Please answer as many as you can Sodium (Z = 11) and calcium (Z = 20) are group
ID: 106344 • Letter: P
Question
Please answer as many as you can
Sodium (Z = 11) and calcium (Z = 20) are group 1 (alkali) and group 2 (alkaline) elements in the periodic table, with strong geochemical and mineralogical affinities. A) Describe, on the basis of the information given, the atomic structures of the two elements and their contrasting properties in the atomic and ionic state (10 marks).
B) The plagioclase feldspar system has a Na (albite) and a Ca (anorthite) end-member, with the general idealized formula I(AlxSiy)4O8. What are the stoichiometric formulas for the two end-member feldspar compositions and the values of x and y in each case? (6 marks).
C). Albite and anorthite are end-members of a perfect solid solution. Is this an example of a simple or a coupled (complex) solid-solution? Provide an explanation for your answer (10 marks).
6) Pyroxene and amphibole group minerals are based on 1-dimensional infinitely long single and double chains of SiO44- ionic groups, with the pyroxenes being defined by a (Si2O6)4- silicate unit and amphiboles being defined by a Si4O11)6- silicate unit.
A) Sketch idealized representations of the two silicate structures (single and double chains), show and label the two sub-cells, and label and explain the two contrasting oxygen environments in the structure (HINT – does each oxygen participate in an ionic or covalent bond type?) (10 marks)
B) The single-chains in the pyroxene are stacked to create two slightly different 6-fold (octahedral) coordination polyhedra. The apices of each polyhedral are oxygen ions, and the polyhedra are occupied by Fe, Mg or Ca. Sketch the coordination polyhedral, and explain what happens to its shape and symmetry when Fe or Mg are substituted by Ca (6 marks).
7) Pyroxene and other ferro-magnesium minerals, as well as calcium plagioclase are common constituents of mafic basaltic (gabbroic) magmatic rocks, which represent the end-members of the common magma series.
A) What is a magma series, and how do you use chemical discrimination diagrams to identify one? (10 Marks).
B). Describe, in detail, two mechanical or chemical processes that may cause a parental magma to evolve and produce a magma series? (16 Marks)
8) Many mafic parental magmas are produced by partial melting of the upper mantle. The partial melting may be caused by changes in temperature, pressure, or water content. Using schematic phase diagrams, your knowledge of Bowen’s reaction series, and tectonic environments, explain how mafic partial melts are created in the upper mantle. (25 marks)
9) Provide, using sketches if necessary, the definition of a bridging versus an apical oxygen in silicate minerals (4 marks)
10) Provide an example of a mineral species or group that contains:
a) Only apical oxygens (1 mark)
Explanation / Answer
A) Describe, on the basis of the information given, the atomic structures of the two elements and their contrasting properties in the atomic and ionic state (10 marks).
The atomic structures of the two elements
Sodium- Sodium (Na) is the Chemical element, symbol: Na, atomic number: 11 and atomic weight 22,9898. It’s a soft metal, reactive and with a low melting point.
Calcium -Calcium (Ca) The nucleus consists of 20 protons (red) and 20 neutrons (orange). 20 electrons (white) occupy available electron shells.
Sodium atoms have 11 electrons, one more than the extremely stable configuration of the noble gas neon. As a result, sodium usually forms ionic compounds involving the Na+ cation. The most common oxidation state for sodium is +1, whereas in Calcium is a chemical element with symbol Ca and atomic number 20 The ion Ca2+ is also the fifth-most-abundant dissolved ion in seawater by both molarity and mass, after sodium.
B) The plagioclase feldspar system has a Na (albite) and a Ca (anorthite) end-member, with the generally idealized formula I(AlxSiy)4O8. What are the stoichiometric formulas for the two end-member feldspar compositions and the values of x and y in each case.
The Two end product in plagioclase feldspar system is through this reaction
KAlSi3O8 – NaAlSi3O8 – CaAl2Si2O8.
For Na (Albite) the stoichiometric formula is NaAlSi3O8
For Ca (anorthite) the stoichiometric formula is CaAl2Si2O8.
Value for x = 1
Value for y = 1
C). Albite and anorthite are end-members of a perfect solid solution. Is this an example of a simple or a coupled (complex) solid-solution? Provide an explanation for your answer
Albite is a plagioclase feldspar mineral. It is the sodium endmember of the plagioclase solid solution series. As such it represents a plagioclase with less than 10% anorthite content. The pure albite endmember has the formula NaAlSi3O8 Whereas Anorthite is the calcium endmember of plagioclase feldspar. Plagioclase is an abundant mineral in the Earth's crust. The formula of pure anorthite is CaAl2Si2O8. Anorthite is the calcium-rich endmember of the plagioclase solid solution series, the other end member being albite, NaAlSi3O8. Anorthite also refers to plagioclase compositions with more than 90 molecular percent of the anorthite endmember.