Magnus Vonsen, prominent California mineral collector and amateur mineralogist, was born in Vallejo, Sonoma County, California on April 17, 1880, the son of Danish immigrants Loretta Moltzen and John Vonsen, a dairy farmer. He received an 8th-grade education there, and then worked as a milker on a dairy farm in Humboldt County in 1920. He became part-owner in a grocery, feed and grain store in Petaluma in 1904, and in 1913 became the sole owner, reorganizing the store as the M. Vonsen Company. The company proved to be very successful, and in time he opened branch stores in several nearby towns. In 1906 he married Bertha E. Petersen of Petaluma; they had one daughter together.
His interest in mineralogy was kindled around 1914 when the demand for metals increased as a result of World War I, and mining activity sprang up around Petaluma. He educated himself in the subject through private study and through an extension course from the University of California. He also built a fully equipped laboratory in his home and became proficient at qualitative chemical analysis and optical mineralogy.
Vonsen's mineral collection rapidly grew over the years. He housed his collection of about 3,500 specimens in museum-style cases in a special room in his home—every specimen being of high quality and well displayed. By 1947 he had added a separate building adjacent to his house, where he could store and display his collection. He was particularly interested in the glaucophane schist minerals, the saline minerals and the borate minerals, and collected at the Death Valley borate mines and neighboring areas every year. He became extremely knowledgeable about California and Nevada localities. He discovered a new Fe-Mg borate near Riverside, California, and it was later named vonsenite in his honor by Arthur Eakle in 1920. He also discovered the new borate mineral teepleite at Borax Lake, and described it with W. A. Gale and William Foshag in 1934.
Vonsen published several other articles in mineralogy, including "Death Valley and the borates of California" (1929) and "Borates of California" (1951) in Rocks & Minerals, and two articles on "The Geyers" (1941 and 1946).
In 1931 Vonsen became a member of the California Academy of Science, and a few years later was named Honorary Curator of Minerals there. He was elected a Fellow of the Mineralogical Society of America in 1937, and died June 16, 1954, by which time his mineral collection was considered to be the finest on the Pacific Coast and one of the finest in the country; it went to the California Academy of Science, becoming California's first world-class public mineral collection
California Death Index.
SWITZER, G. (1955) Memorial of Magnus Vonsen. American Mineralogist, 40 , p. 286-288.
SWITZER, G. S. (1975) Personality sketch: Magnus Vonsen (1879-1954). Mineralogical Record, 6, 169-170.
U.S. Federal Census, 1880, 1900, 1910, 1920, 1930.
World War I Draft Registration Cards, 1917-1918.
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WILSON, Wendell E. (2019)
Biographical Archive, at www.mineralogicalrecord.com.]
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