Six-sided, pyramidal Quartz Crystals.

1. Six-sided, pyramidal Quartz Crystals. Minerals are the building blocks of rock and the solid earth. All minerals 1. have a specific chemical composition, 2. have a specific atomic arrangement (crystal form), 3. are solid, 4. inorganic and 5. naturally occurring.

2. Basalt is composed of minerals that crystallize from magma derived from a partial melt of the asthenosphere. Crystals are too small to see with the naked eye. Andesite is composed of minerals that crystallize from magma derived from a partial melt of basaltic ocean crust. The large plagioclase crystals cooled slowly underground. 1. Minerals can form during crystallization of a magmatic melt.

3. c porphyritic andesite b a Which minerals, crystallized first, which crystallized last? a. Small black minerals; b. larger white minerals; c. smaller minerals in the gray matrix Which of the above minerals have the highest melting temperature in the porphyritic rock, which would have the lowest melting temperature in the porphyritic rock?

4. 2. Minerals can precipitate directly from an aqueous (out of water) solution, such as the salt deposits shown in the image above in the desert playa lake.

5. Quartz crystal The arrangement of the constituent atoms will define the crystal form of a mineral, providing the crystal can grow in an unrestricted environment.

6. Quartz crystal Note that the mineral quartz (glassy appearance) in this granite does not form well-defined pyramidal shaped crystals. Do you think quartz was the first or last mineral to crystallize in granite? What does that tell you about the melting temperature of quartz compared to the other minerals comprising granite?

7. There are six major crystal systems that may form as constituent atoms arrange themselves during the growth of minerals. All of the famous gems of the world represent one of these crystal systems. Can you determine the crystal system to which topaz belongs?

8. Chemical bond strength and the fixed arrangement of atoms will define whether weakness (cleavage planes) will form in minerals. The halite (salt) crystal shown in the image on the left has three cleavage planes at right angles (90°) to one another. The weakness planes develop between the ionic bonds between the sodium (Na) and chloride (Cl) atoms in the crystal lattice.

9. The arrangement of atoms in a mineral is related to the charge state (positive versus negative) and the size (ionic radius) of the atom. To understand the importance of charge state and bonding requires a little chemistry review.

10. Atoms are the smallest particles that define the chemical properties of matter. They are composed of protons (+ yellow)and neutrons (neutral orange) in the nucleusand electrons (- white)surrounding the nucleus in defined energy levels.

11. Important chemistry facts: • The number of protons in the nucleus of the atom defines the chemical properties of the element (e.g., an oxygen atom will always have 8 protons in its nucleus). • The number of protons plus neutrons within the nucleus of the atoms define its mass (electrons have negligible mass), and defines the mass #. • The electrons are arranged around the nucleus of the atom in orbits (or energy levels). Each energy level can hold a specific number of electrons. • (1st- 2 electrons; 2nd – 8 electrons; 3rd – 8 electrons).

12. Elements are arranged in rows (periods) on the periodic table sequentially from upper left to right based on their respective atomic numbers (proton number), which defines the element. The element’s mass corresponding increases as well.

13. The elements are arranged in columns (groups) based on the number of electron valencies in their respective energy levels. The elements in the middle (pink) of the periodic table (transition metals) may have multiple valency states. What is significant about the noble (inert) gases?

14. +1 +3 +4 -2 -1 +2 Atoms can give up, receive or share electrons to fill their valencies. I have provided the ionic charges for the elements that may be found in minerals. Note that N and P and metallic elements can have more than one valency state. To achieve stability atoms that tend to give up electrons (+ charged) will bond with atoms that tend to receive electrons (- charged). Atoms can also bond by sharing electrons to satisfy valencies & achieve a stable configuration.

15. Ionic bonds form as the result of a physical transfer of electrons between a donor cation (+ charge) and receiver anion (- charge).

16. Because sodium (Na) is in the same column as lithium (Li), it will have similar bonding properties (transfers one electron to the chloride (Cl) atom to fulfill valency requirements for both atoms) when it forms NaCl.

17. +1 +2 +3 +4 -2 -1 I have provided the ionic charges for elements that are found in earth minerals. N and P and metallic elements can have more than one valency state. Note the valency states of silicon (Si+4) and oxygen (O-2), as these elements are major components of minerals comprising the Earth’s crust and upper mantle.

18. Covalent Bond Electrons are shared between constituent atoms to achieve charge balance and fill valencies. Covalent Bond Covalent bonds form when electrons are shared between two atoms of a molecule. Covalent bonds tend to be very strong.

19. +1 +2 +3 +4 -2 -1 I have provided the common ionic charges for elements that are common in earth minerals. Note that N and P and metallic elements can have more than one valency state. The elements in the middle of the periodic table (blue color) are transition metals and can also have more than one valency state.

20. The relative hardness of a mineral is controlled by its composition and bond strength between its constituent atoms. Minerals of equal or greater hardness can abrade minerals of equal or less hardness. What mineral do you think is found on a dentist’s drill?

21. Graphite and diamond have the same composition, but different atomic arrangements. The bonds between the carbon sheets in graphite are weak Van der waal bonds and it is very soft.

22. Diamond is also composed of carbon atoms, but they are arranged in a more compacted structure than graphite and have strong covalent bonds between the carbon atoms. Diamond is the hardest natural occurring mineral. It forms very deep within the earth under very high pressures in volcanic pipes.

23. Streak is most diagnostic for metallic minerals. Hematite (Fe2O3) leaves a distinct reddish-brown streak on a porcelain plate.

24. Silicon (Si+4) and oxygen (O-2) are the dominant elements comprising earth rocks. The other common elements are cations (have positive ionic charges (K+1, Na+1, Ca+2, Fe+2+3, Al+3, Mg+2)

25. The silicate tetrahedron is a complex anion with a charge of -4 (SiO4-4). Can you explain why its charge is -4? It achieves charge balance and fills its valencies by ionic bonding with available cationsorcovalent bonds between shared oxygen atomsof adjacent tetrahedra.

26. The olivine structure satisfies its tetrahedral valencies with ionic and metallic bonds with magnesium (Mg+2) and iron (Fe+2) atoms. It is the first silicate mineral to crystallize from a magmatic melt.Olivine has no cleavage planes and will break along fractures. Olivine is a major mineral comprising the upper mantle. It also forms phenocrysts in some basalts. (Mg,Fe)2SiO4

27. What is the ratio of Si:O in the olivine mineral structure? Think about the number of shared (divide by 2) versus unshared (divide by 1) oxygen atoms. One Si atom to 4 unshared (divided by 1) O atoms. Si : O ratio 1 : 4 (Mg,Fe)2SiO4

28. Pyroxene is a single chain silicate that share two oxygen atoms. Pyroxene has two cleavage planes (at right angles). Can you infer which atomic bonds will define these cleavage planes? Pyroxene is one of the main minerals comprising basaltic ocean crust. (Mg,Fe,Ca,Na)(Mg,Fe,Al)Si2O6

29. What is the ratio of Si:O in the pyroxene (single chain silicate) mineral structure? Think about the number of shared (divide by 2) versus unshared (divide by 1) oxygen atoms. One Si atom to 2 unshared (divided by 1) and 2 shared (divided by 2) O atoms. Si : O ratio 1 : 3 shared unshared (Mg,Fe,Ca,Na)(Mg,Fe,Al)Si2O6

30. The amphibole (double chain silicate)structure shares electrons either 2 or 3 of its oxygen atoms in its crystal structure. It will form two cleavage planes (124° and 56°) within the crystal structure. It is a common mineral in subduction zone rocks (andesite, dacite or diorite and granodiorite) Note that water is also present in the crystal structure. Where do you think this water originates? (Na,Ca)2(Mg,Al,Fe)5(Si,Al)8O22(OH)2

31. shared shared What is the ratio of Si:O in the amphibole mineral structure? Think about the number of shared (divide by 2) versus unshared (divide by 1) oxygen atoms. 50% of tetrahedra: One Si atom to 2 unshared (divide by 1) and 2 shared (divide by 2) O atoms. 50% of tetrahedra: One Si atom to 1 unshared (divided by 1) and 3 shared (divided by 2) O atoms. Si : O ratio 1 : 2.75 unshared unshared (Na,Ca)2(Mg,Al,Fe)5(Si,Al)8O22(OH)2

32. Biotite: K2(Mg,Fe)6Si3O10(OH)2 Muscovite: K2Al4(Si6Al2O20)(OH,F)2 Muscovite and biotite micas are sheet silicates and have one cleavage plane at 180°. Based on the relative bond strength between constituent atoms within the crystal lattice can you predict where this cleavage plane will form? Biotite contains Fe and Mg which gives it a dark appearance. Both micas have dissolved water present.

33. Biotite: K2(Mg,Fe)6Si3O10(OH)2 unshared shared Muscovite: K2Al4(Si6Al2O20)(OH,F)2 What is the ratio of Si:O in the sheet silicate mineral structure? Think about the number of shared (divide by 2) versus unshared (divide by 1) oxygen atoms.. One Si atom to 1 unshared (divide by 1) and 3 shared (divide by 2) O atoms. Si : O ratio 1 : 2.5

34. The 3-dimensional framework silicates share electrons between four of the oxygen atoms in each tetrahedron (covalent bonds exist between all tetrahedral oxygen atoms). Quartz has no cleavage planes and will break by conchoidal fracture. Note that the planes shown in the image above are crystal faces and do not repeat throughout the crystal. Quartz is a common mineral in granite or rhyolite. shared

35. What is the ratio of Si:O in the quartz 3-dimensional framework silicate) mineral structure? Think about the number of shared (divide by 2) versus unshared (divide by 1) oxygen atoms. One Si atom to 0 unshared (divide by 1) and 4 shared (divide by 2) O atoms. Si : O ratio 1 : 2 shared Chemical formulas for quartz: SiO2

36. (Ca,Na)AlSi3O8 Plagioclase Feldspar The feldspar structure is similar to quartz except that an aluminum (Al+3) will exchange with a silicon atom (Si+4) within the tetrahedron and require an additional electron to satisfy the extra valency. This bond can be between calcium (Ca+2), sodium (Na+1) and potassium (K+1), depending upon composition of the magma and crystallization temperature. Where would the cleavage plane develop in this mineral group? KAlSi3O8 Potassium Feldspar

37. Other important mineral groups include: the carbonates, halides, oxides and sulfides. You can read about these four mineral groups in your text. I will have two questions on the midterm related to the four non-silicate mineral groups listed above.

38. Other important mineral groups include the carbonates of which calcite belongs. Calcite comprises the marine sedimentary rock, limestone and the metamorphic equivalent marble.

39. Halides (e.g., halite, NaCl, common salt) also form an important mineral group which sustains our lives. Halite often forms as an evaporite deposit in desert lakes (playas).

40. Galena (PbS) is a mineral in the sulfide group. It is an important source of the world’s lead. Hematite (Fe2O3) is a mineral in the oxide group. It is an important source of the world’s iron.