By: Chris Fite, TRMS member
Info from the book, Mineralogy for Amateurs, written by John Sinkankas, 1964
PSEUDOMORPH…the word is Greek, “pseudo” means false, “morph” means shape. A PSEUDOMORPH rock or mineral or crystal has the shape of another mineral instead of its normal shape. Most Limestone is not in the shape of an animal but sometimes you find a fossil rock that is shaped like a clam or a shark’s tooth. So fossils are PSEUDOMORPHS. Likewise, Quartz in the variety called petrified wood is a PSUEDOMORPH. Chemically related minerals can change from one to the other without changing the original crystal shape. Examples of this kind of PSEUDOMORPHISM are Azurite changing to Malachite but preserving faithfully the faces and angles of azurite, or Aragonite converting to Calcite, or Rutile to Brookite. Another kind of PSEUDOMORPHISM is when one kind of mineral replaces a chemically completely different kind of mineral. Examples of this kind of PSEUDOMORPHISM are Quartz replacing Fluorite and native Copper replacing Aragonite.
People often use the word “after” to describe the relationship between PSEUDOMORPH and the mineral that it replaced. For example, opalized petrified wood would be called “Quartz after wood” because first it was wood then later changed to Quartz. The Quartz occurred later or “after” the wood was formed.
Of course, all minerals originally came from other minerals, but the original crystal shape is almost always destroyed being replaced by the new mineral’s characteristic shape. The unique property of a PSEUDMORPH is it retains the shape of the older mineral. Identifying a PSEUDOMORPH is more difficult than the average mineral because it has the “wrong” shape. I had a difficult time identifying some elongated cubic crystals of Goethite because Goethite does not form cubic crystals. But Goethite does occur as a PSEUDOMORPH after Pyrite, and Pyrite does form cubic crystals.
So, here is a list of PSEUDOMORPHS copied form Mr. Sinkankas’ book:
Source: T-Town Rockhound, April 2006
PSEUDOMORPHS
QUARTZ after CALCITE
QUARTZ after ARAGONITE
QUARTZ after FLUORITE
QUARTZ after AZURITE
QUARTZ after DATOLITE
QUARTZ after SERPENTINE
QUARTZ after ASBESTOS
QUARTZ after FOSSILS
OPAL after GLAUBERITE
OPAL after FOSSILS
GOETHITE after PYRITE
GOETHITE after SIDERITE
GOETHITE after MAGNETITE
GOETHITE after CHALCOPYRITE
GOETHITE after SPHALERITE
PYRITE after FOSSILS
CALCITE after ARAGONITE
CALCITE after CELESTITE
CALCITE after FOSSILS
CLAY MINERALS after FELDSPARS
CLAY MINERALS after TOURMALINE
CLAY MINERALS after BERYL
CLAY MINERALS after SPODUMEN
HEMATITE after MAGNETITE
GYPSUM after ANHYDRITE
GYPSUM after ARAGONITE
COPPER after CUPRITE
COPPER after AZURITE
COPPER after ARAGONITE
MALACHITE after CUPRITE
MALACHITE after AZURITE
CHLORITE after GARNET
ANDRATITE after BIOTITE
MUSCOVITE after TOURMALINE
MUSCOVITE after SPODUMENE
RUTILE after BROOKITE
CASSITERITE after QUARTZ
CASSITERITE after FELDSPAR
ANGLESSITE after GALENA
PYROMORPHITE after GALENA
PYROLUSITE after MANGANITE
TALC after QUARTZ
DOLOMITE after ARAGONITE
MAGNESITE after CALCITE