Editing Metamorphism (section)

===Phase change===
{{Annotated image
| image = Al2SiO5 phase diagram.svg
| image-width=210
| height=250
| caption = Phase diagram of Al<sub>2</sub>SiO<sub>5</sub>
 <br />([[nesosilicate]]s)

|annotations = 
{{annotation|040|100|[[Kyanite]]}}
{{annotation|070|170|[[Andalusite]]}}
{{annotation|130|120|[[Sillimanite]]}}
}}

Phase change metamorphism is the creating of a new mineral with the same chemical formula as a mineral of the protolith. This involves a rearrangement of the atoms in the crystals. An example is provided by the [[aluminium silicate]] minerals, [[kyanite]], [[andalusite]], and [[sillimanite]]. All three have the identical composition, {{chem2|Al2SiO5}}. Kyanite is stable at surface conditions. However, at atmospheric pressure, kyanite transforms to [[andalusite]] at a temperature of about {{cvt|190|C||}}. Andalusite, in turn, transforms to [[sillimanite]] when the temperature reaches about {{cvt|800|C||}}. At pressures above about 4 kbar (400 MPa), kyanite transforms directly to sillimanite as the temperature increases.{{sfn|Yardley|1989|pp=32–33, 110, 130–131}} A similar phase change is sometimes seen between [[calcite]] and [[aragonite]], with calcite transforming to aragonite at elevated pressure and relatively low temperature.{{sfn|Yardley|1989|pp=183–183}}