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Cal-Portland Rillito Quarry – TGMS Field Trip - May 11, 2019


Twenty-two members attended the May 11, 2019 TGMS Field Trip to the Cal-Portland Rillito Cement Quarry.  Just off of Twin Peaks Road, a few miles west of I-10, the quarry produces the raw materials for Cal-Portland’s Portland Cement plant about five miles north of north of the quarry off I-10. 

Quarry Manager, Jared Stokes graciously hosted us, describing the general geology of the deposit, the rock units mined for various grades of cement, and the process used to grind and mix the recipes to produce cement with different specifications.  He also explained that we were standing in the hole created by the removal of one of the Twin Peaks, for which the Road and an Elementary School were named.  Mining had begun there in the 1940s.  Besides being a great host, Jared was very interested in any insights we could provide about the geology and mineralogy of the rocks.


The nature of cement quarrying requires very consistent rock properties and composition, which, in general, is not promising for a mineral collecting trip.  Recognizing that, Jared took us to a portion of the quarry where the rock is much more variable, broken by numerous veins and intruded by igneous dikes.  Prior to the trip, he had his equipment operators pull numerous piles of the various rock types out onto the bench, to make them more safely accessible.  This and the fact that we were the only ones in the quarry that day, we were free to wear comfortable shade hats rather than hard hats.  (As he pointed out in his safety lecture, the greatest hazard that day was the sun!)

Members collected a number of interesting specimens, including small water-clear calcite crystals with limonite-goethite inclusions, nice manganese dendrite specimens, a dense caliche-cemented breccia, and distinctly pink calcite vein pieces.  Everyone drooled over a large boulder of the material, out of reach on the bench slope above us.  A few small fluorite crystals were collected, but none of the banded fluorite collected on a previous trip. 

One of the attached pictures shows pink calcite veins, crosscut by later white calcite.  A second picture shows a spectacular manganese dendrite, more than two feet “tall”.  While no-one probably found anything that will win a “best of show” trophy, it was an enjoyable and easy trip, made all the better by the nice weather and the gracious hospitality of Cal-Portland.

Article/photographs by Mark Marikos and Ron Gibbs