Geo-pic of the week: Titanite

(FOV approx. 2 mm, photo courtesy of Stephen Stuart)

The wedge-shaped crystal in the photo above is the mineral titanite. This calcium titanium silicate (formula CaTiSiO5) is commonly found as an accessory mineral in igneous intrusions similar to those present at 3M and Granite Mountain quarries near Sweet Home. This sample was collected from 3M Quarry.

Titanite gets its name from its titanium content, but it was more commonly known by the name “sphene” until 1982 when the new name was officially adopted by the International Mineralogical Association. Sphene was derived from the Greek word “sphenos”, meaning wedge.

Crystals of titanite have a higher dispersion than diamonds. Dispersion is the measurement of refractive properties of a gemstone. The higher the dispersion, the more “sparkle” from the gem. However, gem quality samples of titanite are very rare, and the mineral is relatively soft compared with other gemstones.

Geo-pic of the week: Basal Conglomerate

Pictured above is a little piece of geologic history known as a basal conglomerate.  that’s a rock formed after a period of erosion that marks the boundary between two geologic time periods: in this case, the Mississippian (359-318 million years ago) and the Pennsylvanian (318-299 million years ago).

318 million years ago sea level subsided, bedrock was exposed, and the Mississippian Period came to an end.  When exposed to erosion at the earth’s surface, pieces break off from bedrock.  Flowing water in rivers, streams and oceans wears the edges of those rock fragments till they’re rounded.  Once ocean level rises and deposition resumes, the rounded gravel gets mixed with newly accumulating sediment and forms a rock which is made partly of fragments of the older bedrock.  Geologists call this type of rock a basal (at the base) conglomerate (containing round gravel) because it is the first bedrock signaling the beginning of a new period of geologic time.