in crystalline schists, a metamorphic product of calcium feldspar" is called a clinozoisite.

## Vesuvianites

"Minerals such as harkerite, wilkeite, cuspidine, cancrinite, vesuvianite and phlogopite indicate the intrusive melt had a high volatile content which is in agreement with the very high explosivity index of this volcanic district."
Def. a "yellow, green or brown mineral, a mixed calcium, magnesium and aluminium silicate sometimes used as a gemstone" is called a vesuvianite.

## Cyclosilicates

Def. any group of silicates that have a ring of linked tetrahedra is called a cyclosilicate.

## Inosilicates

Def. "any silicate having interlocking chains of silicate tetrahedra" is called an inosilicate.
To form these chains, each silica tetrahedron shares two oxygens with neighboring tetrahedra.
Single chain tetrahedra are the pyroxenes. Double chains of tetrahedra are the amphiboles.

## Pyroxenes

Def. a group of monoclinic or orthorhombic, single chain inosilicates with the general formula of X Y(Si,Al)$_2$O$_6$, where
    X is calcium, sodium, ferrous iron (Fe$^{2+}$), magnesium, zinc, manganese and lithium;
    Y is chromium, aluminum, ferric iron (Fe$^{3+}$), magnesium, manganese, scandium, titanium, vanadium, and ferrous iron (Fe$^{2+}$)
is called a pyroxene.
"The pyroxenes are a group of important rock-forming inosilicate minerals found in many igneous and metamorphic rocks. They share a common structure consisting of single chains of silica tetrahedra and they crystallize in the monoclinic and orthorhombic systems. Pyroxenes have the general formula XY(Si,Al)$_2$O$_6$ (where X represents calcium, sodium, iron$^{+2}$ and magnesium and more rarely zinc, manganese and lithium and Y represents ions of smaller size, such as chromium, aluminium, iron$^{+3}$, magnesium, manganese, scandium, titanium, vanadium and even iron$^{+2}$). Although aluminium substitutes extensively for silicon in silicates such as feldspars and amphiboles, the substitution occurs only to a limited extent in most pyroxenes."
At right is an image of a very rare, sharp, complete-all-around pyroxene is from Ducktown District, Polk County, Tennessee, USA, circa mid to late 1800s.

## Wollastonites

"Primary silicate–melt and carbonate–salt inclusions occur in the phenocrysts (nepheline, fluorapatite, wollastonite, clinopyroxene) in the 1917 eruption combeite–wollastonite nephelinite at Oldoinyo Lengai."

## Amphiboles

Def. a group of monoclinic or orthorhombic double chain inosilicates with the general formula of
    X$_2$Y$_5$Z$_8$O$_{22}$(OH)$_2$ where
    X is magnesium, ferrous iron (Fe$^{2+}$), calcium, lithium, sodium, and ferric iron (Fe$^{3+}$)
    Y is Al, Mg, or Fe or less commonly Mn, Cr, Ti, Li, etc.
    Z is chiefly Si or Al
is called an amphibole.
Def. a "large group of structurally similar hydrated double silicate minerals, containing various combinations of sodium, calcium, magnesium, iron, and aluminium/aluminum" is called an amphibole.

## Hornblendes

"Hornblende phenocrysts in recent andesites of the Soufrière Hills Volcano display reaction rims of microcrystalline plagioclase, pyroxene, Fe-oxides and interstitial glass, formed by decompression during magma ascent."
A chemical formula for hornblendes is (Ca, Na, K)$_{2-3}$(Mg, Fe$^{2+}$, Fe$^{3+}$, Al)$_5$(Si, Al)$_8$O$_{22}$(OH)$_2$.

## Phyllosilicates

Def. any "silicate mineral having a crystal structure of parallel sheets of silicate tetrahedra" is called a phyllosilicate.
Phyllosilicate tetrahedra share three oxygens with other silica tetrahedra to form two-dimensional sheets.

## Micas

Def. a group of monoclinic phyllosilicates with the general formula
    X$_2$Y$_{4–6}$Z$_8$O$_{20}$(OH,F)$_4$
    in which X is K, Na, or Ca or less commonly Ba, Rb, or Cs;
    Y is Al, Mg, or Fe or less commonly Mn, Cr, Ti, Li, etc.;
    Z is chiefly Si or Al, but also may include Fe$^{3+}$ or Ti;
    dioctahedral (Y = 4) and trioctahedral (Y = 6)
is called a mica.

## Biotites

Biotite has the chemical formula "K(Mg, Fe)$_3$(Al, Fe)Si$_3$O$_{10}$(OH, F)$_2$".
Def. a "dark brown mica; it is a mixed aluminosilicate and fluoride of potassium, magnesium and iron" is called a biotite.

## Muscovites

Def. a "pale brown mineral of the mica group, being a basic potassium aluminosilicate with the chemical formula KAl$_2$(Si$_3$Al)O$_{10}$(OH],F)$_2$" is called a muscovite.
"The strongly peraluminous tuffs contain phenocrysts of andalusite, sillimanite, and muscovite and have high $^{87}$Sr/$^{86}$Sr$_i$ (0.7258 and 0.7226) and δ$^{18}$O (+11‰). Elevated concentrations of Li, Cs, Be, Sn, B, and other minor elements compare with those in “tin granites.”"

## Tektosilicates

Def. a type "of silicate crystal structure characterized by the sharing of all SiO$_4$ tetrahedral oxygens resulting in three-dimensional framework structures" is called a tektosilicate.
Def. any "of various silicate minerals ... with a three-dimensional framework of silicate tetrahedra" is called a tectosilicate.

## Feldspars

Def. "[a]ny of a large group of ... aluminum [tectosilicates] of the alkali metals sodium, potassium, calcium and barium" is called feldspar.
As the sample on the right shows two different cooling rates even in volcanic rocks can yield euhedral feldspar crystals (light yellow) in a rhyolite (tiny crystal) matrix.

## Oligoclases

"The apical parts of large volcanoes along the East Pacific Rise (islands and seamounts) are encrusted with rocks of the alkali volcanic suite (alkali basalt, andesine- and oligoclase-andesite, and trachyte)."

## Orthoclases

"The mineralogical composition of most feldspars can be expressed in terms of the ternary system Orthoclase (KAlSi$_3$O$_8$), Albite (NaAlSi$_3$O$_8$) and Anorthite (CaAl$_2$Si$_2$O$_8$)."
"The minerals of which the composition is comprised between Albite and Anorthite are known as the plagioclase feldspars, while those comprised between Albite and Orthoclase are called the alkali feldspars due to the presence of alkali metals sodium and potassium."
"Volcanic rock fragments, feldspar (orthoclase, andesine, and rare sanidine), and small amounts of quartz, biotite, and hornblende make up the bulk of these rocks."

## Plagioclases

Def. "[a]ny of a group of aluminum silicate feldspathic minerals ranging in their ratio of calcium to sodium" is called plagioclase.

## Feldspathoids

Volcanic sources that have a low silica concentration are more likely to produce feldspathoid-containing rocks than feldspar-containing rocks.
Feldspathoid volcanic rocks occur in "a suite of basanites, olivine nephelinites, and olivine melilite nephelinites from the Raton-Clayton volcanic field, New Mexico."
"Volcanism in the Raton-Clayton field commenced approximately 7.5 Ma ago with the eruption of alkali basalts and continued intermittently until at least 10,000 y.a. with the eruption of the Capulin Mountain silicic alkalic basalt (Stormer 1972a; Baldwin and Muehlberger 1959). The entire volcanic sequence was erupted onto the high plains east of the Sangre de Cristo Mountains, and as such, represents the eastern limit of late Cenozoic volcanism in the western U.S. Volcanic activity in the Raton-Clayton field was contemporaneous with volcanism in the Rio Grande rift, and the Raton-Clayton volcanic field is interpreted as part of the Rio Grande rift system."
Def. any of a group of silicates "that did not contain enough silica to satisfy all the chemical bonds" of the framework is called a feldspathoid.

## Analcimes

The image on the right contains analcime, or analcite, as colorless sharply formed undamaged crystals to 25 mm in diameter on a 78 mm x 65 mm x 53 mm matrix. They are associated with numerous black prismatic terminated crystals of aegirine, as well as smaller colorless prismatic terminated crystals of natrolite, these from 3 mm to 10 mm in length. Aegirine is a pyroxene.
Def. a "mineral, a sodium aluminosilicate [with a chemical formula NaAlSi$_2$O$_6$·H$_2$O,] having a zeolite structure, found in alkaline basalts" is called an analcime.

## Kalsilites

Def. "a rare [feldspathoid] mineral, a form of KAlSiO$_4$, found in volcanic rocks in parts of Uganda" is called a kalsilite.
"Kaliophilite [occurs in] blocks of biotite-pyroxenite volcanic ejecta from Mte. Somma, Vesuvius."

## Leucites

Def. a feldspathoid "mineral of silica-poor igneous, plutonic and volcanic rocks" is called a leucite.

## Natrolites

Natrolite is another feldspathoid like analcime of the zeolite group.
"Occurs chiefly in cavities in basalt".
Def. a "fibrous zeolite mineral, being a sodium aluminosilicate, of the chemical formula Na$_2$Al$_2$Si$_3$O$_{10}$·2H$_2$O" is called a natrolite.

## Nephelines

Def. a (Na,K)AlSiO$_4$ "feldspathoid mineral of silica-poor igneous, plutonic and volcanic rocks" is called a nepheline.

## Quadridavynes

Def. a feldspathoid, tektosilicate "mineral found in volcanic ash", chemical formula Na$_6$Ca$_2$Si$_6$Al$_6$O$_{24}$Cl$_4$, is called a quadridavyne.
Quadridavyne is a tektosilicate (feldspathoid), chemical formula Na$_6$Ca$_2$Si$_6$Al$_6$O$_{24}$Cl$_4$, with a type locality of Ottaviano, Monte Somma, Somma-Vesuvius Complex, Naples Province, Campania, Italy.

## Quartzes

Red "thermoluminescence (RTL) emission from quartz, as a dosimeter for baked sediments and volcanic deposits, [from] older (i.e., >1 Ma), quartz-bearing known age volcanic deposits [can use as standards] independently-dated silicic volcanic deposits from New Zealand, ranging in age from 300 ka through to 1.6 Ma."

## Alpha quartzes

Def. "a continuous framework [tectosilicate] of SiO$_4$ silicon–oxygen tetrahedra, with each oxygen being shared between two tetrahedra, giving an overall [chemical] formula [of] SiO$_2$ ... [of] trigonal trapezohedral class 3 2", usually with some substitutional or interstitial impurities, is called α-quartz.
When the concentration of interstitial or substitutional impurities becomes sufficient to change the space group of a mineral such as α-quartz, the result is another mineral. When the physical conditions are sufficient to change the solid space group of α-quartz without changing the chemical composition or formula, another mineral results.
"Shocked quartz is associated with two high pressure polymorphs of silicon dioxide: coesite and stishovite. These polymorphs have a crystal structure different from standard quartz. Again, this structure can only be formed by intense pressure, but moderate temperatures. High temperatures would anneal the quartz back to its standard form."
"Short-rived bottle-green or blue luminescence colours with zones of non-luminescing bands are very common in authigenic quartz overgrowths, fracture fillings or idiomorphic vein crystals. Dark brown, short-lived yellow or pink colours are often found in quartz replacing sulphate minerals. Quartz from tectonically active regions commonly exhibits a brown luminescence colour. A red luminescence colour is typical for quartz crystallized close to a volcanic dyke or sill."
Referring to the image on the right: "Lake County experienced incredible volcanic activity. The heat melted the quartz but temperatures and pressures were just right so it was not destroyed. Rather, the melted quartz was carried along with the lava flows."

## Beta quartzes

Beta quartz (β-Quartz) is stable "between 573° and 870°C".

## Seifertites

Def. a polymorph of α-quartz formed at an estimated minimum pressure of 35 GPa up to pressures above 40 GPa with a orthorhombic space group Pmmm no. 47 is called seifertite.

## Tridymites

Def. a polymorph of α-quartz formed at temperatures from 22-460°C with at least seven space groups for its forms with tabular crystals is called tridymite.
Def. a "rare [tektosilicate] mineral of volcanic rocks that solidified at a high temperature, [with the chemical composition of silicon dioxide, SiO$_2$2,] chemically identical to quartz, but has a different crystal structure" is called a tridymite.
α-tridymite is orthorhomic and β-tridymite is hexagonal.
Crystal phases of tridymite :
| Name | Symmetry | Space group | T (°C) |
| HP (β) | Hexagonal | P6$_3$/mmc | 460 |
| LHP | Hexagonal | P6$_3$22 | 400 |
| OC (α) | Orthorhombic | C222$_1$ | 220 |
| OS | Orthorhombic | | 100–200 |
| OP | Orthorhombic | P2$_1$2$_1$2$_1$ | 155 |
| MC | Monoclinic | Cc | 22 |
| MX | Monoclinic | C1 | 22 |

## Coesites

Alpha-quartz (space group P3$_1$21, no. 152, or P3$_2$21, no. 154) under a high pressure of 2-3 gigapascals and a moderately high temperature of 700°C changes space group to monoclinic C2/c, no. 15, and becomes the mineral coesite.
Coesite is "found in extreme conditions such as the impact craters of meteorites".

## Stishovites

Def. a polymorph of α-quartz formed by pressures > 100 kbar or 10 GPa and temperatures > 1200 °C is called stishovite.
Stishovite may be formed by an instantaneous over pressure such as by an impact or nuclear explosion type event.
"[M]inute amounts of stishovite has been found within diamonds".

## Cristobalites

Def. a high-temperature (above 1470°C) polymorph of α-quartz with cubic, Fd3m, space group no. 227, and a tetragonal form (P4$_1$2$_1$2, space group no. 92) is called cristobalite.
Def. a "mineral of volcanic rocks that solidified at a high temperature [...] chemically identical to quartz, with the chemical formula SiO$_2$, but has a different crystal structure" is called cristobalite.

## Kaliophilites

Def. "a rare mineral, a form of KAlSiO$_4$, found in volcanic rocks in parts of Italy" is called a kaliophilite.

## Stilbites

Stilbite (Desmine), a zeolite group, has the chemical formula NaCa$_2$Al$_5$Si$_{13}$O$_{36}$•16(H$_2$O).
Def. a "tectosilicate zeolite mineral consisting of hydrated calcium aluminium silicate, common in volcanic rocks" is called a stilbite.
Stellerite has the chemical formula CaAl$_2$Si$_7$O$_{18}$•7(H$_2$O).
Def. a "hydrated calcium aluminosilicate zeolite, similar to stilbite" is called a stellerite.
Barrerite can have the chemical formula Na$_4$Al$_4$Si$_{14}$O$_{36}$•13(H$_2$O).
"Ca may be calcium and/or potassium." Barrerite can have the chemical formula K$_4$Al$_4$Si$_{14}$O$_{36}$•13(H$_2$O).
Def. a "white to pinkish tectosilicate zeolite mineral" is called a barrerite.
Stilbite-Na can have the chemical formula Na$_3$Ca$_3$Al$_8$Si$_{28}$O$_{72}$•30(H$_2$O).
Stilbite-Ca can have the chemical formula NaCa$_4$Al$_8$Si$_{28}$O$_{72}$•30(H$_2$O).

## Sassolites

Def. a "volcanic mineral form of boric acid originally found in Sasso, [near Volterra in] Italy" is called sassolite.
Sassolite has the chemical formula H $_3$BO $_3$.
Sassolite is a borate mineral, and is the mineral form of boric acid, occurs in volcanic fumaroles and hot springs, as well as in bedded sedimentary evaporite]] deposits.
Its mineral form was first described in 1800, and was named after Sasso Pisano, Castelnuovo Val di Cecina, Pisa Province, Tuscany, Italy where it was found. The mineral may be found in lagoons throughout Tuscany and Sasso. Usually coloured white to gray, it is colourless in transmitted light, and can also take on a yellow colour from sulfur impurities, or brown from iron oxides.

## Sal ammoniacs

Def. "a rare mineral composed of ammonium chloride found around volcanic fumaroles and guano deposits" is called a sal ammoniac, or salammoniac.
Salammoniac has the chemical formula NH $_4$Cl.
Salammoniac, also sal ammoniac or salmiac, is a rare naturally occurring mineral composed of ammonium chloride, NH$_4$Cl. It forms colorless, white, or yellow-brown crystals in the isometric-hexoctahedral class. It has very poor cleavage and is brittle to conchoidal fracture. It is quite soft, with a Mohs hardness of 1.5 to 2, and it has a low specific gravity of 1.5. It is water-soluble. Sal ammoniac is also the archaic name for the chemical compound ammonium chloride.
Sal ammoniac crystal from Ravat Village, Tajikistan, in the image second down on the left is one of many unusual sal ammoniac crystal specimens found in the area of Ravat Village, near Yaghnob River, where the crystals have grown in a feather-like or three-dimensional arborescent.

## Mascagnites

Def. an "orthorhombic-dipyramidal mineral containing hydrogen, nitrogen, oxygen, and sulfur; [a native sulfate of ammonia, found in volcanic districts]" is called a mascagnite.
Mascagnite has the chemical formula (NH $_4$) $_2$SO $_4$.
It occurs in fumaroles, as at Mount Vesuvius and associated with coal seam fires. It was named for Italian anatomist Paolo Mascagni (1752–1815) who first described the mineral.

## Hypotheses

* Volcanic minerals can be formed at room temperature using electrochemistry.