Wednesday, September 8, 2021

Pyrite in the Gravel

One of the sources of crushed gravel in Skagit County is a rock quarry. The quarry has mined large blocks of rock for levy projects, but the process of breaking the large intact blocks of rocks creates smaller rock. Hence a side product is angular crushed rock gravel for driveways and parking areas. The angular rock along with the fines within the crushed material forms a hard drive and parking surface. 

Crushed rock gravel parking area

Walking across one of these driveway areas on a moonlit night I noted remarkably bright spots in the gravel. My geology brain clicked in and I recalled the likely source of the gravel being a rock quarry excavated into some altered ocean floor igneous gabbro. The ocean floor rocks are a Jurassic age accreted terrain slab of ocean floor. Hot fluids flowing through and old ocean floor precipitated minerals. The bright spots I was seeing in the gravel was moonlight reflecting off of the very smooth surfaces of cubic pyrite minerals.       

A larger pyrite mineral - most are much smaller
I spotted this during the day when a very bright reflection struck my eye 

Pyrite is an iron sulfide mineral and is pretty common secondary mineral. Its breakdown via weathering in pyrite rich crushed mining tailings is a source of acid mine drainage. Its presence in some limestone deposits or the rock aggregates mixed into concrete is detrimental as the pyrite breaks down the sulfur released will form acids that will damage wiring, pipes or other metal in the concrete and can also form hydrogen sulfide foul odors.  


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