A Few Rocks That Include Silicate Materials

Obsidian
©Daniela White Images / Getty Images
01
of 36

Amphibole (Hornblende)

Hydrous metal silicates
The Silicate Minerals. Photo (c) 2007 Andrew Alden, licensed to About.com ( fair use policy)

The silicate minerals make up the great majority of rocks. Silicate is a chemical term for the group of a single atom of silicon surrounded by four atoms of oxygen, or SiO4. They come in the shape of a tetrahedron. 

Amphiboles are part of the dark (mafic) minerals in igneous and metamorphic rocks. Learn about them in the amphibole  gallery. This is hornblende.

Hornblende, the most common amphibole, has the formula (Ca,Na)2-3(Mg,Fe+2,Fe+3,Al)5(OH)2[(Si,Al)8O22]. The Si8O22 part in the amphibole formula signifies double chains of silicon atoms bound together with oxygen atoms; the other atoms are arranged around the double chains. (Learn more about hornblende.) The crystal form tends to be long prisms. Their two cleavage planes create a diamond-shaped (rhomboid) cross section, sharp ends with a 56-degree angle and the other two corners with 124-degree angles. That is the main way to distinguish an amphibole from the other dark minerals like pyroxene.

Other amphiboles include glaucophane and actinolite. 

02
of 36

Andalusite

Aluminum silicate
The Silicate Minerals. Photo courtesy -Merce- of Flickr.com under Creative Commons license

Andalusite is a polymorph of Al2SiO5, along with kyanite and sillimanite. This variety, with tiny carbon inclusions, is chiastolite. 

03
of 36

Axinite

Hydrous metal borosilicate
The Silicate Minerals. Photo (c) 2009 Andrew Alden, licensed to About.com ( fair use policy)

Axinite is (Ca, Fe, Mg,Mn)3Al2(OH)[BSi4O15], an uncommon mineral popular with collectors. (more below)

Axinite is not common, but it is worth watching for near granite bodies in metamorphic rocks. Collectors like it because it is a triclinic mineral that often has good crystals displaying the peculiar symmetry, or lack of symmetry, typical of this crystal class. It's "lilac brown" color is distinctive, showing here to good effect against the olive-green of ​epidote and the milky white of calcite. The crystals are strongly striated, though that isn't evident in this photo (which is about 3 centimeters across).

Axinite has an odd atomic structure consisting of two silica dumbbells (Si2O7) bound by a boron oxide group; it was formerly thought to be a ring silicate (like benitoite). It forms where granitic fluids alter surrounding metamorphic rocks, and also in veins within granite intrusions. The Cornish miners called it glass schorl; a name for hornblende and other dark minerals.

04
of 36

Benitoite

Barium titanium silicate
The Silicate Minerals. Photo (c) 2005 Andrew Alden, licensed to About.com ( fair use policy)

Benitoite is barium titanium silicate (BaTiSi3O9), a very rare ring silicate named for San Benito County, California, the only place it's found. 

Benitoite is a rare curiosity found almost exclusively in the great serpentine body of the New Idria mining district of central California. Its sapphire-blue color is unusual, but it really comes out in ultraviolet light where it shines with a bright blue fluorescence.

Mineralogists seek out benitoite because it's the simplest of the ring silicates, with its molecular ring being composed of only three silica tetrahedra. (Beryl, the most familiar ring silicate, has a ring of six.) And its crystals are in the rare ditrigonal-bipyramidal symmetry class, their molecular arrangement displaying a triangle shape that geometrically is actually a bizarre inside-out hexagon (this is not correct technical crystallographic language, you understand).

Benitoite was discovered in 1907 and was later named the state gemstone of California. The benitoite.com site displays luscious specimens from the Benitoite Gem Mine.

05
of 36

Beryl

Beryllium aluminum silicate
The Silicate Minerals. Photo (c) 2010 Andrew Alden, licensed to About.com ( fair use policy)

Beryl is beryllium silicate, Be3Al2Si6O18. A ring silicate, it is also a gemstone under various names including emerald, aquamarine, and morganite. 

Beryl is commonly found in pegmatites and is usually in well-formed crystals like this hexagonal prism. Its hardness is 8 on the Mohs scale, and it usually has the flat termination of this example. Flawless crystals are gemstones, but well-formed crystals are common at rock shops. Beryl can be clear as well as various colors. Clear beryl is sometimes called goshenite, the bluish variety is aquamarine, red beryl may sometimes be called bixbyite, green beryl is better known as emerald, yellow/yellow-green beryl is heliodor, and pink beryl is known as morganite.

06
of 36

Chlorite

Hydrous metal silicate
The Silicate Minerals. Photo (c) 2009 Andrew Alden, licensed to About.com ( fair use policy)

Chlorite is a soft, flaky mineral that is something between mica and clay. It often accounts for the green color of metamorphic rocks. It is usually green, soft (Mohs hardness 2 to 2.5), with a pearly to the glassy luster and micaceous or massive habit.

Chlorite is very common in low-grade metamorphic rocks like slate, phyllite, and greenschist. However, chlorite can appear in higher-grade rocks as well. You'll also find chlorite in igneous rocks as an alteration product, where it sometimes occurs in the shape of the crystals it replaces (pseudomorphs). It looks like a mica, but when you split off its thin sheets, they are flexible but not elastic -- they bend but do not spring back -- whereas mica is always elastic.

Chlorite's molecular structure is a stack of sandwiches consisting of a silica layer between two metal oxide (brucite) layers, with an extra brucite layer laced with hydroxyl between the sandwiches. The general chemical formula reflects the wide range of compositions in the chlorite group: (R2+,R3+)4–6(Si,Al)4O10(OH,O)8 where R2+ can be Al, Fe, Li, Mg, Mn, Ni or Zn (usually Fe or Mg) and R 3+ is usually Al or Si.

07
of 36

Chrysocolla

Hydrous copper silicate
The Silicate Minerals. Photo (c) 2009 Andrew Alden, licensed to About.com ( fair use policy)

Chrysocolla is a hydrous copper silicate with the formula (Cu, Al)2H2Si2O5(OH)4·nH2O, found around the edges of copper deposits. 

Where you see bright blue-green chrysocolla, you'll know that copper is nearby. Chrysocolla is a hydroxylated copper silicate mineral that forms in the alteration zone around the edges of copper ore bodies. It almost always occurs in the amorphous, noncrystalline form shown here.

This specimen has an abundance of chrysocolla coating the grains of a breccia. Real turquoise is much harder (Mohs hardness 6) than chrysocolla (hardness 2 to 4), but sometimes the softer mineral is passed off as turquoise.

Other Diagenetic Minerals

08
of 36

Dioptase

Hydrous copper silicate
The Silicate Minerals. Photo courtesy Craig Elliott of Flickr.com under Creative Commons license

Dioptase is a hydrous copper silicate, CuSiO2(OH)2. It usually occurs in bright green crystals in the oxidized zones of copper deposits.

Other Diagenetic Minerals

09
of 36

Dumortierite

Hydrous aluminum borosilicate
The Silicate Minerals. Photo courtesy Quatrostein via Wikimedia Commons

Dumortierite is a borosilicate with the formula Al27B4Si12O69(OH)3. It's typically blue or violet and found in fibrous masses in gneiss or schist.

10
of 36

Epidote

Hydrous calcium iron silicate
The Silicate Minerals. Photo (c) 2008 Andrew Alden, licensed to About.com ( fair use policy)

Epidote, Ca2Al2(Fe3+, Al)(SiO4)(Si2O7)O(OH), is a common mineral in some metamorphic rocks. Typically it has a pistachio- or avocado-green color.

Epidote has a Mohs hardness of 6 to 7. The color is usually enough to identify epidote. If you find good crystals, they show two strongly different colors (green and brown) as you rotate them. It might be confused with actinolite and tourmaline, but it has one good cleavage where those have two and none, respectively.

Epidote often represents an alteration of the dark mafic minerals in igneous rocks such as olivine, pyroxene, amphiboles, and plagioclase. It indicates a level of metamorphism between greenschist and amphibolite, particularly at low temperatures. Epidote thus is well known in subducted seafloor rocks. Epidote also occurs in metamorphosed limestones.

11
of 36

Eudialyte

Hydrous alkali metal silicate
The Silicate Minerals. Photo courtesy Piotr Menducki via Wikimedia Commons

Eudialyte is a ring silicate with the formula Na15Ca6Fe3Zr3Si(Si25O73)(O, OH, H2O)3(Cl, OH)22. It's usually brick-red and is found in the rock nepheline syenite.

12
of 36

Feldspar (Microcline)

Metal silicates
The Silicate Minerals. Photo (c) 2007 Andrew Alden, licensed to About.com ( fair use policy)

Feldspar is a closely related mineral group, the most common rock-forming mineral of the Earth's crust. ​This is microcline.

13
of 36

Garnet

Metal silicates
The Silicate Minerals. Photo (c) 2009 Andrew Alden, licensed to About.com ( fair use policy)

Garnet is a set of closely related red or green minerals that are important in igneous and high-grade metamorphic rocks. Learn more about the garnet minerals.

14
of 36

Hemimorphite

Hydrous zinc silicate
The Silicate Minerals. Photo courtesy Tehmina Goskar of Flickr.com under Creative Commons license

Hemimorphite, Zn4Si2O7(OH)2·H2O, is a zinc silicate of secondary origin. It forms pale botryoidal crusts like this or clear flat plate-shaped crystals.

Other Diagenetic Minerals

15
of 36

Kyanite

Aluminum silicate
The Silicate Minerals. Photo (c) 2009 Andrew Alden, licensed to About.com ( fair use policy)

Kyanite is a distinctive mineral, Al2SiO5, with a light sky-blue color and bladed mineral habit that is popular with collectors. 

Generally, it is closer to gray-blue, with a pearly or glassy luster. The color is often uneven, as in this specimen. It has two good cleavages. An unusual feature of kyanite is that it has Mohs hardness 5 along the length of the crystal and hardness 7 across the blades. Kyanite occurs in metamorphic rocks like schist and gneiss.

Kyanite is one of three versions, or polymorphs, of Al2SiO5. Andalusite and sillimanite are the others. Which one is present in a given rock depends on the pressure and temperature that the rock was subjected to during metamorphism. Kyanite signifies medium temperatures and high pressures, whereas andalusite is made under high temperatures and lower pressures and sillimanite at high temperatures. Kyanite is typical in schists of pelitic (clay-rich) origin.

Kyanite has industrial uses as a refractory in high-temperature bricks and ceramics such as those used in spark plugs.

16
of 36

Lazurite

Sodium aluminum sulfur silicate
The Silicate Minerals. Photo (c) 2006 Andrew Alden, licensed to About.com ( fair use policy)

Lazurite is the important mineral in lapis lazuli, a gemstone prized since ancient times. Its formula is Na3CaSi3Al3O12S.

Lapis lazuli generally consists of lazurite and calcite, although bits of other minerals like pyrite and sodalite may be present as well. Lazurite is also known as ultramarine from its use as a brilliant blue pigment. Ultramarine was once more precious than gold, but today it is easily manufactured, and the natural mineral is used today only by purists, restorers, forgers and art maniacs.

Lazurite is one of the feldspathoid minerals, which form instead of feldspar when there is either not enough silica or too much alkali (calcium, sodium, potassium) and aluminum to fit into feldspar's molecular structure. The sulfur atom in its formula is unusual. Its Mohs hardness is 5.5. Lazurite forms in metamorphosed limestones, which accounts for the presence of calcite. Afghanistan has the finest specimens.

17
of 36

Leucite

Potassium aluminum silicate
The Silicate Minerals. Photo courtesy Dave Dyet via Wikimedia Commons

Leucite, KAlSi2O6, is also known as white garnet. It occurs in white crystals of the same shape as garnet crystals. It's also one of the feldspathoid minerals.

18
of 36

Mica (Muscovite)

Alkali metal aluminum silicates
The Silicate Minerals. Photo (c) 2009 Andrew Alden, licensed to About.com ( fair use policy)

Micas, a group of minerals that split in thin sheets, are common enough to be considered rock-forming minerals. This is muscovite. Learn more about the micas.

19
of 36

Nepheline

Sodium aluminum silicate
The Silicate Minerals. Photo courtesy Eurico Zimbres via Wikimedia Commons

Nepheline is a feldspathoid mineral, (Na, K)AlSiO4, found in certain low-silica igneous rocks and metamorphosed limestones. 

20
of 36

Olivine

Iron magnesium silicate
The Silicate Minerals. Photo courtesy Gero Brandenburg of Flickr.com under Creative Commons license

Olivine, (Mg, Fe)2SiO4, is a major rock-forming mineral in the oceanic crust and basaltic rocks and the most common mineral in the Earth's mantle.

It occurs in a range of compositions between pure magnesium silicate (forsterite) and pure iron silicate (fayalite). Forsterite is white and fayalite is dark brown, but olivine is usually green, like these specimens found in the black basalt pebble beach of Lanzarote in the Canary Islands. Olivine has a minor use as an abrasive in sandblasting. As a gemstone, olivine is called peridot.

Olivine prefers to live deep in the upper mantle, where it makes up about 60 percent of the rock. It does not occur in the same rock with quartz (except in the rare fayalite granite). It is unhappy at the Earth's surface and breaks down fairly rapidly (geologically speaking) under surface weathering. This olivine grain was swept to the surface in a volcanic eruption. In olivine-bearing rocks of the deep oceanic crust, olivine readily takes up water and metamorphoses into serpentine.

21
of 36

Piemontite

Manganese epidote
The Silicate Minerals Specimen from Squaw Peak, Arizona. Photo (c) 2013 Andrew Alden, licensed to About.com ( fair use policy)

Piemontite, Ca2Al2(Mn3+, Fe3+)(SiO4)(Si2O7)O(OH), is a manganese-rich mineral in the epidote group. Its red-to-brown-to-purple color and thin prismatic crystals are distinctive, although it can also have blocky crystals.

22
of 36

Prehnite

Hydrous calcium aluminum silicate
The Silicate Minerals. Photo courtesy fluor_doublet of Flickr.com under Creative Commons license

Prehnite (PREY-nite) is Ca2Al2Si3O10(OH)2, related to the micas. Its light-green color and botryoidal habit, made of thousands of tiny crystals, is typical.

23
of 36

Pyrophyllite

Hydrous aluminum silicate
The Silicate Minerals. Photo courtesy Ryan Somma of Flickr.com under Creative Commons license

Pyrophyllite, Al2Si4O10(OH)2, is the white matrix in this specimen. It looks like talc, which has Mg instead of Al but may be blue-green or brown. 

Pyrophyllite gets its name ("flame leaf") for its behavior when heated on charcoal: it breaks into thin, writhing flakes. Although its formula is very close to that of talc, pyrophyllite occurs in metamorphic rocks, quartz veins and sometimes granites whereas talc is more likely to be found as an alteration mineral. Pyrophyllite may be harder than talc, reaching Mohs hardness 2 rather than 1. 

24
of 36

Pyroxene (Diopside)

Mixed metal silicates
The Silicate Minerals. Photo courtesy Maggie Corley of Flickr.com under Creative Commons License

Pyroxenes are important in dark igneous rocks and are second to olivine in the Earth's mantle. Learn more about the pyroxenes. This is diopside.

Pyroxenes are so common that together they are considered rock-forming minerals. You can pronounce pyroxene "PEER-ix-ene" or "PIE-rox-ene," but the first tends to be American and the second British. Diopside has the formula CaMgSi2O6. The Si2O6 part signifies chains of silicon atoms bound together with oxygen atoms; the other atoms are arranged around the chains. The crystal form tends to be short prisms, and cleavage fragments have a nearly square cross-section like this example. That is the main way to distinguish a pyroxene from the amphiboles.

Other important pyroxenes include augite, the enstatite-hypersthene series and aegirine in igneous rocks; omphacite and jadeite in metamorphic rocks; and the lithium mineral spodumene in pegmatites. 

25
of 36

Quartz

Silica
The Silicate Minerals. Photo (c) 2007 Andrew Alden, licensed to About.com ( fair use policy)

Quartz (SiO2) is the chief rock-forming mineral of the continental crust. It was once considered one of the oxide minerals. Learn more about quartz.

26
of 36

Scapolite

Alkali aluminum silicate with carbonate/sulfate/chloride
The Silicate Minerals. Photo courtesy Stowarzyszenie Spirifer via Wikimedia Commons

Scapolite is a mineral series with the formula (Na, Ca)4Al3(Al, Si)3Si6O24(Cl, CO3, SO4). It resembles feldspar but usually occurs in metamorphosed limestones.

27
of 36

Serpentine (Chrysotile)

Hydrous magnesium silicate
The Silicate Minerals. Photo (c) 2009 Andrew Alden, licensed to About.com ( fair use policy)

Serpentine has the formula (Mg)2–3(Si)2O5(OH)4, is green and sometimes white and occurs only in metamorphic rocks. 

The bulk of this rock is serpentine in a massive form. There are three main serpentine minerals: antigorite, chrysotile, and lizardite. All are generally green from a significant iron content replacing the magnesium; other metals may include Al, Mn, Ni, and Zn, and silicon may be partly replaced by Fe and Al. Many details of the serpentine minerals are still poorly known. Only chrysotile is easy to spot.

Chrysotile is a mineral of the serpentine group that crystallizes in thin, flexible fibers. As you can see on this specimen from northern California, the thicker the vein, the longer the fibers. (See a closeup.) It is one of the several different minerals of this type, suitable for use as fireproof fabric and many other uses, that together are called asbestos. Chrysotile is the dominant form of asbestos by far, and in the home, it is generally harmless although asbestos workers must beware of lung disease due to chronic overexposure to the fine airborne fibers of powdered asbestos. A specimen like this is completely benign.

Chrysotile is not to be confused with the mineral chrysolite, a name given to off-green varieties of olivine.

28
of 36

Sillimanite

Aluminum silicate
The Silicate Minerals. U.S. Geological Survey photo

Sillimanite is Al2SiO5, one of three polymorphs along with kyanite and andalusite. See more under kyanite.

29
of 36

Sodalite

Sodium aluminum silicate with chlorine
The Silicate Minerals. Photo courtesy Ra'ike via Wikimedia Commons

Sodalite, Na4Al3Si3O12Cl, is a feldspathoid mineral found in low-silica igneous rocks. The blue color is distinctive, but it may also be pink or white.

30
of 36

Staurolite

Hydrous iron aluminum silicate
The Silicate Minerals. Photo (c) 2005 Andrew Alden, licensed to About.com ( fair use policy)

Staurolite, (Fe, Mg)4Al17(Si, Al)8O45(OH)3, occurs in medium-grade metamorphic rocks like this mica schist in brown crystals.

Well-formed staurolite crystals are commonly twinned, crossing at 60- or 90-degree angles, that are called fairy stones or fairy crosses. These large, clean staurolite specimens were found near Taos, New Mexico.

Staurolite is fairly hard, measuring 7 to 7.5 on the Mohs scale, and is used as an abrasive mineral in sandblasting.

31
of 36

Talc

Hydrous magnesium silicate
The Silicate Minerals. Photo (c) 2009 Andrew Alden, licensed to About.com ( fair use policy)

Talc, Mg3Si4O10(OH)2, is always found in metamorphic settings. 

Talc is the softest mineral, the standard for hardness grade 1 in the Mohs scale. Talc has a greasy feel and a translucent, soapy look. Talc and pyrophyllite are very similar, but pyrophyllite (which has Al instead of Mg) may be slightly harder.

Talc is very useful, and not just because it can be ground into talcum powder -- it's a common filler in paints, rubber, and plastics too. Other less precise names for talc are steatite or soapstone, but those are rocks containing impure talc rather than the pure mineral.

32
of 36

Titanite (Sphene)

Calcium titanium silicate
The Silicate Minerals. Photo courtesy Ra'ike via Wikimedia Commons

Titanite is CaTiSiO5, a yellow or brown mineral that forms a characteristic wedge or lozenge-shaped crystals. 

It is typically found in calcium-rich metamorphic rocks and scattered in some granites. Its chemical formula often includes other elements (Nb, Cr, F, Na, Fe, Mn, Sn, V or Yt). Titanite has long been known as sphene. That name is now deprecated by the mineralogical authorities, but you may still hear it used by mineral and gem dealers, collectors and geological old-timers.

33
of 36

Topaz

Aluminum fluosilicate
The Silicate Minerals. Photo (c) 2009 Andrew Alden, licensed to About.com ( fair use policy)

Topaz, Al2SiO4(F, OH)2, is the standard mineral for hardness 8 in the Mohs scale of relative hardness. (more below)

Topaz is the hardest silicate mineral, along with Beryl. It is usually found in high-temperature tin-bearing veins, in granites, in gas pockets in rhyolite, and in pegmatites. Topaz is tough enough to endure the pounding of streams, where topaz pebbles can occasionally be found.

Its hardness, clarity, and beauty make topaz a popular gemstone, and its well-formed crystals make topaz a favorite of mineral collectors. Most pink topazes, especially in jewelry, are heated to create that color.

34
of 36

Willemite

Zinc silicate
The Silicate Minerals. Photo courtesy Orbital Joe of Flickr.com under Creative Commons license

Willemite, Zn2SiO4, the reddish mineral in this specimen, has a wide range of color. 

It occurs with white calcite and black franklinite (a Zn and Mn-rich version of magnetite) in the classic locality of Franklin, New Jersey. In ultraviolet light, the willemite glows bright green and the calcite shines red. But outside collectors' circles, willemite is a scarce secondary mineral that forms by oxidation of zinc vein deposits. Here it may take massive, fibrous or radiating crystal shapes. Its color ranges from white through yellow, bluish, green, red and brown to black. 

Other Diagenetic Minerals

35
of 36

Zeolites

Low-T,P authigenic silicates
The Silicate Minerals. Photo (c) 2009 Andrew Alden, licensed to About.com ( fair use policy)

Zeolites are a large set of delicate, low-temperature (diagenetic) minerals best known filling openings in basalt. See the common zeolites here.

36
of 36

Zircon

Zirconium silicate
The Silicate Minerals. Photo (c) 2008 Andrew Alden, licensed to About.com ( fair use policy)

Zircon (ZrSiO4) is a minor gem, but a valuable source of zirconium metal and a major mineral for today's geologists. It always occurs in crystals that are pointed at both ends, although the middle may be stretched into long prisms. Most often brown, zircon also can be blue, green, red, or colorless. Gem zircons are usually turned blue by heating brown or clear stones.

Zircon has a very high melting point, is fairly hard (Mohs hardness of 6.5 to 7.5), and is resistant to weathering. As a result, zircon grains can remain unchanged after being eroded from their mother granites, incorporated into sedimentary rocks, and even metamorphosed. That makes zircon valuable as a mineral fossil. At the same time, zircon contains traces of uranium suitable for age dating by the uranium-lead method