The most uncommon mineral known to exist in geodized brachiopods, from the Georgetown area, is Barite. In fact, good Barite crystals from the State of Ohio, are extremely difficult to find. Barite, a sulfate similar to Celestine, has been observed as coarsely crystalline masses in limestone and ironstone septaria throughout the state, however only a few Ohio localities have produced any crystal specimens of the mineral.
Although Barite shares many physical characteristics with Celestine including weight, luster, crystal system and hardness, the two are easily distinguishable at this locality. Celestine tends to form bluish gray to white bladed crystals. Barite, at this locality, forms cream yellow to white rosette shaped crystal clusters. Upon careful inspection, the crystals comprising these rosettes are somewhat translucent to water clear, however the overall appearance of the rosette is generally cream to white.
However, the vitreous nature of the small crystal faces causes the rosette to glisten. The photo to the left displays one of these Barite rosettes. Notice the association with Dolomite and Celestine. This photo relays the ease of differentiating the two sulfates; Barite in the foreground under and in front of unterminated Celestine crystals. This particular piece is a four mineral specimen, also a rare find for the locality and Ohio in general.
Pyrite FeS, Marcasite FeS and Goethite Fe3+O(OH)
Good crystal specimens of Iron Sulfide minerals are becoming increasingly difficult to find at this locality. This, in part, is due to the fact that many of the exposed geodized brachiopods have undergone some amount of weathering. The result is that most of the Iron Sulfides, Pyrite and Marcasite, have deteriorated into rusty masses of Goethite. Both cubic and octahedral Pyrite are common in these brachiopods. Although rarely seen, Marcasite blades also occur in Calcite geodized brachiopods.
In most cases, the iron sulfide’s crystal form has been blanketed by a coating of botryoidal Goethite, although occasionally the crystal is completely replaced by the Oxide. The photo to the lower right exhibits a common example of this replacement. It is likely that this Goethite needle was at one time a Marcasite blade. The more common corrosion of Iron Sulfide is displayed in the photo to the upper right.
A Pyrite cube has been partially altered to Goethite and was, at one time, completely coated with the Oxide. Part of the Goethite coating has crumbled away revealing the cube. In the photo, one can clearly see both the cube, and the rind of corroded Goethite. Also of interest in this photo, a small clear Dolomite crystal has begun to develop on the Pyrite cube, indicating that at least one generation of Dolomite was introduced after the Pyrite. Marcasite blades and are less frequently observed than either Pyrite or Goethite.
This is perhaps due to the delicate nature of Marcasite blades. Blades of Marcasite have been observed up to 4 millimeters tall and 1 millimeters wide, although the thickness of these crystals is nearly unmeasurable. When viewed from the side, Marcasite blades, like these, are virtually undetectable. A slight breath of air can easily damage a Marcasite blade.
Unfortunately, this thin fragile crystal shape rarely survives trimming. Marcasite blades, from this locality can be reddish brown to black in color and often have a colorful mirror-like iridescent coating. To the lower right is an example of a Marcasite blade. This crystal is approximately .6 millimeters wide but less than .05 millimeters thick.