Silicate Minerals

Isolated Tetrahedra

Olivine

Chemical formula: (Mg,Fe)₂SiO₄

Structure: Iron and magnesium atoms are cations (positively charged). The tetrahedron is an anion, with its overall -4 charge. The tetrahedra are bonded to the iron and magnesium atoms because of the way the octet rule can be satisfied. The tetrahedra are totally isolated from one another, as the iron and magnesium atoms take up the space between the tetrahedra, where they form bonds with the tetrahedra.

Hammer behavior: When struck with a hammer, olivine fractures conchoidally (with curved fractures, as seen in a broken coke bottle). This reflects the evenness of the strength of bonds in olivine, where there aren't planes of weakness along which the hammer blow could produce shear faces (cleavage faces).

Color: usually olive-green, but it the iron can oxidize ("rust"), giving a reddish tinge

Streak: white

Luster: vitreous (glassy)

Diaphaneity: transparent to translucent

Hardness: 6.5 to 7, on a scale of 1 to 10, with diamond being 10; this is called Mohs Scale of Hardness:

Occurrence: forms much of volume of Earth's mantle; found in mafic (iron and magnesium rich) igneous rocks; also in moon rocks, Mars rocks, meteorites.

Garnet

Chemical formula: similar to olivine's

Structure: similar to olivine's

Hammer behavior: conchoidal to uneven fracture; often brittle; no cleavage (like olivine, no planes of weakness)

Color: most commonly red in color but can be found in a variety of colors, including purple, red, orange, yellow, green, brown, black, or colorless

Streak: white

Luster: vitreous to resinous (resin like)

Diaphaneity: transparent to translucent

Hardness: 6.5 to 7.5

Occurrence: metamorphic rocks

Single Chains of Tetrahedra

Augite (Pyroxene Group)

Chemical formula: (Mg,Fe)₂SiO₃

Structure: single chains of tetrahedra bonded together with magnesium and iron atoms.

Hammer behavior: The bonds between the magnesium or iron atoms and the chains of tetrahedra are weaker than the strong covalent bonds holding the chains together. For this reason, a hammer blow will cleave the structure between the chains, and not across the chains. The angle of cleavage faces is at right angles, giving augite a blocky appearance in hand specimen:

Color: dark green to black, sometimes brownish

Streak: greenish-gray

Luster: fairly dull, although some vitreous specimens give the name augite, which in Greek means "brightness"

Diaphaneity: translucent to opaque

Hardness: 5.5 to 6

Occurrence: mainly found in mafic (magnesium and iron rich) igneous rocks such as gabbro and basalt

Double Chains of Tetrahedra

Hornblende (Amphibole Group)

Chemical forumla: Ca₂(Fe,Mg)₅Si₈O₂₂(OH)₂

Structure: double chains of tetrahedra bonded together with magnesium and iron atoms

Hammer behavior: The bonds between the iron and magesium atoms are weaker than those within the double chains. Just as augite has good cleavage, so does hornblende, but in hornblende specimens the cleavage angles are a combination of wide (about 120 degree) and narrower (about 60 degree) angles. Instead of having a blocky appearance, as with augite, hornblende specimens have a "splintery" appearance. Observe the cleavage angles in the illustration:

Color: generally dark, as greenish or brownish black

Streak: white

Luster: vitreous to pearly

Diaphaneity: translucent to opaque

Hardness: 5 to 6

Occurrence: mainly found in mafic igneous rocks, but is found more generally as an accessory mineral in igneous rocks; also common in certain types of metamorphic rocks.

Sheets of Tetrahedra

Biotite

Chemical formula: K(Mg,Fe)₃AlSi₃O₁₀(OH)₂

Structure: sheets of tetrahedra, with sheets sandwiched together with bonds to potassium, magnesium, iron, and aluminum atoms.

Hammer behavior: You don't even need a hammer; usually you can peel thin cleavage sheets with your fingernails (these aren't individual sheets, as atoms are tiny!).

Color: dark brown, greenish brown

Streak: gray

Luster: vitreous to pearly

Diaphaneity: transparent to opaque

Hardness: 2.5 to 3

Occurrence: found in igneous rocks such as granite

Muscovite

Chemcial formula: KAl₂(AlSi₃O₁₀)(OH)₂

Structure: sheets of tetrahedra, with sheets sandwiched together with bonds to potassium and aluminum atoms.

Hammer behavior: same as biotite

Color: white to gray, with various tones such as brown, green, silver

Streak: white

Luster: vitreous

Diaphaneity: transparent to translucent

Hardness: 2 to 2.5

Occurrence: found in igneous rocks such as granite

Framework of Tetrahedra

Orthoclase (Potassium feldspar)

Chemical formula: KAlSi₃O₈

Structure: three dimensional framework of tetrahedra, with potassium and aluminum atoms bonded between the tetrahedra

Hammer behavior: Bonds are strong within the framework, but in some planes there are weaker bonds, leading to cleavage at 90 degrees. Orthoclase tends to break into blocky specimens. There are two directions of cleavage, not three. Look at a cardboard box and observe that if you put your hand on top of the box, that is the same plane as the bottom of the box. Take a pen or marker and place a check on the top and bottom of the box. Now move your hand from the top of the box to one of the sides; there is an opposite side of the box that is also in the same plane. Place a check on these two sides. Now, as you move the box around, you will see that there are two unmarked sides. If you look at specimens of orthoclase, you may find cleavage angles at 90 degrees between any two cleavage planes, but like the cardboard box, when you look at the "ends" relative to those two planes, you will not see cleavage. The specimen will not show super smooth surfaces here, and will show uneven fracture.

Color: often pinkish, to white, to colorless

Streak: white

Luster: vitreous

Diaphaneity: transparent to translucent

Hardness: 6

Occurrence: igneous rocks, and metamorphic rocks

Plagioclase (Calcium/sodium feldspar)

Chemical formula: (Ca, Na)AlSi₃O₈

Structure: three dimensional framework of tetrahedra, with calcium, sodium, and aluminum atoms bonded between the tetrahedra

Hammer behavior: Same as orthoclase, but plagioclase can have well-developed striations

Color: white to gray, with bluish, greenish, or reddish tinting

Streak: white

Luster: vitreous

Diaphaneity: transparent to translucent

Hardness: 6 to 6.5

Occurrence: igneous rocks

Quartz

Chemical formula: SiO₂

Structure: three dimensional framework of tetrahedra, in pure forms usually without atoms of other elements, but there are commonly other elements

Hammer behavior: Bonds in the framework of quartz are strong in all directions, so when hit with a hammer, even if the specimen is a beautiful crystal, it just fractures conchoidally.

Color: highly variable

Streak: white

Luster: vitreous

Diaphaneity: transparent

Hardness: 7

Occurrence: all rocks

Other Silicate Minerals

Talc

Chemical formula: Mg₃Si₄O₁₀(OH)₂

Structure: foliated (alignment of crystals from pressure) to masses of tiny crystals

Hammer behavior: Talc is very soft, so it comes apart into pieces easily. Technically, there is a perfect direction of cleavage when you can see it.

Color: white with various tintings, to greenish

Streak: white

Luster: vitreous to pearly

Diaphaneity: translucent

Hardness: 1

Occurrence: in metamorphic rocks

Kaolinite

Chemical formula: Al₂Si₂O₅(OH)₄

Structure: clay-like masses, appearing smooth

Hammer behavior: Kaolinite is very soft, so it comes apart into blocky, irregular pieces easily.

Color: white, with various tintings

Streak: white

Luster: earthy (dull)

Diaphaneity: transparent to translucent

Hardness: 1.5 to 2

Occurrence: weathered aluminum-silicate rocks

Antigorite (Serpentine)

Chemical formula: (Mg,Fe)₃Si₂O₅(OH)₄

Structure: occurs in masses of small crystals, in blocky specimens displaying brittle fracture

Hammer behavior: See comment above; in microscopic view, can see good cleavage in one direction.

Color: green, gray, black

Streak: greenish white

Luster: vitreous to greasy

Diaphaneity: transparent to opaque

Hardness: 3.5 to 4

Occurrence: metamorphic rocks