Pfeiffer Purple Sand EarthCache

Bits of manganese garnet eroded out of the surrounding hills create a purple sand on this beach. Though not all of the sand on this beach is purple, it can be found in patches on the dry dunes and in rivulets in the wave wash zone.

Access to the beach is by a 2 mile one-lane road down from PCH 36.24023 / -121.77706. Additionally, there is a $10 parking fee which, unless you park up on PCH, there is no way around. It is also a very popular beach, so on weekends you will often meet a line to get in as the attendants limit entrance when the parking lot fills, allowing only one car in for every car that leaves.

The sands on this beach are not all purple. Manganese garnet sand grains accumulate in various areas on this beach and where there are enough grains, the sand turns purple. Though sometimes the color has been described as pink or lavender depending upon the lighting. The grains of garnet have a different density than the lighter colored sand. Due to this difference in density, wind and water transport segregate the different sand grains making various concentrations of color. The concentration of purple sand increases as you walk up the beach.

The garnet sand did not come from the beach cliffs or the deep valley that Sycamore Creek currently drains as garnets are not found in the rocks drained by Sycamore Creek. Instead, the garnets were eroded out of the Santa Lucia Range (located just east of here on the other side of the highway) and transported to this beach by the Big Sur River. However, the Big Sur River no longer drains through Sycamore Canyon but instead drains cuts north and drains out through Andrew Molera State Park to the beach further north. This change in drainage occurred through a process called stream capture (another EarthCache will be developed for this).

Garnet is a generic name given to a series of minerals share a common crystal structure and a generalized chemical composition of A₃B₂(SiO₄)₃ where A can be Ca, Mg, Fe²⁺ or Mn²⁺, and B can be Al, Fe³⁺, Mn³⁺, V³⁺ or Cr³⁺. Garnets can be a wide range of colors depending upon its chemical makeup. These minerals are often formed during the metamorphosis of rock to schist or gneiss, but can also be found in igneous rocks. Garnets are usually durable, so they can also be found in sedimentary rocks after being eroded from a metamorphic or igneous rock. The garnets from this beach likely formed in the granites and gneisss of the Santa Lucia Range.

The particular form of garnet on this beach is likely spessartine Mn₃Al₂(SiO₄)₃. Spessartine is relatively dense, hard, and heavy for garnet. While most references show this type as orange, you can see it is purple here.

Logging questions:

1. The text "GC5DXJT Pfeiffer Purple Sand" on the first line.
2. The number of people in your group.
3. Compare how the garnet grains accumulate on the dry sand vs. the wave wash zone?
4. In general what size are the garnet grains?
5. Since the source of new garnet grains is cut off, why are there still garnet grains on this beach?

The above information was compiled from the following sources:

• http://www.coastal.ca.gov/publiced/newsletter/summer12/destination.html
• http://es.ucsc.edu/~geoclub/bigsurpics.html
• http://pubs.usgs.gov/bul/1261b/report.pdf
• http://geology.com/minerals/garnet.shtml
• http://www.geo.arizona.edu/tectonics/Ducea/Salinia%20Web%20Page.htm

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