Matter and Minerals The spectacular scenery of the Sierra Nevada Mountains in California results from glaciers that carved high peaks and deep valleys into a massive body of granite -- a common type of igneous rock . Igneous rocks are rocks that form the cooling and solidification of molten magma.
The Earth consists of four interconnected systems: the solid earth or lithosphere (rock and soil); the atmosphere ; the hydrosphere ; the biosphere ; These systems interact and influence each other.
The earth’s crust consists of rocks made of various types of minerals. Minerals are classified into different groups according to their chemistry. The silicate mineral group makes up most rock of the crust.
Rocks are composed on one or more minerals. There are three main classes of rocks:
Igneous rocks form from solidification of magma (molten rock)
Sedimentary rocks form from solidification of sediment (particles formed from the erosion of older rocks, or by chemical or biological processes at the earth’s surface).
Metamorphic rocks form from other rocks changed by heat and pressure
One type of rock can be transformed into another type over geologic time; a process known as the rock cycle .
The earth’s crust is made of various kinds of rock , and rock is composed of one or more minerals . For example, as this picture shows, the rock granite is composed mostly of the minerals quartz, biotite, and feldspar.
Smallest unit of matter that cannot be broken down into other substances (i.e. it’s the smallest part of the element that retains the properties of that element).
Periodic Table of the Elements http://www.chemicalelements.com
Minerals are made of various chemical elements , or atoms . Atoms, which form all matter, consist of three kinds of particles. -- protons with a positive charge and neutrons with a neutral charge -- occupy the center, or nucleus , of the atom. The nucleus contains nearly all the mass of the atom. -- electrons with a negative charge -- orbit around the nucleus. Electrons have almost zero mass. All atoms have this same general structure . By changing the numbers of protons we make different chemical elements !
Negatively charged sub-atomic particles that move very quickly around the nucleus.
Electrons are arranged in ‘shells’ or ‘orbitals’ around the nucleus. Electron shells are merely SPACE surrounding the nucleus at different levels. Electrons within a shell can move anywhere within this space.
The shells are layered where the first shell can only hold up to two electrons. Each successive shell can hold up to 8 (as you get up in atomic number, the number or electrons/shell varies…we will not worry about that).
For an atom to be STABLE, that is, not ‘bond’ or chemically react with another atom, its VALENCE shell (outermost shell) must be full.
Electrons and valence shells are involved in chemical BONDING.
Chemical bonding between elements forms minerals. Covalent and Ionic Bonds
An ionic bond is the chemical bond that results from the electrostatic attraction between positive ions (cations) and negative ions (anions). It can form when two atoms meet and an electron is permanently transferred from one to the other. These two ionized atoms then stick very tightly together to make sodium chloride -- also known as table salt (or the mineral halite).
So basically electric glue holds NaCl together. This bond is not that strong (as we can dissolve NaCl easily in water). For any element, be able to determine the valence electrons and what charge it would have if it would become an ion.
Okay so what if you have 4 valence electrons. Would you give up or obtain 4 electrons. Probably neither. Your best bet in this situation is to just SHARE the electrons.
Covalent chemical bonds involve the sharing of a pair of valence electrons by two atoms. A Covalent bond is formed when the valence shells of two atoms overlap and the electrons are free to roam between both shells.
The number of covalent bonds an atom can have depends on the number of valence electrons.
Covalent Bonding Note: the more covalent bonding a mineral has in its structure, the more stable (stronger) that mineral is, and the more difficult to weather or break it down.
Okay so if there are thousands of minerals, why only a few common ones? The answer is that there are only a few elements that make up MAJORITY of the earths minerals (and minerals make up rocks).
O (oxygen) Si and O form the silica tetrahedron
Na (sodium) +1
K (potassium) +1
Ca (calcium) +2
Mg (magnesium) +2
Al (aluminum) +3
Fe (iron) +2
98% of the earth’s crust are made of combinations of these 8 elements.
92% of the earth’s crust are made of ‘silicate’ minerals, those containing mostly Si and O.
For example: The oxide group consists of all minerals formed from various elements bonded to negatively charged oxygen (O -2 ) atoms. The carbonate group consists of all minerals formed from elements bonded to negatively charged carbonate ((CaCO 3 ) -2 ) molecules. The silicate minerals are the MOST COMMON minerals in the earth’s crust. Geologists classify minerals into groups that share the same negatively charged atoms or molecules .
Silicate Minerals The silicates are the most abundant and important mineral group. Silicates are minerals composed of silicate tetrahedra (SiO 4 ) - 4 bonded to each other and/or to other elements. A single silicate tetrahedron consists of 1 silicon (Si) atom surrounded by 4 oxygen (O) atoms, forming a tetrahedron shape. These tetrahedra bond readily to each other, and to other elements, forming several kinds of abundant minerals, including quartz and feldspar. feldspar quartz hornblende muscovite mica
The chemical formula Si04 will then have a net negative 4 charge. This is because, each of four oxygens have one vacancy in their valence shells. Each Oxygen therefore needs to obtain one more electron to fill its outer shell.
If the MOLECULE accepts four electrons to become STABLE (and it will), the tetrahedron itself will have obtained four electrons (have full valence shells) and will have an overall charge of -4.
So where does it get these other 4 electrons? From atoms that want to get rid of their valence electrons (to become cations). Of the remaining abundant elements of the earth’s crust, you can see that there are many combinations of cations (bottom right) that will form ionic bonds to the tetrahedron and thus form a variety of different minerals. So, eight elements can make many different SILICATE minerals that make up the earth’s crust.
The silica tetrahedron itself is covalently (strong) bound, but each tetrahedra is electrically glued to adjacent tetrahedra via the cations. If the ‘silica tetrahedra backbone’ is arranged in this way, we call this the isolated silicate structure (see next slide). The OTHER way the silica tetrahedra can ‘fill’ its valence shells is by covalently bonding to adjacent tetrahedra. If it forms two, three, or four covalent bonds (oxygens sharing electrons with oxgyens of adjacent tetrahedra) and subsequent demand for electrons drops, the net negative charge on the tetrahedron becomes less and less. If all four oxygens share electrons, all covalent bonds are formed. There will be no charge (and thus no bonding to cations).
The figure shows the shape of the silicate tetrahedron, and shows how progressively more complex bonding between silicate tetrahedra forms the different types of common silicate minerals.
Suppose we covalently bind three oxygens with adjacent tetrahedra. The only oxygen that had accepted an electron is the top oxgyen (see diagram—it is the one pointed at you).
The silica is arranged in sheets, and the sheets are electrically glued together by cations. These minerals break easily between the ionic bonds between the sheets.
Si:O = 1-2.5
+4:-5, net charge –1
Positive ions IONICALLY bond sheets together.
Framework Silicates Each tetrahedron shares all four oxygens with adjacent tetrahedra. Si:O = 1:2. All four oxygens shared with adjacent tetrahedron forms a complex 3-D network, all covalently bound. Quartz as an example. Quartz therefore is a pretty stable mineral as it is only SiO2 all covalently bound. The reason that the formula is SiO2 is because each silicon ‘owns’ half of four oxygens (sharing with other silicon atoms). Quartz (and glass) will not ‘break’ in a straight line or preferential direction (minerals tend to break along planes of weakness resulting from ionic bonds). Four covalent bonds means strength between the atoms is equal in all directions. Hence, glass (and quartz) breaks in curved patterns (wherever the force was the strongest).
High Medium Low Lower Lowest Stability Low Medium High Higher Highest Density Low Medium High Higher Highest Ionic Bonds High Medium Low Lower Lowest Covalent Bonds +4-4=0 +4-5=-1 +4-5.5=-1.5 +4-6=-2 +4-8=-4 Net Charge 1:2 1:2.5 1:2.75 1:3 1:4 Si:O Low Felsic Medium Felsic High Mafic Higher Mafic Highest Ultramafic Percent Fe, Mg High Medium Low Lower Lowest Percent SiO2 Quartz Mica Hornblende Augite Olivine Example Framework Sheet Double Chain Single Chain Isolated Silicate Structure
Mineral Identification Minerals take the form of crystals of different colors and shapes. The different colors result from the ways that different atoms absorb and reflect light. The different shapes result from the different ways that bonded atoms pack together in particular geometric arrangements.
Mineral Identification These terms describe the way a mineral breaks. If a mineral fractures , it breaks along rough edges. If it cleaves , then it breaks along smooth, flat surfaces or definite planes as determined by their crystal structure. Fracture and Cleavage
Mineral Identification Types of Cleavage and typical minerals in which they occur. Note: quartz is covalent bonds (framework silicate) equal in all directions. Quartz does not break in a preferential direction or plane so it doesn’t have cleavage. It has fracture (breaks any which way).
Mineral Identification This Biotite cleaves into flat sheets on the top, but it fractures on the sides. When some minerals fracture, they appear to break along long curved surfaces (that look like a shell) like that shown left.
Mineral Identification Hardness This is how resistant a mineral is to being scratched. We use the Mohs scale to classify a given minerals hardness. The technique is rather simple. All you do is try to scratch the unknown mineral with minerals of the mohs hardness scale or various items of known hardness, such as a fingernail (hardness of about 2.5), a penny (3), and a steel nail (5.5). The item that first scratches the unknown is harder than the unknown so then you must estimate. Mohs hardness scale TALC 1 GYPSUM 2 CALCITE 3 FLUORITE 4 APATITE 5 FELDSPAR 6 QUARTZ 7 TOPAZ 8 CORUNDUM 9 DIAMOND 10
The color of a mineral when it is powdered is called the streak of the mineral. Streak can be determined for any mineral by rubbing the mineral across the surface of a hard, unglazed porcelain material called a streak plate.
The color of the powder left behind on the streak plate is the mineral's streak.
So the take home message is that mineral chemistry determines the ‘strength of the mineral’ and contributes to the mineral’s properties.
The more covalent bonding a mineral has, the less the ionic bonding and the stronger it is.
Minerals with higher percentages of iron and magnesium (and therefore a lot of ionic bonds) have weaker silicate structures (isolated, single, double) and are denser. These minerals make up denser layers of the earth.
Minerals with lower percentages of iron and magnesium (due to more covalent bonding between adjacent tetrahedra) have stronger silicate structures (sheet, framework) and are lower density.
We will DEFINITELY revisit this stuff during igneous rocks, metamorphic rocks, weathering, etc.