GEOL 333 #8 - Silicate Minerals

Overview - Over next 2+ weeks we will discuss chemical composition, crystal structure, geologic occurrence and origin, and environmental significance (uses, hazards) selected important minerals. For 27 minerals, you need to know name, chemical formula, general crystal structure, uses, and environmental significance (indicated in on-line notes with *; summary sheet will be distributed after we finish). For 8 other minerals, you need to know name, crystal structure, uses, and environmental significance (indicated in on-line notes with ^). Also, we will introduce phase diagram, which indicates what minerals will form (most stable minerals) under different conditions of temperature, pressure, and chemical composition (T - P - X).

Silicate Minerals - most important rock-forming minerals. Nearly all minerals in igneous and metamorphic rocks are silicates and many sedimentary rocks (e.g., sandstone and shale) consist mostly of silicates.

Isolated Tetrahedra (Nesosilicates) subclass includes Olivine (link #2) and Garnet (link #2) groups.
Olivine* general formula = (Mg, Fe2+)2 SiO4*, consists of end-members Forsterite* (Fo - Mg2SiO4*) and Fayalite* (Fa - Fe2+2 SiO4*). Most common composition in nature is ~80% Fo/20% Fa. Complete solid solution (continuous range in composition) from Mg- to Fe-end members. Crystal structure of olivine = isolated tetrahedra (that alternate pointing up and down) bonded by metal cations.

Origin of olivine = major rock-forming mineral in igneous rock of mafic and ultramafic composition (rocks rich in Mg/Fe-silicates); sometimes in metamorphic rocks. Olivine forms over range of temperatures (Fig. 5.3, T-X or temperature-composition phase diagram, see figure for generic version of one). In white area above upper (liquidus) curve, system is completely molten. In gray area below lower (solidus) curve, system is completely solid olivine. In white area between two curves, system is partly molten and partly solid olivine. In white area between two curves, horizontal dashed lines link solid olivine composition (points X1, X2, and X3 along solidus) with melt composition (intersections between dashed horizontal lines and liquidus on right side).

Consider completely molten rock with composition of 50% Fo/50% Fa. What are liquid and solid compositions at temperatures listed below?

 Temperature  State (liquid vs. solid)  Solid Composition  Liquid Composition
 > 1,700C      
 ~1,700C      
 ~1580C      
 < 1450C      

If crystals keep in contact with molten rock, they change composition as cooling continues (equilibrium crystallization). Olivine solidifies over broad range of temperatures (~200C), not single temperature.

Other features of olivine: high specific gravity (for silicate), no cleavage (isolated tetrahedra), common varieties (Fo-rich) = clear green. Used as gemstone (peridot, link #2) and refractory sand (resists melting due to very high melting point).

Garnet (link to one specific type of garnet) = compositionally variable (Ca, Mg, Fe, Al-silicate), often rich in Fe and Mg (2 solid solution series, red garnet and green garnet); Occurs in metamorphosed Al-rich rocks as equant 12-sided crystals. Used as abrasive and inexpensive gemstone.

Al2SiO5 Group includes 3 polymorphs - kyanite, sillimanite, and andalusite, form during metamorphism of Al-rich rocks; diagnostic metamorphic minerals that form under different T and P conditions. Kyanite forms at high P, andalusite forms at ~low T (and low P), silimanite forms at high T (and low P). Use = refractory minerals.

Sorosilicates (link #2, pairs of Si-tetrahedra) epidote (hydrous/OH, Ca, Fe, Al-silicate, link #2). Forms in several metamorphic settings including hydrothermal (hot water) alteration of igneous rocks. Pistachio green is common color.

Cyclosilicates (link #2, cyclic rings of 6 Si-tetrahedra) gemstones beryl and tourmaline (chemically complex with extensive solid solutions including boron, link to one type of tourmaline, link #2, gemstone link). Beryl (link #2, technically framework silicate, contains rings) commonly blue-green (aquamarine, link #2) or dark green (emerald, link #2, #3); elongate hexagonal crystals. Economic source of Be. Forms in pegmatite (very coarse-grained silicic igneous rock).

Inosilicates (chain) long chains of linked Si-tetrahedra; 2 important mineral groups - Pyroxene* (single chain) and amphibole (double chain structure). Both = silicate chains bonded by divalent (and monovalent) cations.

 

Pyroxenes are simpler in composition and do not contain water (anhydrous), whereas chemically complex amphiboles contain OH- water (and F- and Cl-). Certain pyroxenes and amphiboles form in igneous rocks, pyroxene forms at higher temperature.
Does that mean mafic or silicic rock composition?

Both have varieties that form in metamorphic rocks, pyroxene at higher metamorphic grade (higher T and P). Both usually are dark with 2 cleavage directions. Pyroxene cleaves at 90, amphibole cleaves at ~60 and 120. Pyroxene forms blocky crystals, amphibole forms longer, thin crystals.

Pyroxene* (link #2) general formula = (Ca, Mg, Fe2+)2Si2O6. Major end-member compositions and 2 major solid solution series for pyroxene on Ca-Mg-Fe compositional triangle (Fig. 5.11). Sometimes only bottom part of triangle is shown (pyroxene quadrilateral). 2 major solid solution series = Orthopyroxene (orthorhombic symmetry, Ca-poor) and Clinopyroxene (monoclinic symmetry, Ca-rich).
Orthopyroxene* (opx) = (Mg, Fe2+)2Si2O6* with enstatite = Mg-end member (link #2), ferrosilite = Fe-end member.

Solid-solution with respect to Fe, Mg, and Ca?

Opx = major rock-forming mineral in mafic and ultramafic igneous rocks (gabbro, peridotite, basalt). Pigeonite = more Ca-rich variety of opx, forms at higher T in quickly cooled igneous rocks.
Clinopyroxene* (cpx) = Ca(Mg, Fe2+)Si2O6*, diopside = Mg-end member (link #2), hedenbergite = Fe-end member (link #2), and augite (link #2) = highly variable intermediate composition.

Solid-solution with respect to Fe, Mg, and Ca?

Cpx major rock-forming mineral in mafic and ultramafic igneous rocks and several metamorphic rocks. Usually both opx and cpx coexist in same mafic rock due to lack of solid solution. 4 straight lines in Fig. 5.11 connect coexisting cpx and opx compositions for 4 different rock compositions. Jadeite (one of varieties of gemstone jade, green color) = Na-rich pyroxene (link to Chicago area museum specializing in jade).

Amphibole (link #2) = much more complicated and highly variable chemical formula
(Na, K)0-1(Ca, Na, Fe2+, Mg)2(Fe, Mg, Al)5(Si, Al)8O22(OH, F)2. Fig. 5.23 shows some chemical variations and end-member compositions (Ca-Mg-Fe triangle). Like pyroxene, Ca-poor and Ca-rich varieties of amphibole.

Solid-solution with respect to Fe, Mg, and Ca?

Hornblende^ (^ = responsible for knowing characteristics of mineral but not chemical formula) = ~any black amphibole (need chemical analysis to specify more). Amphibole = common in igneous rocks of intermediate to mafic composition and metamorphic rocks - common in high-grade metamorphic rock, amphibolite. 3 amphibole minerals (cummingtonite, riebeckite/crocidolite and tremolite) = minor sources of asbestos, that can be major health hazard if breathed into lungs.

Sheet (Phyllo) Silicates (link #2) = flat (2-d) sheets of linked Si-tetrahedra, e.g., mica, serpentine, chlorite, and clay minerals (kaolinite, illite, and smectite)

 

major rock-forming mineral in fine-grained sedimentary rocks and soil; many environmental issues. Clay minerals = very fine-grained (<2m = 2 x 10-6 m) sheet silicates, sedimentary origin. Sheet silicates = platy shape, excellent cleavage along 1 direction (basal cleavage), weak bonds between sheets. Clay minerals = massive in hand sample, using high magnification electron microscope also platy with basal cleavage (see scanning electron microscope/SEM photos of kaolinite and chlorite #1, #2). Crystals of sheet silicates often have hexagonal shape (chlorite photo, kaolinite photo) (or even triangular shape, another chlorite SEM photo), reflecting hexagonal network of Si-tetrahedra.

Low hardness (talc = softest mineral, very weak bonding between sheets). Basic building blocks = tetrahedral sheets (Si2O52- and octahedral sheets (Al, Mg, or Fe octahedra linked by edges to form sheet structure, Fig. 4-1), which are strongly bonded together. Anions of octahedra = OH- and O2-. 2 types of octahedral sheets: trioctahedral sheets (Mg3(OH)6 = brucite) - 3 out of 3 octahedral cation sites are filled and dioctahedral sheets (Al2(OH)6 = gibbsite) - 2 out of 3 octahedral cation sites are filled and one is vacant. Symbol for tetrahedral sheet = trapezoid, octahedral sheet = rectangle. Sheet silicates can be represented by proportion and nature of tetrahedral/octahedral sheets (T-O-T diagrams).

Serpentine and Kaolinite = 1:1 layer silicates (1 tetrahedral sheet:1 octahedral sheet) bonded by weak hydrogen bonds. Kaolinite^ = Al2Si2O5(OH)4, dioctahedral 1:1 layer silicate. Forms in low-temperature alteration (soils) of Al-rich minerals like feldspar. Used for ceramics (porcelain and fine china) and as filler/coating in glossy paper. Can be molded when wet and hardens when heated to high T (>500C).

Serpentine^ group = Mg3Si2O5(OH)4, trioctahedral 1:1 layer silicate. Several polymorphs including chrysotile (link #2) = major source of silky asbestos, not most harmful variety. Commonly green, forms in hydrothermal (100 - 300C) alteration of Mg-silicates (e.g., mafic and ultramafic rocks). Crystal shape = ~rolled rug.

 

Mica Group = 2:1 layer silicate structure (2 tetrahedral sheets surrounding central octahedral sheet). 2:1 layers have negative charge, balanced by tightly held, fixed cations (usually K) in area between 2:1 layers (interlayer). Examples = Muscovite^ (K[Al3Si3O10(OH)2]1-, link #2), dioctahedral 2:1 layer silicate, very common in silicic igneous rocks (granite) and metamorphic rocks (schist and gneiss); Illite^ (dioctahedral 2:1 layer silicate), ~ muscovite in composition and structure, second most abundant mineral in sedimentary rocks, present in most shale; Biotite^ (K[(Mg, Fe)3(Al1Si3)O10(OH)2]), trioctahedral 2:1 layer silicate, very common in igneous and metamorphic rocks. Biotite = extensive Mg-Fe solid solution and usually dark.

Smectite^ ("swelling" clay, link to one kind of smectite, link #2, #3) 2:1 layer silicate structure in which 2:1 layers have negative charge, balanced by loosely held, exchangeable cations (usually Na or Ca) in region between 2:1 layers*. There are >1 planes of water in between 2:1 layers, resulting in very weak bonds between 2:1 layers and interlayer cations. Ability to exchange (take up) cations = cation exchange capacity. Chemical formula = (Na0.3 . nH2O[(Al, Mg, Fe)2Si4O10(OH)2]0.3- where n = ~ 4), making it dioctahedral although trioctahedral varieties exist. Swelling (and shrinking) during wetting and drying involves uptake (or loss) of water between layers, which causes damage to overlying buildings and roads (cracks in foundations and pavement). It is very weak earth material. Origin = low-temperature alteration (soils) of Al-rich minerals such as feldspar and volcanic glass. It is used as drilling mud (photo, swells to make thick suspension that keeps rock chips from settling back into bottom of drill hole, sketch) and low permeability (and high cation exchange capacity) liner for waste disposal. Many other uses of smectite in everyday household items.

Chlorite (link to one kind of chlorite) has 2:1 layer structure with octahedral sheet in interlayer (~ 2:2 layer silicate). Chemical formula = (Mg, Fe, Al)6(Al1Si3)O10(OH)8 (trioctahedral) with extensive solid solutions; typically massive scaly appearance and green; common in metamorphic rocks (greenschist facies - slate, phyllite, schist, metamorphosed mafic rocks/greenstones), high temperature hydrothermal alteration of Mg-Fe silicates.

Framework (tecto) Silicates 3-dimensional network of shared tetrahedra (all 4 oxygens), Si (andAl(4)):O ratio of 1:2. Framework silicates include 2 most abundant minerals in Earth's crust (quartz and feldspar, ~63% of minerals in crust) and zeolites (environmentally important).

Silica (SiO2) Group ~10 polymorphs, SiO2 = electrically neutral, no extra cations.

Quartz (low-quartz) most common polymorph, forms at "low" temperatures (0 - 550C) in many rocks, especially silicic igneous (granite), most metamorphic, and sedimentary clastic (sandstone and shale). Quartz is most abundant sedimentary mineral. Why?

 

Small amounts of impurity elements give different colors of quartz (e.g., rose, purple-amethyst, yellow-citrine, brown). Names for fine-grained (microscopic) quartz = agate (banded, multi-colored, often occurring as cavity filling), jasper (red), chalcedony (fibrous), chert or flint (light gray to dark, breaks with smooth fractures). Uses = inexpensive gemstone (link #2, #3, #4, #5, #6, #7), glass manufacture, computer chips, and watches. Other SiO2 polymorphs = high (temperature) quartz - rare and usually reverts to low quartz in cooling silicic igneous rock; coesite and stishovite = high pressure forms that occur at meteorite impact sites and high pressure igneous and metamorphic rocks in mantle.

Feldspars* - most abundant minerals in crust; consist of two solid solution series: plagioclase feldspar and alkali feldspar. Feldspars = most abundant mineral in most igneous rocks and used to classify igneous rock composition. Feldspars also major constituent of most metamorphic rocks and commonly occur in clastic sedimentary rocks (sandstone and shale) but usually not as abundant as quartz.
Why?

Plagioclase* = chemical formula (Na, Ca) (Al, Si)4O8 with albite* = Na-end member (Ab - NaAlSi3O8), anorthite* = Ca- end member (An - CaAl2Si2O8), and complete solid solution between; coupled substitution to maintain charge balance Ca2+ and Al3+ substitutes for Na+ and Si4+. Various mineral names for plagioclase minerals of different Ab/An content.

An-rich compositions form at higher temperatures than Ab-rich compositions in similar manner to olivine minerals. Examine plagioclase phase diagram.

Sometimes crystallization occurs quickly so plagioclase crystals do not have time to react with melt, so early-formed plagioclase has different (more An-rich) composition than later-formed plagioclase. This can produce zoned plagioclase crystals with inner part An-rich and outer part Ab-rich (scroll down to photo set #6, photo #2, #3).

 

 

Continuous series in plagioclase compositions ranging from mafic to silicic composition igneous rocks with Ca- plagioclase in mafic rocks and Na- plagioclase in silicic rocks.

Feldspars = 2 directions of cleavage at right angles, usually light colored (white, salmon, light gray) and rectangular/blocky crystals. Distinguishing characteristic of plagioclase = polysynthetic (multiple) twins, represent single crystal with different regions having different atomic orientation. Easily recognized in thin section as regular alternating black and white stripes (see numerous photos all on right side) and sometimes in hand sample as striations (parallel lines).

Alkali feldspars* have chemical formula (K, Na) AlSi3O8 with albite* as Na-end member (Ab - NaAlSi3O8) and several polymorphs (e.g., orthoclase - Or) as K- end member (Or - KAlSi3O8). Alkali feldspars = major feldspar in silicic igneous rocks, lesser feldspar (< plag) in intermediate igneous rocks, and absent in mafic rocks.

Complete solid solution between Na- and K-end members only at high temperatures. At low temperatures (<~800C) immiscibility (separation into 2 different minerals), which results in formation of Na-end member and K-end member. Process of single feldspar separating into two compositions = exsolution.
Two ways that exsolution can occur, either as separate grains of K-rich feldspar and Na-rich feldspar or (more frequently) alternating zones of albite and K-feldspar within single grain (exsolution lamellae). If original alkali feldspar is K-rich, after it exsolves = perthite (most common). If original alkali feldspar is Na-rich, after it exsolves = antiperthite.

 

3 polymorphs of K-feldspar: sanidine* (high temperatures, crystals in extrusive igneous rocks), orthoclase* (intermediate temperatures, quickly cooled granites fine-grained), and microcline* (low temperatures, slowly cooled granite coarse-grained).

Feldspathoids (link #2): Similar to feldspars, but contain less silica. Occur in anomalous igneous rocks (Na- and K-rich, but Si-poor) that lack quartz. Examples = leucite (K-rich) and nepheline (Na-rich).

Zeolites^: Hydrous (water-bearing) framework silicates with large cavities that contain loosely bound (exchangeable) cations and water, similar to smectite but zeolites do not swell due to framework structure. Highly variable compositions, many minerals = clinoptilolite (link #2), laumontite (link #2), and analcime (link #2, sometimes considered feldspathoid). Usually fine-grained and difficult to identify in hand sample. General formula = Xz[AlzSi1-zO2] . nH2O, where X = Na+, K+, Ca2+; z = 0.15 - 0.5 (negative charge developed by Al3+ for Si4+ substitution), and n = 0.3 - 1.1. Important sedimentary minerals that occur in altered volcanic ash and rhyolite and in cavities of igneous rocks. Environmental applications = use as molecular sieves (ion exchange), sewage treatment (remove NH4), water softener, and oil refining (catalyst).