Magnesium cations have a radius of 0.73 Å, and iron cations have a radius of 0.62 Å. Magnesium and iron can substitute for each other because they both have a charge of +2, and they are similar in size. Olivine can be pure Mg 2SiO 4 or Fe 2SiO 4, or a combination of the two, written as (Mg,Fe) 2SiO 4. In olivine, the –4 charge of each silica tetrahedron is balanced by two iron or magnesium cations, each with a charge of +2. The simplest silicate structure, that of the mineral olivine (Figure 5.23), is composed of isolated tetrahedra bonded to iron and/or magnesium ions (Figure 5.23 left). Source: Steven Earle (2015) CC BY 4.0 view source Isolated Tetrahedra If you’re feeling ambitious, make several tetrahedra and and use toothpicks through the corners to make the configurations discussed below. If you don’t have glue or tape, make a slice along the thin grey line and insert the pointed tab into the slit. If you have glue or tape, secure the tabs to the tetrahedron to hold it together. Cut around the outside of the shape (solid lines and dotted lines), and then fold along the solid lines to form a tetrahedron. Download this PDF file with the tetrahedron pattern below.
