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Minerals in Geodes

 

Where do the minerals come from?

 

The Four Most Common Minerals... Quartz, Calcite, Gypsum, and Dolomite

Quartz

SiO2

Formed by dissolved silica in ground water.

    Quartz geode (colored by iron)       Amethystine Quartz with Celestine

                       Quartz (colored by iron)                             Amethystine Quartz (with Celestine)

                        Meade Co., Kentucky                            Kings Mountain, Lincoln Co., Kentucky

    

     Smoky Quartz      Tabular Quartz crystal

Microphotos: Smoky Quartz (field of view 8 mm)             Tabular Quartz crystal (view 1 cm)

                                                   Halls Gap, Lincoln Co., Kentucky

Chalcedony with an orange line

Microphoto: Chalcedony with an orange line

(Field of view 1cm)

                                                           Halls Gap, Lincoln Co., Kentucky

Calcite

CaCO3

Formed by dissolved calcium carbonate in ground water.

   Calcite Crystal in Quartz Geode      Pink Calcite in geode

            Calcite Crystal in Quartz Geode                                            Pink Calcite

                                                              Monroe Co., Indiana

    Calcite in chalcedony geode     Calcite suspended on Millerite

              Calcite crystals in chalcedony                    Microphoto: Calcite suspended on millerite

            Halls Gap, Lincoln Co., Kentucky                                   Field of view ~ 1 cm

                        Halls Gap, Lincoln Co., Kentucky                          

Gypsum (and selenite)

CaSO4.2H2O

Primary (from bedded gypsum deposits shortly after deposition) and secondary origin (more recently)

      Geode filled with transparent gypsum selenite         Geode with partially dissolved Gypsum selenite

    Geode filled with transparent Gypsum selenite      Geode with partially dissolved Gypsum selenite

                         Green Co., Kentucky                                                Hardin Co., Kentucky     

The following minerals are less common as a general rule. Some are common within a small geographic area and those are designated 'Locally Common.' Uncommon indicates that the mineral is widespread, but is not frequently encountered. Rare or very rare mean seldom encountered or rare within a local geographic area and not found elsewhere in the region.

Dolomite

CaMg(CO3)2   

Formed by ground water precipitation.

   Ferroan dolomite in quartz geode      Ferroan dolomite in iron-stained quartz geode

   Ferroan dolomite - colored reddish-orange by iron (from decomposing pyrite or marcasite)

Monroe Co., Indiana

   Iron-stained Dolomite     Dolomite with Pyrite & Millerite

           Microphoto: Iron-stained Dolomite              Microphoto: Dolomite with pyrite & millerite

                         1 cm field of view                                                1.5 cm field of view 

                                                    Halls Gap, Lincoln Co., Kentucky

The minerals described and illustrated below follow the classification used scientists.

Native Elements

Minerals composed of a single element on the periodic chart. In geodes, such minerals are very rare.

Sulfur

S

Decomposition of sulfides, petroleum(?) (Very Rare)

   Sulfur in geode        Sulfur in geode

                                                                   Meade Co., Kentucky

Sulfide Minerals

Contain metallic elements like iron, nickel, zinc, and copper. They are associated with sulfur.

Chalcopyrite

CuS2

Copper in ground water (Uncommon)    

   Iridescent Chalcopyrite        Iridescent Chalcopyrite

Microphotos: Iridescent Chalcopyrite

Field of view ~6 mm

Halls Gap, Lincoln Co., Kentucky

Microphoto: Twinned Chalcopyrite

Field of view ~5 mm

Halls Gap, Lincoln Co., Kentucky

Galena

PbS

From by lead in ground water (Very rare)

   Galena on Quartz       Galena on Quartz

Microphotos: Galena on Quartz

Field of view ~3 mm

Halls Gap, Lincoln Co., Kentucky

Marcasite

FeS

From by iron in ground water (Uncommon)

    Marcasite blades         Marcasite in wedge-shaped crystals

     Microphoto: Marcasite blades (1 cm field)      Microphoto: Marcasite in wedge-shaped crystals

                                                       Halls Gap, Lincoln Co., Kentucky

Millerite

NiS

Nickel from meteorites(?) (Uncommon)

    Millerite Halls Gap, KY        Millerite in Calcite (1 cm field)

                                   Millerite                                        Microphoto: Millerite in calcite (1 cm field)

              Halls Gap, Lincoln Co., Kentucky                                    Monroe Co., Indiana

    Millerite and pyrite on chalcedony       Millerite in red chalcedony

 Microphoto: Millerite and pyrite on chalcedony                         Millerite in red chalcedony

                                                       Halls Gap, Lincoln Co., Kentucky

    Millerite on chalcopyrite         Millerite on chalcopyrite 

Microphotos: Millerite on chalcopyrite 

                                                           Halls Gap, Lincoln Co., Kentucky

          

Microphotos: Millerite on chalcopyrite 

                                                           Halls Gap, Lincoln Co., Kentucky

    Millerite crystal in a loop        Millerite crystal in rings

Microphotos: Millerite crystal in a loop                                    Millerite crystal in rings

                                                           Halls Gap, Lincoln Co., Kentucky

    Millerite crystal with eshelby twists       Millerite crystal with eshelby twists

Millerite crystal with eshelby twists

Microphotos: Millerite crystal with eshelby twists 

                                                           Halls Gap, Lincoln Co., Kentucky

Pyrite

FeS

From by iron in ground water (Common)

Octahedral Pyrite in chalcedony geode       Pyrite on Dolomite

              Octahedral pyrite in chalcedony                 Microphoto: Pyrite on dolomite (field 6 mm)

                                                       Halls Gap, Lincoln Co., Kentucky

 

   Pyrite cube - iridescent

Microphoto: Pyrite cube - iridescent (field 6 mm)

           Crystal on left center is calcite.

        Halls Gap, Lincoln Co., Kentucky

Sphalerite

ZnS

Zinc in ground water (Locally Common)

    Sphalerite with Millerite on Chalcopyrite              Sphalerite on Millerite

Microphotos: Sphalerite with millerite on chalcopyrite                    Sphalerite on millerite

                                  ~2 cm field                                                                    1 cm field

                                                       Halls Gap, Lincoln Co., Kentucky

Smythite

(Fe,Ni)3+S4

Iron and nickel in groundwater (Locally Common, always microscopic)

   Smythite in Quartz

    Microphoto: Smythite in Quartz (1 cm field)

             Halls Gap, Lincoln Co., Kentucky  

Sulfate Minerals

Minerals with sulfur and oxygen (SO4).

Barite

BaSO4 

Barium in the groundwater (Locally Common)

  barite in quartz geode       barite with pyrite in geode

                  Yellow Barite in Quartz                                              Yellow Barite with Pyrite

Monroe Co., Indiana

Celestine

SrSO4

From strontium in the groundwater (Locally Common)

  Celestine in quartz geode     Celestine in quartz geode

                                                                 In pyramidal crystals

                                                               Meade Co., Kentucky                                 

   Celestine in bladed crystals

                         In bladed crystals

                     Lincoln Co., Kentucky

Gypsum

See common minerals, above.

 

Halide Minerals

Minerals with fluorine and chlorine, the latter is extremely rare in geodes.

Fluorite

CaF2

Fluorine gas from the earth’s interior (Rare)

  Fluorite in quartz geode         Fluorite and calcite in geode

       Fluorite, pale yellow cube in Quartz                       Fluorite (lower left), brown, with calcite

                                                                  Meade Co., Indiana

 

Oxide - Hydroxide Minerals

Minerals with oxygen and hydroxide radical (OH).

"Goethite" or "Limonite"

Iron hydroxide minerals

Decomposing pyrite or marcasite

See examples under "Aragonite" below.

Jamborite

(Ni2+,Ni3+,Fe)(OH2)(OH,S,H2O)

Decomposition of the rare mineral millerite (Very Rare)

    Jamborite altering from millerite        Jamborite altering from millerite - close-up

         Jamborite altering from millerite                                Microphoto of crystals on the left

Monroe Co., Indiana

Hydrocarbons

Petroleum

Migration of oil through bedrock from source rock (Uncommon)

 

    Close up of strontianite stained by petroleum

Microphoto of the strontianite geode below, right, 

      stained by natural oil associated with the

      Muldraugh dome in Meade Co., Kentucky.

 

Carbonate Minerals

Minerals with carbon and oxygen as CO3.

Aragonite

CaCO3

Calcium from ground water

   Elongated aragonite crystals with Goethite     Elongated aragonite crystals with Goethite 

                                                  Elongated aragonite crystals with Goethite                           

Monroe Co., Indiana

Cerussite

PbCO3

Decomposition of galena - which is itself very rare ( very, very rare!)

   Cerussite in microscopic white grains on Chalcopyrite         

    Microphoto: Cerussite in tiny white grains                  

          on Chalcopyrite (larger crystal) and                             

                    galena (small gray cube)

            Halls Gap, Lincoln Co., Kentucky

Malachite

Cu2CO3(OH2)

Alteration of chalcopyrite (Rare)  

   Malachite on Chalcopyrite

        Microphoto: Malachite on chalcopyrite

               Field about 4 mm wide field

          Halls Gap, Lincoln Co., Kentucky

Siderite

FeCO3

Decomposition of  pyrite (Uncommon)

   Elongated aragonite crystals with siderite?

Aragonite with Siderite(?), Monroe Co., Indiana

Strontianite

SrCO3

Decomposition of celestine (Uncommon)

   'Puff-balls' of Strontianite on Celestine   Strontianite stained by natural petroleum       

        'Puff-balls' of Strontianite on Celestine          Strontianite stained by natural petroleum

                                                                    Meade Co., Kentucky  

                 

Photographs by Alan Goldstein

Created January 24, 2012, Updated May 27.