packing of atoms in minerals

how do ions arrange themselves?

surround themselves with ions of opposite charge to form regular repeating 3D structures

what is the coordination number?

number of nearest neighbour ions

what is a coordination polyhedron?

the shape obtained by joining centres of these nearest numbers

what determines the way ions pack togeher?

the size of the ion

what are larger anions or cations?

anions

how does ionic radius change down a group

increases

if there are two ions of the same element will the more highly charged one be smaller or larger?

smaller

what is the formula for radius ration?

r+/r-

what is the coordination number for a radius ratio of <0.155?

2

what is the coordination number for a radius ratio of 0.155-0.255?

3

what is the coordination number for between 0.255 to 0.414?

4

what is the coordination number for a radius ratio of between 0.414 to 0.732?

6

what is the coordination number for a radius ratio of between 0.732 and 1?

8

what shape does a coordination of 2 make?

linear

what shape does a coordination of 3 make?

equilateral triangle

what shaped does a coordination number of 4 make?

tetrahedron

what shape does a coordination of 6 make?

octahedron

what shape does a coordination number of 8 make?

cube

what are complex ions?

many minerals possess covalently bonded complex ions which usually remain as such even in solution

what is ionic substitution?

one ion can replace another of a similar ionic radius (± 15%) e.g. in olivine Fe²⁺ and ⁺ can replace each other

what is incomplete substitution?

in some minerals not all the atoms can be substituted .e.g. in Sphalerite only 50% by weight of the zinc atoms can be replaced by iron atoms

what is coupled ionic substitution?

it is not necessary that substituting ions should have the same charge however electrical neutrality must be maintained by simultaneous substitution elsewhere in the structure

what is the general formula for Plagioclase?

Na_1xCa_xAl_1+xSi_3-xO₈

what is exsolution?

in the Alkali Feldspar’s ionic Subsitution of Na and K is much more extensive at higher temperatures than at low temperatures

what is camouflage?

Although Rubidium is a relatively abundant element there are no rubidium minerals but its ionic radius is very close to that of potassium so all available Rubidium.