California’s Golden Coast - Historic Coronado Beach
Note: For this EarthCache it is best to visit on a Sunny Day, it will be easier to complete.
Answer the following Questions and send the answers to the CO at the top of the page. You will be taking 2 photos, and answering 3 Questions.
1) Wade into the ocean, and using your hands, a makeshift net (perhaps a nylon will work) attempt to collect a sample of the floating mica in the water. I was able to make a video, and screen capture the floating mica. Include a photo, with your log, of the sample you collected.
2) Describe the mica that you collected, size, shape, number of pieces.
3) What type of Mica do you believe you collected, Muscovite, Biotite, or Lepidolite. Why do you believe you are correct with your Mica Choice?
4) How far (in miles) has this Mica traveled to reach this beach?
5) Include a second photo, with your log, with the Coronado Beach Shoreline in the background.
My Story
In January 2025, we flew into San Diego, with the intent of spending some time in San Diego and seeing the sights and then traveling on to the Block Park in Yuma Arizona later in the week. One of our stops was to see the Beautiful Historic Coronado Hotel. Well, for me no trip can be complete without getting wet in the local water hole, in this case, it was just a calf high wade in the water. That’s when I saw something in the water, “GOLD”. Instantly, I thought of the nickname for California, “The Golden Coast”.
Common sense told me it was real gold, but then I wondered what it could be. “The glitter swooshed around in the water like a snow globe, and the metallic flecks, which were softer and flatter than regular sand, stuck to my skin when I stepped out of the waves.” I suspected, perhaps flakes of pyrite, or perhaps mica glittering in the bright sunlight. Wow! I told the rest of the group, but no one believed me. So I made a video, with the intent of doing research when I got home.
Here is what I learned:
A Google search revealed that those shiny flecks are actually a mineral called mica - the same type of mineral used to produce glittery substances prized by cave painters (40,000 years ago), cosmetic companies (1960s), and David Bowie as Ziggy Stardust (1972).
In her book, Coronado, author Leslie Hubbard Crawford attributes the gold and silver specks to naturally occuring mica deposits in mountains over the Mexican border. The mica was washed down the mountains by the Tijuana River, which eventually empties into the Pacific Ocean. From there, the shiny mineral is deposited onto the beach by the littoral currents moving up the coastline.
It seems, It doesn’t take much mica to make a beach sparkle. The sheet-like atomic arrangement, specks of mica end up being lightweight and flat, meaning that they’re easily washed away by waves. These shiny flat flakes act like millions of tiny mirrors thrown into the sand, and are so reflective in the sunlight that they are visible even when mica accounts for less than one percent of the sand grains on a beach.
What is Mica?
What types of mica are found in granite?
• Muscovite: A type of mica that can reflect quartz and give granite a silver appearance
• Biotite: A type of mica that can reflect quartz and give granite a black appearance
• Lepidolite: A type of mica that can give granite a violet or pink color
Source of Information:
A Beach So Beautiful, I Thought it Was Polluted
Muscovite - Geology.com