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Santa Cruz County History - Making a Living
Mineral Survey of Santa Cruz County - Black Sand
by C. McK. Laizure
Prepared by C. McK. Laizure,
Mining Engineer of the California State Mining Bureau
There are extensive stretches along the coast of California where the heavier constituents of the beach sands have been concentrated by wave action into deposits of so-called 'black sand'. The composition of this heavy concentrate varies somewhat with the locality. In general the following commercial mierals are present in greater or less amounts: gold, magnetite, ilmenite (oxide of iron and titanium), garnet, zircon, hematite, chromate, and the platinum group metals.
Could each of these minerals be readily and completely separated from the others and from the olivine, quartz and other worthless constituents, they would be readily marketable. Gold and platinum are found in relatively minute amounts only, but on account of their high value, practically all past efforts to work these sands have been confined to the recovery of these metals.
Besides the uncertain profitable production of gold and platinum from this source, the use of black sand in the manufacture of artificial iron castings, where great strength is not required, has been suggested. Some experimental work has been carried on along this line including the making of sashweights by moulding or briquetting the material into the desired form with oxy-chloride (magnesite) cement, portland cement, or other binder. Black sand may also be utilized in concrete mixes where a concentrated weight is wanted, as in concrete balance-weights used on bascule-type bridges and similar structures.
By subjecting the magnetite in black sand to an oxidizing roast, it may be changed to the ferric state, producing a red oxide (Fe2O3), which forms when finely ground a mineral pigment suitable for the manufacture of paint. The pigment so made is said to be superior to ground hematite or the natural soft red oxide.
Magnetite in the black sands is obviously also an ore of iron. With reference to this, Day and Richards (Day. David T. and Richards, R.H., Useful Minerals in the Black Sands of the Pacific Slope, U.S. Geological Survey Mineral Resources, 1905, pp. 1175-1258) in their report on the useful minerals in the black sands of the Pacific slope state,
"It was found that the magnetite contained in the black sands of the Pacific slope constitutes a greater supply of useful iron ore than any other available source known on the Pacific slope. This magnetite usually contains from 5 to 10 per cent of titanium. It was found that this titanium offered no obstacle to the production of high-grade cast iron in the electric furnace and that in a modification of this electric furnace this cast iron could even by decarburized to a very soft iron of high quality."
Well-known deposits of black sand occur in Santa Cruz County along many of the beaches, particularly on the northern shore of Monterey Bay from the mouth of Pajaro River to Soquel point. The deposits are found both on the present beaches and on the older marine terraces back from the present shore line. They occur in strata form a few inches to several feet in thickness, interstratified with light beach sand. They are said to carry small amounts of platinum, as well as gold, and have been washed in the vicinity of Leonard Station near Aptos at intervals for many years, though probably with little if any profit. Typical analyses of the natural black sand near Aptos, according to Day and Richards, [see previous cite] show their content in the following minerals to be:
|Magnetite||502 to 1120 pounds|
|Ilmenite||224 to 576 pounds|
|Chromite||8 to 126 pounds|
|Garnet||1 to 80 pounds|
|Quartz||216 to 1046 pounds|
|Zircon||18 to 26 pounds|
|Unclassified||104 to 189 pounds|
|Gold||none to 62 cents|
At the time of visit, no one was found attempting to extract gold or platinum, but there is now a plant near Aptos utilizing the black sand as an iron ore. For a description of this plant see under Iron.
|Bituminous Rock||Gold||Mineral Water|
|Black Sand||Granite||Moulding Sand & Peat|
|Cement||Iron||Petroleum & Potash|
The Mineral Survey of Santa Cruz County was printed in the Santa Cruz Chamber of Commerce Annual Report, 1926. Reproduced by permission of the Santa Cruz Area Chamber of Commerce.
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