Lab 2: Minerals I - Silicate Structure


Silicon-Oxygen Tetrahedron


We have already met the silicon-oxygen tetrahedron in description. Let's take a look at a 3D model of this structure:



The silicon-oxygen tetrahedron has a pyramidal shape, with three faces on top and a base, for a total of four faces.



The silicon atom is considerably smaller than an oxygen atom, and because of the way the electrons are shared between the four oxygens and one silicon, the silicon sits in the "heart" of the pyramidal structure, nestled between the oxygen atoms.



Imagine if you had four basketballs (O atoms) and one baseball (Si atom). You could place the three basketballs at the base of a pyramidal structure (on the floor) and you could rest the baseball at the junction between the three basketballs. And, you could take the fourth basketball and place it on top, to complete the tetrahedron.


Silicate Structures


Recall that an accounting exercise for the sharing of electrons between the silicon and the four oxygens indicates that the overall structure is four electrons shy of fulfilling the octet rule. This results in the tetrahedron having a -4 charge (an anion). The tetrahedron, with its negative charge, bonds with positively charged atoms and other molecules (cations) to form the silicate minerals. The silicate structures, built of tetrahedra, are as follows:


isolated (single tetrahedron) In the mineral olivine, (Mg,Fe) 2 SiO 4 , the magnesium and iron atoms are surrounding single tetrahedra, isolating the tetrahedra from each other. The tetrahedra in olivine are not "kissing" at the corners of the tetrahedra, as you'll see below in the other structures; the tetrahedra are not touching one another, they are isolated within the mineral structure. Olivine is a dark green mineral, rather hard and blocky in appearance.



In the other structures, the tetrahedra are "kissing" at the corners. The tetrahedra themselves are covalently bonded to one another in this fashion.


single chains In silicate minerals formed of single chains of tetrahedra, the tetrahedra are linked at the corners by oxygen-to-oxygen covalent bonds ("kissing" corners), which are strong bonds, making the single chain quite strong down its length. The chains are bonded with positively charged atoms to form the higher level structure of the mineral. A good example of a silicate mineral with this structure is augite, within the pyroxene group. Augite has the formula (Mg,Fe) 2 SiO 3 . Augite is a dark greenish-black mineral with a "blocky" appearance, owing to the 90 degree angles between the single chains of tetrahedra.



double chains In silicate minerals formed of double chains of tetrahedra, the tetrahedra are linked at the corners along single chains and the single chains are, in turn, bonded together by corner "kissing" of the sort discussed above. The mineral hornblende, of the amphibole group, has this structure. Hornblende has the formula Ca 2 (Fe,Mg) 5 Si 8 O 22 (OH) 2 . The double chains are bonded with positively charged atoms to form the higher level structure. Hornblende is a dark greenish-black mineral with a "splintery" appearance, owning to the combination of wide and narrow angles between the double chains.



sheets In silicate minerals formed of sheets of tetrahedra, the tetrahedra lie in a plane, and are "kissing" corners extensively. Sheets of tetrahedra are sandwiched together by bonds with positively charged atoms. The mica minerals, such as biotite and muscovite, have this sheet structure. Biotite, K(Mg,Fe) 3 AlSi 3 O 10 (OH) 2 , is a dark-colored mica. Muscovite, KAl 2 (AlSi 3 O 10 )(OH) 2 , is light in color, and is even clear to translucent. The micas occur in distinctly sheeted form, such that you can often peel a mica specimen apart into thin sheets. The bonds between the sheets of tetrahedra are weaker than the strong covalent bonds holding the sheets together internally, so that micas, if in a thin enough piece, are quite flexible.



three-dimensional frameworks The structures with the most "kissing" corner bonds are those structures in which the tetrahedra bond together in intricate 3D structures. The feldspars are the most common minerals. Potassium feldspar is the mineral orthoclase, KAlSi 3 O 8 , which tends to be pinkish, or light in color, and exhibits a blocky appearance with right angles common. Plagioclase is a calcium-sodium feldspar with the formula (Ca, Na)AlSi 3 O 8 . Plagioclase tends to be light in color, leaning toward white at one end of the extreme, but grades to gray and darker gray, depending on the sodium-to-calcium ratio. Plagioclase, like orthoclase has a blocky appearance with right angles common. Quartz, SiO 2 , is highly variable in color, and is consistently hard, like the feldspars.