# Geominerals/Sorosilicates

Def. any group of silicates that have structurally isolated double tetrahedra (the dimeric anion Si$_2$O$_7$$^{6-}$) is called a sorosilicate.
"There are four distinct Si positions, all of which are tetrahedrally coordinated; one of the silicate tetrahedra is an acid silicate group: SiO $_3$(OH). The four Si tetrahedra form a linear [Si $_4$O $_{12}$(OH)] cluster [that is] a sorosilicate."
Let 𝞍 be an unspecified anion.
"The silicate tetrahedra polymerize to form an [Si $_4$𝞍$_{13}$, 𝞍:O$^{2−}$ , OH$^−$ ] sorosilcate fragment. These sorosilicate fragments matchup with the large interstices between the strips of edge-sharing octahedra [...], linking to both the octahedra of adjacent strips and to the shared edge of the [Mn $_2$O $_6$] dimer of tetrahedra."
"The detailed stereochemistry of extended [T $_n$𝞍$_{(3n+1)}$], n > 2 sorosilicate clusters was considered by Hawthorne (1984)".
All "sorosilicate clusters with n > 3 have acid tetrahedral groups."
"Sorosilicate structures with short clusters ([Si $_2$𝞍$_7$], [Si $_3$𝞍$_{10}$]) are based on sheets or walls of polyhedra (octahedra or octahedra and tetrahedra) that are cross-linked by sorosilicate clusters that extend perpendicular to the sheets. Sorosilicate structures with longer clusters ([T $_6$𝞍$_{19}$] in medaite, Mn $_6$[VSi $_5$O $_{18}$(OH)], Gramaccioli et al. 1981) consist of sheets or ribbons of octahedra in which the sorosilicate cluster extends parallel to the sheets or ribbons. In ruizite (Ca $_2$Mn $_2$[Si $_4$O $_{11}$(OH) $_2$](OH) $_2$(H $_2$O) $_2$: Hawthorne 1984), discontinuous sheets of edge-sharing octahedra are linked by [Si $_4$𝞍$_{13}$] clusters orthogonal to the sheets. On the other hand, in tiragalloite [Mn $_4$AsSi $_3$O $_{12}$(OH): Gramacciolei et al. 1979] and akatoreite, the sorosilicate clusters are oriented parallel to the sheets of octahedra, and show more structural affinity with the inosilicate structures, in which infinite (T𝞍$_3$) chains generally extend parallel to sheets of edge-sharing octahedra. Thus the sorosilicate composition [T $_4$𝞍$_{13}$] seems to be a transitional point between these two structural themes."

## Akatoreites

Akatoreite has the chemical formula Mn$^{2+}$ $_9$Al $_2$Si $_8$O $_{24}$(OH) $_8$ and crystallizes in the triclinic system.
Empirical formula: Mn$^{2+}$ $_{8.61}$Fe$^{2+}$ $_{0.19}$Mg $_{0.09}$Ca $_{0.05}$Al $_{2.09}$Si $_{7.75}$O $_{23.17}$(OH) $_{8.83}$.
"Results of chemical analyses reported by Read & Reay (1971) indicate that akatoreite has the formula (Mn $_{8.61}$Fe $_{0.19}$Mg $_{0.08}$Ca $_{0.05}$)Al $_{2.09}$Si $_{7.75}$O $_{23.17}$(OH) $_{8.83}$."
Common impurities: Ti,Fe,Mg,Ca.
The International Mineralogical Association (IMA) symbol is Aka.
Akatoreite is a sorosilicate.
Geological setting: "in manganiferous potassium-rich felsic metavolcanics (Norberg, Sweden)."
Occurrence: "as vitreous, orange-brown, massive to radiating twinned columnar aggregates in a manganiferous metachert and carbonate lens on the southeastern margin of the Haast Schist Group, South Island, New Zealand."
Environment: "In a manganiferous metachert and carbonate lens in schists (Akatore Creek, New Zealand)".
"Locality: In New Zealand, three km south of Akatore Creek, east Otago, South Island."
Associated minerals at type locality: Alabandite, Apatite, Pyroxmangite, Quartz, Rhodochrosite, Rhodonite, Spessartine, Tinzenite, Todorokite.
"Akatoreite is associated with rhodochrosite, rhodonite, spessartine, quartz, hübnerite, alabandite, pyroxmangite, tinzenite, apatite, psilomelane, pyrolusite and todorokite."
Common associates: Ganophyllite - (K,Na,Ca) $_2$Mn $_8$(Si,Al) $_{12}$(O,OH) $_{32}$ · 8H $_2$O, Pyrolusite - Mn$^{4+}$ O $_2$ and Rhodochrosite - MnCO $_3$.

## Åkermanites

Structures of Ca $_2$CoSi $_2$O $_7$, Ca $_2$MgSi $_2$O $_7$, and Ca $_2$(Mg $_{0.55}$Fe $_{0.45}$)Si $_2$O $_7$ contain the double tetrahedra chain shown in the diagram on the left.

## Allanites

Allanite has the general formula A $_2$M $_3$Si $_3$O $_{12}$(OH), where the A sites can contain large cations such as Ca$^{2+}$ , Sr$^{2+}$ , and rare-earth elements, and the M sites admit Al$^{3+}$ , Fe$^{3+}$ , Mn$^{3+}$ , Fe$^{2+}$ , or Mg$^{2+}$ among others.
The age of allanite grains that have not been destroyed by radiation can be determined using different techniques.
Allanite is usually black in color, but can be brown or brown-violet, often coated with a yellow-brown alteration product, likely limonite.
It was discovered in 1810 and named for the Scottish mineralogist Thomas Allan (1777–1833). The type locality is Aluk Island, Greenland, where it was first discovered by Karl Ludwig Giesecke.

## Allanite-(Ce)

Allanite-(Ce) has the formula (Ca, Ce) $_2$(Al, Fe$^{2+}$ , Fe$^{3+}$ )(SiO $_4$)(Si $_2$O $_7$)O(OH).
Crystal system: Monoclinic.
Mineral Group: Epidote group.
Member of Allanite Group > Epidote Supergroup.
Occurrence: An accessory in some granites and granite pegmatites, syenites, more rarely in gabbroic pegmatites. Rarely in schists, gneisses, and some contact metamorphosed limestones; a clastic component of sediments.
Association: Epidote, muscovite, fluorite.
Often slightly radioactive due to minor U and/or Th contents; therefore often metamict.
The Ca analogue of uedaite-(Ce).
Geological Setting:A common accessory mineral in many igneous rocks, particularly those that are felsic, and metamorphosed igneous rocks. Also in pegmatites, skarns, and tactites. Not common in volcanic rocks. A detrital mineral.

## Axinites

Axinite-(Mg) or magnesioaxinite, Ca $_2$MgAl $_2$BOSi $_4$O $_{15}$(OH) magnesium rich, can be pale blue to pale violet
| Composition | Ca $_2$(Mn$^{2+}$ , Fe$^{2+}$ , Mg)Al $_2$(BO $_3$)Si $_4$O $_{12}$(OH) |
| Class | Silicates (sorosilicates) |
| Crystal system | Triclinic |
| Mohs' hardness | 6-7.5 |
| Fracture | Conchoidal |
| Cleavage | Good |
| Lustre | Vitreous |
| Streak | White |
| Geological Setting | Low to high grade regionally metamorphosed rocks, contact metamorphic rocks, pegmatites. |
| Associated minerals | Quartz, Epidote, Albite, Actinolite, Calcite, Chlorite Group, Prehnite, Smoky Quartz, Adularia, Microcline. |

## 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.

## 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.

## Harstigites

Harstigite is an extremelly rare sorosilicate (of calcium, manganese and beryllium). Harstigite forms masses colorless to beige in tone. Species presents red fluorescence under SW-UV light.

## Medaites

The medaite in the image on the right has sparkling luster and a rose red color, and is from Molinello Mine, Graveglia Valley, Ne, Genova Province, Liguria, Italy.
"Sorosilicate structures with longer clusters ([T $_6$𝞍$_{19}$] in medaite, Mn $_6$[VSi $_5$O $_{18}$(OH)], Gramaccioli et al. 1981) consist of sheets or ribbons of octahedra in which the sorosilicate cluster extends parallel to the sheets or ribbons."
Formula: Mn$^{2+}$ $_6$V$^{5+}$ Si $_5$O $_{18}$(OH).

## Rosenhahnites

Rosenhahnite has the formula Ca $_3$Si $_3$O $_8$(OH) $_2$.

## Ruizites

Ruizite has the formula Ca $_2$Mn $_2$Si $_4$O $_{11}$(OH) $_4$·2H $_2$O.
Ruizite is a sorosilicate mineral discovered at the Christmas mine in Christmas, Arizona, and described in 1977. The mineral is named for discoverer Joe Ana Ruiz.
Ruizite is translucent and orange to red-brown in color with an apricot yellow streak. The mineral occurs as euhedral prisms up to 1 mm (0.039 in) or as radial clusters of acicular (needle-like) crystals.
Ruizite is common at the Christmas mine. The mineral is known from Arizona, Pennsylvania, and Northern Cape Province, South Africa.
Association: Apophyllite, kinoite, junitoite, wollastonite, grossular, diopside, vesuvianite, calcite, chalcopyrite, bornite, sphalerite, smectite (Christmas, Arizona, USA); apophyllite, pectolite, datolite, inesite, orientite, quartz (Wessels mine, South Africa).
Occurrence: In veinlets and on fracture surfaces in metalimestone, formed during cooling of a high-temperature calc-silicate assemblage under oxidizing conditions (Christmas, Arizona, USA). The mineral formed by retrograde metamorphism during cooling of a calc–silicate skarn assemblage in an oxidizing environment.
Ruizite crystallizes in the monoclinic crystal system and twinning is common along the miller index {100} plane between exactly two crystals. Ruizite's formula was originally identified as CaMn(SiO $_3$) $_2$(OH) $_2$·2H $_2$O in 1977. The formula was revised to Ca $_2$Mn$^{3+}$ $_2$Si $_4$O $_{11}$(OH) $_4$·2H $_2$O.
Ruizite's structure consists of edge-sharing Mnφ$_6$ octahedra, connected at corners into sheets and together into a lattice by clusters of Si $_4$O $_{11}$(OH) $_2$.
Nitric acid, hydrochloric acid, and potassium hydroxide have little effect on ruizite at low temperatures but readily dissolve the mineral at elevated temperatures.
During the investigation of junitoite at the Christmas mine in Christmas, Arizona, Joe Ana Ruiz and Robert Jenkins discovered an unknown brown mineral. Mine geologist Dave Cook located better specimens, and it was determined to be a new mineral species. The mineral was named ruizite in honor of Joe Ruiz as discoverer. Ruizite's properties were analyzed using a sample provided by Joseph Urban, and it was described in the journal Mineralogical Magazine in December 1977. The International Mineralogical Association approved the mineral as IMA 1977-077. Type specimens are housed in the University of Arizona, Harvard University, the National Museum of Natural History, and The Natural History Museum, London.

## Tiragalloites

"Tiragalloite belongs to a rare class of compounds with As atoms replacing Si atoms in silicate anions, the so-called arsenosilicates (analoguous to the more common alumosilicates and borosilicates, in which Si atoms are replaced by Al atoms and B atoms, respectively). It contains isolated chains consisting of one (AsO $_4$) tetrahedron and three (SiO $_4$) tetrahedra, which are linked by sharing corners. One oxygen atom is protonated, resulting in the anion formula (HAsSi $_3$O $_{13}$)$^{8−}$ . The tetrahedral sites in the anion are ordered in that the As atoms are only found at the end of the chains, and there is exactly one As atom in each anion."
Tiragalloite is a sorosilicate with an orange, brownish orange color.
"Named after Paolo Onofrio Tiragallo (1905, Carro, La Spezia, Liguria, Italy - 1987), distinguished amateur mineralogist. Conservator and curator of the mineralogical collection of the University of Genoa."
Tiragalloite has the formula Mn$^{2+}$ $_4$As$^{5+}$ Si $_3$O $_{12}$(OH).
Common impurities: Ti,Al,V,Fe,Ca,Na,K.
Common associates: areniopleites NaCaMn$^{2+}$ Mn$^{2+}$ $_2$(AsO $_4$) $_3$ and rhodonites CaMn$^{2+}$ $_3$Mn$^{2+}$ [Si $_5$O $_{15}$].
The end member compositions are areniopleites NaCaMn$^{2+}$ Mn$^{2+}$ $_2$(AsO $_4$) $_3$ and caryinites CaNaCa$^{2+}$ Mn$^{2+}$ $_2$As $_3$O $_{12}$.

## 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.

## Zoisites

On the right is both a rough stone and a cut stone of tanzanite. Tanzanite is the blue/purple variety of the mineral zoisite (a calcium aluminium hydroxy silicate) with the formula (Ca$_2$Al$_3$(SiO$_4$)(Si$_2$O$_7$)O(OH))]. Tanzanite is noted for its remarkably strong trichroism, appearing alternately sapphire blue, violet and burgundy depending on crystal orientation. Tanzanite can also appear differently when viewed under alternate lighting conditions. The blues appear more evident when subjected to fluorescent light and the violet hues can be seen readily when viewed under incandescent illumination. A rough violet sample of tanzanite is third down at left.
Tanzanite in its rough state is usually a reddish brown color. It requires artificial heat treatment to 600 °C in a gemological oven to bring out the blue violet of the stone.
Tanzanite is found only in the foothills of Mount Kilimanjaro.
Tanzanite is universally heat treated in a furnace, with a temperature between 550 and 700 degrees Celsius, to produce a range of hues between bluish-violet to violetish-blue. Some stones found close to the surface in the early days of the discovery were gem-quality blue without the need for heat treatment.

## Hypotheses

* Most minerals on Earth are oxides.