Hydroxylellastadite with Monticellite

specimen number:
2282031

location:
Crestmore, Riverside Co., California, USA

description:
Rich blue massive hydroxylellastadite with yellowish green monticellite.

The Crestmore quarry, located in the Jurupa Mountains of Riverside County, California, is a premier geological locality renowned for its incredibly complex contact metamorphic suites. Geologically, the deposit was formed during the Cretaceous period when a granodiorite magma body intruded into a thick sequence of late Paleozoic Chino Limestone and Sky Blue Limestone formations. This intense igneous intrusion initiated massive contact metamorphism and metasomatism, creating distinct, highly mineralized skarn zones that yielded over one hundred and fifty distinct mineral species. Commercial extraction at the site began in 1909 under the Riverside Portland Cement Company, which established extensive open-pit and underground block-caving operations to mine the high-purity limestone for industrial cement manufacturing. Mining and quarrying activities remained continuously active throughout the mid-twentieth century until the primary economic stone reserves were depleted and operations finally ceased around 1986.

Monticellite is an uncommon calcium magnesium silicate mineral that belongs to the olivine group and crystallizes in the orthorhombic system. Geologically, it forms as a characteristic high-temperature mineral during the contact metamorphism of siliceous dolostones or limestones that have been intruded by low-silica mafic magmas. The mineral typically occurs as granular aggregates, compact masses, or distinct prismatic crystals that range in color from colorless or white to gray, pale yellow, or light brown, presenting a vitreous to greasy luster. At the Crestmore locality, monticellite occurs abundantly within the specialized sky-blue calcite zones, where it is frequently found closely associated with other diagnostic skarn minerals like vesuvianite, gehlenite, and merwinite.

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