Shelly This is a very pretty bit of Ordovician aged limestone…


This is a very pretty bit of Ordovician aged limestone found in western Ohio. The fossil assemblage is common in limestones from throughout the central United States – mostly stems of crinoids ( and shells of brachiopods, cemented together by a carbonate matrix into a rock.

450 million years ago, the eastern side of the continent we call Laurentia (the core of which today makes up North America) was just finishing a mountain building event we call the Taconic Orogeny. A volcanic arc collided with the eastern portion of this continent, causing uplift of mountains that would run from Canada to the southeastern US. These mountains shed sediments into a basin formed behind them, creating sands and shales that today hold oil and gas deposits.

Farther offshore, away from these sedimentary inputs from the eroding mountains ocean-dwelling organisms inhabited the water. The waters were warm as that part of the continent was near the tropics, and they were fairly quiet as there was a mountain range on both sides that limited ocean currents. These settings are great for the formation of carbonate minerals, making this an easy place to form shells. These rocks are one of several huge limestones that occupy the center of the country, formed under similar conditions as the full Appalachian Mountain chain grew to the east.


Image credit: Kyle Hartshorn


Preisingerite, Polybasite-Tac Bi3O(AsO4)2(OH),…

Preisingerite, Polybasite-Tac

Bi3O(AsO4)2(OH), (Ag,Cu)6(Sb,As)2S7][Ag9CuS4   


Clara Mine, Rankach valley, Oberwolfach, Wolfach, Black Forest, Baden-Württemberg, Germany

Field of View: 0.65 mm

A creamy white preisingerite aggregate together with tabular pseudohexagonal polybasite-Tac crystals. 

Photo: Michael Förch

Preisingerite is a rare secondary mineral in the oxidized zone of some arsenic-bearing
hydrothermal mineral deposits. Forms translucent white to grey crystals.  

Polybasite-Tac is a polytype of Polybasite, with the same chemical formula but different crystal structure. Polybasite-Tac is therefore classified as an individual mineral species. Polybasite-Tac was previously known as Antimonpearceite, and was renamed by the IMA in 2006.  It is almost always found in known silver deposits.