GY 111 Lecture Notes D Haywick 2007 08 1 GY 111 Lecture Note Series Bonding and Packing in Minerals Lecture Goals A Bonding B Packing of atoms ions in crystals Reference Press et al 2003 Chapter 3 Grotzinger et al 2007 Chapter 3 A Bonding Last time we met we were discussing basic chemistry This was necessary in order to examine the chemical characteristics of the mineral groups Chemical reactions are largely driven by movements of electrons between different atoms Ions result when electrons are gained anions or lost cations during these reactions Ions are different sizes than their parent atoms In general atoms that become cations become smaller and atoms that become anions become larger The size of the ions atoms is one of the principle factors regulating how crystals form I am jumping ahead just now Let s get back to the first topic of today chemical bonding There are 3 major types of chemical bonds that glue together atoms and ions in minerals 1 Ionic bonding 2 covalent bonding and 3 metallic bonding A 4th type van der waal or hydrogen bonding is relatively weak but does play an important role in some of the minerals e g the micas Ionic bonding is electromagnetic and occurs when ions bond together consider this a bond resulting from the exchange of electrons Remember the mineral halite from last time This mineral is composed of Na and Cl and the resulting ionic bond is very strong because of the charge difference between the two ions Other minerals characterized by ionic bonding include fluorite calcite and many others Covalent bonding is another very strong type of chemical bond This type electrons are shared between 2 or more ions The classic examples illustrating this type of bonding are the molecules H2 and O2 hydrogen gas and oxygen gas 1 GY 111 Lecture Notes D Haywick 2007 08 2 Many of the sulfide and silicate minerals display covalent bonding A very simple and largely incorrect way to view the covalent bonding in silicate minerals like quartz is as follows The third type of bonding metallic is relatively weak As the name applies it is characteristic of metals such as gold silver and copper It also occurs in several other types of native elements most of which you will not see in GY 111 In metallic bonding there is free movement of electrons around atomic nuclei and it is this property that makes most metals conduct electricity To explain how the electrons were associated with the nuclei my High School chemistry teacher asked us to first imagine a pipe packed with gumballs the atomic nuclei and then to imagine sugar electrons passing through the spaces between the gumballs Once again the analogy isn t all that accurate but it more or less captures the gist of metallic bonding it also did wonders for gumball sales in the school store Hydrogen bonding results from weak attraction between ions of opposite charge such as illustrated below As previously stated it is a very weak bond and results in some interesting mineral properties where it occurs The minerals that were produced at least in 2 GY 111 Lecture Notes D Haywick 2007 08 3 part due to hydrogen bonding include the micas biotite and muscovite other platy minerals such as chlorite and most of clays e g kaolinite Bonding does more than simply glue atoms ions into minerals it also dictates some of the physical properties of those minerals Note the following are generalized properties They are not inclusive Bond Ionic Covalent Metallic Mineral properties strong bond high melting points vitreous lusters higher solubility strong bonds hard minerals very high melting points vitreous adamantine lusters weak bonds low to high melting points metallic lusters electrically conductive B Packing of atoms ions crystallography I hate teaching this section to GY 111 students as it requires artistic skills which I simply do not have The problem is that we need to envision 3D space A chalk board only permits 2D drawings of 3D objects If you have a good eye for perspectives you will probably be okay If not well Google Crystal Systems All objects are made of atoms and ions In minerals these particles are put together in a precise manner depending upon 1 the amount of space you have and 2 the relative size of the atoms ions that are bonded together It is largely the ionic ratio that determines how the particles are packed in space For ions that are the same size the packing is relatively easy to envision Picture again the gumballs that I discussed earlier in this lecture Picture a glass filled with them All of the gumballs will shift slightly to best fill the space provided by the glass There will be space between them but not nearly as much as if the packing were random 3 GY 111 Lecture Notes D Haywick 2007 08 4 Unfortunately ions are seldom the same size in a mineral and consequently the way they pack together is seldom as simple as that portrayed above Take halite again Na is much smaller than Cl meaning that rather than the perfect packing in gum balls the packing in halite looks rather more like this Note that the Cl ions largely dictate the size of the resulting package Incidentally crystallographers those geologists who study how crystals form call these packages unit cells The actual definition of a unit cell goes something like this the smallest reproducible structure that records both chemical composition and packing arrangement of ions atoms in a mineral Real crystals are nothing but stacked together identical unit cells The unit cell of halite is this Many of you may have envisioned this as the unit cell 4 GY 111 Lecture Notes D Haywick 2007 08 5 If so you d be wrong This cell is not identical and repeatable the corner ions are different The correct cell can be stacked with others to make larger and larger identical assemblages It is ultimately this stacking that gives rise to the macroscopic crystal forms that we see in nature The unit cell of halite has a face centered structure Fluorite s unit cell is also cubic but it is different from halite s It is said to be body centered because the fluoride ions which comprise part of the mineral are wholly contained within the unit cell Note Dr Haywick will show 3D models to illustrate the crystallography of halite and fluorite in the lecture A lot of this stuff is pretty abstract but if you are following it you probably have several questions about mineral chemistry One that I asked when I was a student was Why is the formula of halite NaCl if the unit cell has a bunch of Na and