Some metamorphic reactions produce new minerals that are scattered about the rock and the minerals that reacted away either disappear entirely or, if relics survive, they may look a bit ragged. In the case of pseudomorphs, the shape of a reacted mineral is retained by the mineral(s) that replaced it. Pseudomorphs are like petrified wood -- it still looks like wood, but it's now something else (silica). Metamorphic pseudomorphs are of great interest because they contain a lot of information for reconstructing the reaction and pressure-temperature history of a rock.

An important point about pseudomorphs is that the replacement is not necessarily isochemical. That is, the replacing minerals may have chemical components not present in the mineral that was replaced.

The elongate mineral (~ 3 cm long) just below the region with garnet + quartz + plagioclase was kyanite but is now sillimanite. Thor-Odin dome region, British Columbia. Photo by C. Teyssier.

Below are some photographs of pseudomorphs. Clicking on some images will lead to larger versions of the image.

Thin section, plane light: This is a pseudomorph of chlorite + epidote after garnet in a retrograded eclogite, Sivirhisar, Turkey. There are still relics of garnet (high relief, lighter pieces) in the pseudomorph. Elsewhere in the Sivrihisar Massif, pristine eclogites occur (Whitney & Davis, 2006, Geology), with garnets that have similar composition to the relics in these retrograded samples. Thin section, plane light: These pseudomorphs of chlorite + epidote after garnet are from the same rock as the image to the left. In the garnet on the left in this image, the high relief minerals in the pseudomorph may preserve an inclusion trail that was present in the garnet. Sivrihisar, Turkey. The matrix is dominated by epidote + chlorite + oxides. Thin section, plane light: The tabular region containing fine-grained (fuzzy looking) minerals used to be something else, but is entirely gone. It has been replaced by fine-grained muscovite, sillimanite, and garnet. The darker colored irregular rings/circular regions are garnets. The matrix is cordierite + biotite + quartz + garnet. A possibility for the original mineral is staurolite. Sample collected by F. Sengun, Menderes Massif, Turkey.
Thin section, plane light: The clear mineral in the center used to by kyanite (we see relics of kyanite in some pseudomorphs in this rock, so we know it was kyanite), but now it is entirely replaced by numerous grains of prismatic sillimanite. The dark ring is mostly spinel (symplectite), surrounded by colorless cordierite, in a matrix of biotite (zoned) and gedrite + cordierite. Thor-Odin dome, British Columbia. Thin section, crossed polars: Prismatic sillimanite after kyanite, with corona of spinel + cordierite, Thor-Odin dome, British Columbia. The original outline of the kyanite may have been at the outer edge of the cordierite corona, or it may have been at the boundary of the cordierite and spinel layers. Replacement of kyanite by spinel indicates involvement of other phases in the reaction, not just Ky =Sil. Thin section, plane light: Pseudomorph of muscovite after garnet in graphitic schist.
Thin section, crossed polars: Same as image above.
Thin section, plane light: Possible pseudomorphs of calcite after elongate aragonite, blueschist facies marble, Sivrihisar, Turkey.