Pseudomorphs

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