Chap 7/8: Phase Diagrams

rock-forming minerals

mineral group composed of the most common minerals found in earth

major element

chemical elements that occur in large amounts

minor elements

elements with abundances btwn 1.0 and 0.1 percent

trace elements

elements that occur in smaller amounts <0.1 weight %

triangular diagram

a diagram that allows for graphic representation of mineral chemistry in terms of three end member groups

Al2SiO5 polymorphs and their conditions

• Kyanite, lower temperature and high pressure

• sillimanite, high temperature

• andalusite, low to mid temperature and low pressure

polymorph definition

a group of minerals that have the same chemical composition but different structure

two uses of the phase diagram

• predict which phase we will find

• use the phases to understand the temperature and pressure history

tectosilicates

SiO2 polymorphs, feldspars, sodalite

chain silicates (inosilicates)

tetrahedral chains

• single - pyroxenes

• double - amphiboles

sheet silicates (phyllo)

infinitely extending tetrahedral sheets

• talc, micas, chlorite,

nesosilicates

corner oxygens link to other cation polyhedra instead of link with other silica tetrahedra

• (SiO4)4+, andalusite polymorphs

sorosilicates

double tetrahedral units

• epidote

cyclosilicates

ring structures

• beryl, tourmaline, and cordierite

invariant point

can only exist at one temperature and pressure

univariant line

one variable can change but all others would be fixed

divariant field

both temperature and pressure can be varied independently without changing the number of phases

liquidus meaning

Temperature at which a mixture of minerals is completely liquid

solidus

Temperature below which a mixture of materials is completely solid

tie line

line in phase diagram joining coexisting minerals

fractional crystallization

a realistic crystallization, doesnt matter where the starting composition is as it will always end up at the same place

solid solution

homogenous mixture of two different kinds of atoms in solid state and have a single crystal structure

miscibility gap

a region in a phase diagram where the mixture exists as two or more phases, forms exsolution lamella upon cooling

zoning

composition changes from the inside out

eutectic

minimum-melting composition of 2 or more components, each of which melts and freeze congruently forming a mixture of the component crystals during crystallization