l>Inosilcates
EENS 212

Petrology

Prof. Stephen A. Nelson rebab.net University
Amphiboles & Phyllosilicates

Inosilicates (Double Chain Silicates) - The Amphiboles

The amphibole group of mineral is based on the double-chain silicate structure as displayed here. The simple structural unit is (Si4O11)-6. The structural formula can be written as: W0-1X2Y5Z8022(OH,F)2

where W = Na+1 or K+1 in the A website with 10 to 12 wrinkles coordination.

You are watching: Hornblende and the other amphiboles have what type of silicate structure?

X = Ca+2, Na+1, Mn+2, Fe+2, Mg+2, Fe+3, in one M4 website with 6 come 8 fold coordination.

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Y = Mn+2, Fe+2, Mg+2, Fe+3, Al+3. Or Ti+4 in an M1 octahedral coordination site. Z = Si+4 and Al+3 in the tetrahedral site.

There is complete solid solution in between Na and Ca end members and amongst Mg and also Fe finish members, with partial substitution that Al+3 for Si+4 in the tetrahedral site, and partial substitution of F for five in the hydroxyl site.

The composition of the typical (non-sodic) amphiboles are shown in the diagram here. Note the similarity to the pyroxene compositional diagram, above. Actinolite is the solid solution between Tremolite and also Ferroactinolite Cummingtonite - Grunerite is a solid solution in between Anthophyllite and also Grunerite .
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Hornblende is the most usual amphibole and has much more in usual with the Tremolite - Ferroactinolite series, through Al substituting right into the Y sites and the tetrahedral site. The thus has actually the complex formula: (Ca,Na)2-3(Mg,Fe,Al)5Si6(Si,Al)2O22(OH,F)2

The sodic amphiboles have actually the adhering to formulae:

Glaucophane - Na2Mg3Al2Si8O22(OH)2

Riebeckite - Na2Fe3+2Fe2+3Si8O22(OH)2

Arfvedsonite - NaNa2Fe4+2Fe+3Si8O22(OH)2

All the the amphiboles except Anthophyllite room monoclinic, and also all present the excellent prismatic cleavage on 110. The angles in between the cleavages, yet are 56o and also 124o making every amphiboles straightforward to distinguish from the pyroxenes. Looking at encounters that show only a solitary cleavage map would show inclined extinction, except in Anthophyllite.
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Occurrence and difference of the Amphiboles Tremolite - Occurs virtually exclusively in low grade metamorphic rocks, particularly those v a high Ca concentration, such as meta-dolomites, meta-ultrabasic rocks. Tremolite in hand specimen is white in color and shows a fibrous habit and the characteristic amphibole cleavage. In thin section it is differentiated from wollastonite and also diopside by its amphibole cleavage. In thin section it is clear through no pleochroism, which distinguishes that from other amphiboles. It reflects high relief, inclined extinction, and is optically an adverse with a 2V of about 85o.

Actinolite - Also occurs virtually exclusively in low grade metamorphic rocks, particularly in meta-basalts and meta-gabbros where it is commonly associated with chlorite. That is eco-friendly in hand specimen and shows the characteristics amphibole cleavage, usually mirroring an elongated habit. In thin ar it mirrors a characteristics pale yellow to environment-friendly pleochroism, has actually high relief, and also is optically an unfavorable with a 2V that 60 to 85o.

Hornblende - is a common mineral in both igneous and metamorphic rocks. In igneous rocks it is uncovered in andesites, dacites, and rhyolites, and in gabbros, diorites, and granites. In metamorphic rocks it is a common constituent that meta-basalts that have been metamorphosed to intermediate grades of local metamorphism (amphibolites). The is likewise found in some ultrabasic rocks. In hand specimen it is dark brown to black in color and shows the characteristic amphibole cleavage. In slim section, it reflects high relief with a characteristic environment-friendly - brown - yellow pleochroism. Optic sign and 2V angle cover a wide variety and not very useful in the difference of hornblende.

Basaltic Hornblende (also referred to as Oxy-hornblende)- is a dark brown to reddish brown variety of hornblende that results from oxidation throughout crystallization the basalts, andesites, dacites, and also rhyolites. It usually has a dark reaction rim that is composed of opaque oxide, and also is characteristically pleochroic in yellow come brown to reddish brown colors.

Anthophyllite - does not occur in igneous rocks, however is a constituent of metamorphic rocks. The is the only orthorhombic amphibole so the is easily characterized by that is parallel extinction relative to the 110 cleavage.

Cummingtonite - Grunerite -is an ext common in metamorphosed igneous rocks wherein members of the collection occur v hornblende. It has actually been uncovered in siliceous volcanic rocks as well. Cummingtonite is optically positive, while grunerite is optically negative. Members that this series can be identified from orthorhombic Anthophyllite through the inclined die out of the monoclinic Cummingtonite-Grunerite series, and also can be differentiated from tremolite and actinolite by the greater refractive indices and higher birefringence the the Cummingtonite Grunerite series.

Glaucophane - Riebeckite - Glaucophane is a common mineral in blueschist facies metamorphic rocks that an outcome from low temperature, high pressure metamorphism along old subduction zones. Riebeckite is found in alkali granites, syenites, and also peralkaline rhyolites. Glaucophane is easily differentiated from the various other amphiboles by its characteristics blue-lavender pleochroism. Glaucophane is length slow, whereas Riebeckite is size fast.

Arfvedsonite - occurs most frequently in peralkaline volcano rocks and also alkaline plutonic igneous rocks, where it generally occurs through the sodic pyroxene aegerine. That blue environment-friendly to yellow environment-friendly pleochroism distinguish it native the other amphiboles.

The chart below, also found in her lab assignments, summarizes the properties offered to identify the amphiboles.

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Phyllosilicates (Sheet Silicates)

The phyllosilicates, or sheet silicates, are an essential group of minerals that consists of the micas, chlorite, serpentine, talc, and also the clay minerals.

The basic structure the the phyllosilicates is based upon interconnected 6 member rings of SiO4-4 tetrahedra that extend outward in unlimited sheets. 3 out the the 4 oxygens from every tetrahedra are mutual with various other tetrahedra. This leader to a an easy structural unit that Si2O5-2.
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most phyllosilicates save hydroxyl ion, OH-, with the OH located at the center of the 6 membered rings, as presented here. Thus, the group becomes Si2O5(OH)-3. Once other cations space bonded to the SiO4 sheets, lock share the apical oxygens and also the (OH) ion which bond come the other cations in octahedral coordination. This forms a class of cations, typically Fe+2, Mg+2, or Al+3, that happen in octahedral coordination through the O and OH ions of the tetrahedral layer. Together shown, here, the triangles come to be the deals with of the octahedral teams that deserve to bind to the tetrahedral layers.

The octahedral layers take on the framework of either Brucite if the cations space +2 ions choose Mg+2 or Fe+2, or Gibbsite if the cations space +3 choose Al+3. In the brucite structure, all octahedral web page are occupied and all of the anions are all OH-1. In the Gibbsite structure every 3rd cation website is unoccupied and every one of the anions are OH-1.
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This provides rise come 2 teams of paper silicates: The trioctahedral paper silicates whereby each O or five ion is surrounding by 3 divalent cations, choose Mg+2 or Fe+2. The dioctahedral sheet silicates where each O or oh ion is surrounded by 2 trivalent cations, commonly Al+3.

We can construct the structures of the various sheet silicates by starting with the octahedral layers comparable to the structures of brucite or gibbsite, as questioned last semester in planet Materials. Review your notes from the clay minerals discussion.

Serpentine Group

The serpentine team of minerals has actually the formula - Mg3Si2O5(OH)4. Three ranges of serpentine space known. Antigorite and also Lizardite space usually massive and also fine grained, while Chrisotile is fibrous. As disputed above, the imperfect right of the octahedral layers and also the tetrahedral layers causes the crystal structure to have to bend.

In Antigorite the bending the the sheets is no continuous, however occurs in sets, comparable to corrugations, as displayed here.
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In Chrisotile, the bending the the sheets is more continuous, result in constant tubes that give the mineral it"s fibrous habit. The Chrisotile variety is frequently referred to as asbestos.
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Occurrence - Serpentine is uncovered as an alteration product that Mg-rich silicates like pyroxene and also olivine. It results as result of hydration. Because that example: 2Mg2SiO4 + 3H2O Mg3Si2O5(OH)4 + Mg(OH)2 Olivine water Serpentine Brucite

Thus, serpentine is typically found pseudomorphed after ~ olivines and pyroxenes in altered an easy and ultrabasic igneous rocks, like altered peridotites, dunites, and also sometimes basalts and gabbros. The is commonly associated with minerals choose magnesite (MgCO3), chromite, and magnetite. If the absent is make up practically entirely that serpentine, it is referred to as a serpentinite.

Properties - due to the fact that the serpentines usually occur either as fine-grained aggregates or fibrous crystals, optical nature are complicated to determine. Many of the time, serpentine have the right to be differentiated by its characteristic pseudomorphing of various other crystals favor olivines and pyroxenes. In hand specimen it usually tends to have actually a dark green shade with a greasy luster. In thin ar it is clean to pale eco-friendly to pale yellow, yet does not display pleochroism, mirrors a generally low relief contrasted to minerals like olivine and also pyroxene through which the is associated, and also show very low bespeak interference colors as result of its low birefringence.

Talc

Talc has the chemical formula - Mg3Si4O10(OH)2. That is probably finest know because that its short hardness. Return it has actually a micaceous structure, it is so quickly deformed, the crystals are seldom seen.

Occurrence - favor serpentine, talc needs an atmosphere rich in Mg. That is therefore found in short grade metamorphic rocks that originated as ultrabasic to straightforward igneous rocks. Rocks composed virtually entirely the talc have actually a greasy feel and also are described as soapstone.

Properties -Talc is many easily distinguished in hand specimen by its short hardness, greasy feel, and association with other Mg-bearing minerals. Once crystals are present they display the properties micaceous cleavage ~ above 001. In thin section, talc is colorless, biaxial an adverse with a 2V that 0 come 30o. Like other sheet silicates, it reflects the well occurred 001 cleavage. Maximum interference colors, continual with a birefringence of 0.05 is 3o yellow. Muscovite has a higher birefringence and higher 2V, properties which easily identify the 2 minerals.

Mica Group

The micas can be divided into the dioctahedral micas and also the trioctahedral micas, as discussed above. Muscovite, Paragonite, and Margarite space the white micas, and also represent the trioctahedral group, and Biotite and Clintonite (Xanthophyllite) the black or brown mica, to represent the dioctahedral group. Muscovite and Biotite space the most usual micas, but the Lithium- rich, pink mica, Lepidolite, K(Li,Al)2AlSi3O10(OH)2 is also common, being discovered mostly in pegmatites.

Muscovite

Muscovite, KAl3Si3O10(OH)2, and Paragonite, NaAl3Si3O10(OH)2, are two potential finish members the the hard solution collection involving K and Na. But, there is a large miscibility gap in between the two end members through Muscovite being between 65% and 100% of K-rich finish member, and also Paragonite reflecting compositions between about 80% and also 100% of the Na-rich end member.

Occurrence - Muscovite is usual constituent that Al-rich tool grade metamorphic rocks whereby is discovered in Al-rich schists and also contributes to the schistose foliation uncovered in this rocks. Muscovite is likewise found in siliceous, Al-rich plutonic igneous rocks (muscovite granites), but has not been discovered as a ingredient of volcanic rocks. In this rocks that is typically found in association through alkali feldspar, quartz, and also sometimes biotite, garnet, andalusite, sillimanite, or kyanite.

Properties - Muscovite is easily determined in hand specimen through its white to periodically light brownish color and also its perfect 001 cleavage. In slim section, the 001 cleavage is conveniently seen and it"s high birefringence is exhibited by the large change in relief on rotation the the stage and also it"s 2nd to fourth order interference colors. The is clear and shows no pleochroism (which distinguishes that from Biotite), and also it is biaxial an adverse with a 2V in between 28 and 50o. Among the most diagnostic properties of the micas, consisting of muscovite, is the mottled or birds-eye extinction displayed by these minerals.

Biotite

Biotite is a heavy solution in between the finish members Phlogopite KMg3AlSi3O10(OH)2 and Annite KFe3AlSi3O10(OH)2, return pure Annite does not take place in nature. In addition, little amounts of Na, Rb, Cs, and also Ba might substitute for K, and like in various other minerals, F can substitute because that OH and increase the security of Biotite to higher temperatures and also pressures.

Occurrence - nearly pure phlogopite is found in hydrous ultrabasic rocks choose kimberlite, and also is also found in metamorphosed dolomites. Biotite, with more Fe-rich compositions is usual in dacitic, rhyolitic, and also trachytic volcano rocks, granitic plutonic rocks, and a wide selection of metamorphic rocks. In metamorphic rocks, biotite usually shows a wanted orientation with its 001 develops parallel come the schistose foliation.

Properties - In hand specimen, Biotite is brown to black and shows the perfect 001 micaceous cleavage. In slim section, it reflects the perfect cleavage and mottled extinction common of all micas. It"s most characteristic building is that is pleochroism, showing yellow to brown to environment-friendly colors. Hornblende shows comparable pleochroic colors, however is identified from biotite through the differences in cleavage that the 2 minerals. Biotite is biaxial an unfavorable with a short 2V that 0o come 25o.

Chlorite Group As discussed above, the Chlorite group has a structure that is composed of phlogopite T-O-T class sandwiching brucite-like octahedral layer. Over there is considerable substitution that Mg for Fe, and also Al deserve to substitute because that (Mg, Fe) in both the octahedral sites, and also for Si in the tetrahedral sites. Thus, chlorite have the right to have a rather facility formula - (Mg,Fe,Al)3(Si,Al)4O10(OH)6.

Occurrence- Chlorite is a usual mineral in short grade metamorphic rocks, whereby it occurs in association with minerals like actinolite, epidote, and biotite. It likewise forms together an change product the pyroxenes, amphiboles, biotite, and garnet in igneous too a metamorphic rocks.

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Properties - In hand specimen, chlorite is recognized by its green color, micaceous habit and cleavage, and also association with other minerals choose actinolite and epidote. In slim section, Chlorite mirrors low relief and also low birefringence, through a characteristic midnight blue to black color anomalous interference color. It reflects some pleochroism in the selection of environment-friendly to pale yellow. That is easily identified from biotite by its lower relief and anomalous interference color.