A Titaniferous Magnetite Boulder from Southern California

A few days ago, a 1-foot-long blackish boulder weighing about 40 or 50 pounds was shown to me in order to get my opinion as to its origin.

The definitive answer about this boulder came from fellow geologist Dave Liggett, whose was kind enough to help me and whose knowledge of igneous petrology (especially of the southern California area) is phenomenal. The rock you see below is actually a "meteor-wrong" instead of a meteorite.

A close-up of the rock specimen shown above.

Here are the particulars. It is a worn boulder of titaniferous magnetite (iron + titanium + oxygen), probably with intergrown crystals of the mineral ilmenite. The boulder is most likely from the western San Gabriel Mountains, in the Los Angeles National Forest, north of Pasadena, southern California. The geologic age of this rock is Precambrian (several billion of years old). Mineralization in this area is hosted by the rock anorthosite, which will be the subject of one of my future posts. This anorthosite body comprises a 82-square mile area, in which there are many old mines, including some abandoned gold mines (now on private property). The magnetite in this host rock occurs mostly as irregular or dike-like bodies with either sharp or gradual contacts with the anorthosite.

F.Y.I., it seemed to me at first glance that it could be a meteorite. It is very heavy, hard, and magnetic. Those features by themselves, however, do not prove that it is a meteorite. To see a very useful and educational "yes/no" flow chart for trying to decipher whether a rock is a meteorite, I highly recommend a website maintained by Randy Korotev, who used as a basis for comparison, images of meteorites housed in the Department of Earth and Planetary Sciences at the Washington University in St. Louis.