April Speaker: "The Geologic Origin of Serpentinites and Unique Associated Minerals" by Dr. C. Leigh Broadhurst

by Cindy Schmidtlein, MSDC Vice President

Serpentinite is a metamorphic rock composed of one or more of the serpentine group minerals.

Serpentinites are metamorphic rocks that originate from the Earth’s mantle. The mantle rock precursors are generally categorized as “ultramafic”. Serpentinites of more recent origin are found in active or recently active subductive plate margins. Older serpentinites or “alpine ultramafics” are found within ancient regionally metamorphosed piedmont or shield locales, but mainly have similar origins. Their distinctive mineralogy helps geologists reconstruct past plate margins, and how plate tectonics worked when the Earth was much younger.

Serpentinite rocks are of immense commercial importance and also contain a suite of minerals of interest to collectors. We will discuss the various materials that are mined from serpentinites and which minerals are commercially important. We will also discuss the mineralogy of the mantle rocks that are metamorphosed to form the suite of serpentine minerals, and the distinctive soils developed from serpentine rocks.

Serpentinite with lilac-colored masses of stichtite, a rare hydrous magnesium chromium hydroxy-carbonate mineral from Tasmania.

Our speaker for the evening, Dr. C. Leigh Broadhurst holds a Ph.D. Planetary Geochemistry and Physical Chemistry from the University of Arizona, and a B.S in Geology from the University of Maryland. Her geological research has been focused on rocks of mantle origin — including serpentinites — that help to categorize the origin of the Earth and its plate tectonic system. Dr. Broadhurst recently retired from the Dr. Rufus Chaney research group at the U.S. Department of Agriculture Agricultural Research Service.