After stress is applied, the very same electrostatic forces that held the crystal together now cause it to fly apart. The structure of a sodium chloride crystal consists of alternating Na+ and Cl- ions, arranged in a cube. When this happens the ions in the compound undergo electrostatic repulsion, splitting the compound. These positive and negative bonds create crystals in rigid, lattice structures. Ionic compounds are brittle due to the strong bond between the positive and negative ions that make up the molecules. Covalent compounds do not conduct electricity this is because covalent compounds do not have charged particles capable of transporting electrons. In a covalent bond, the shared electrons contribute to each atom’s octet and thus enhance the stability of the compound. Why do simple covalent bonds not conduct electricity? Very little energy is needed to overcome the intermolecular forces, so simple molecular substances usually have low melting and boiling points. These intermolecular forces are much weaker than the strong covalent bonds in molecules. There are intermolecular forces between simple molecules. Note: octahedral (zig zag) chains! 3 repeat chainsĬovered in previous lecture: similar to olivine structure, but with 4 octahedra in zig and zag.Why do simple covalent bonds have low melting points? These are arranged so as the unsatisfied charge on tetrahedral O is distributed in a pattern that creates octahedral sites (M1, M2). All O in structure associated with Si cations. Olivine, Fe, Mg rather random between these. M1 and M2 differ in number of shared edges. Structure: two types of octahedral sites: M1 and M2. Fayalite-rich olivine more commonly found in more felsic and alkaline plutonic rocks.Īlso found in metamorphic rocks (e.g., metasediments such as metacrabonates) - essentially pure forsterite is possible (not likely in igneous rocks). ( - discuss next lecture its behavior as a function of P and T). Occurrence: common in mafic and ultramafic rocks, important mantle mineral (Mg,Fe)2 SiO4 (color changes with Fe content - see peridot. Of these, we will only focus on the very important forsterite - fayalite solid solution series. One basic (low-temperature) structure, several compositions Si tetrahedra link Ti octahedra (which form corner linked chains)Ī common accessory mineral in igneous and metamorphic rocks. Structure: Zr in large, 8-coordinated sites, linked by Si tetrahedra (distorted) - why? Heat and radiation treatment change its color Note kyanite slabs separated by Al, Fe - hydroxide layers.ĪlO4F2 octahedral chains parallel to c, crosslinked by Si tetrahedra.įluorine-bearing vapors in late crystallization of igneous rocks. Often associated with kyanite, which bears a special relationship to it. Layer stagger results in a monoclinic unit cell.Ĭomposition: complex, hydrous, Fe,- aluminosilicate Sheets (chains, rings) of octahedral cations (Fe2+, Al) support these tetrahedra.
The structure (234K) is characterized by sheets of ISOLATED tetrahedra Structurally and physically similar to a layer silicate! Phase transformations require rebonding of Al. Same octahedral chains!Ĭlearly, changes in structure are in response to changing P and T.
(Al avoidance principal - not much likelyhood of Al-Si disorder in tetrahedral sites!ĥ-coordinated sites link Si tetrahedra. THUS: structure contains chains in which tetrahedral cations alternate:Īl,Si,Al,Si. Orthorhombic: Same octahedral chains are crosslinked by both Si and Al tetrahedra! Octahedral Al-bearing chains // c are cross-linked by Si tetrahedra, which are separated by the second type of Al octahedra. Triclinic: Al is octahedrally coordinated. 6-coordinated sites (octahedra) form chains parallel to the c axis. Polymorphs: One Al in 4, 5, or 6 coordination, the other in 6 coordinated THIS GROUP CALLED " PYR AL SP ITE" (pyralspite)Ī third group: HYDROGARNETS - involves the 'hydrogarnet substitution":ĤH+ Si4+. THIS GROUP CALLED " U GR AND ITE" (ugrandite) OVERALL: structure is cubic (except for some special cases) M2+ sites are large, 8-coordinated sites.