Introduction to minerals of Latah County, Idaho

Latah County covers approximately 1080 square miles in northern Idaho. The geology of the county is very diverse and complex. From oldest to youngest, the rock types are the metamorphic rocks of the Belt Supergroup, intrusive rocks of the Idaho Batholith, extrusive rocks of the Columbia River Basalts, and sedimentary deposits. The catalyst for this project came from a mineralogy class I took as an

undergraduate. One of the requirements for completing the course was to collect and identify at least 25 minerals from the county. The first question that came to mind was "where do I go to find these minerals?"

The first part of this project consisted of extensive field work in Latah County. The primary emphasis was on abandoned mining areas, numerous basalt quarries, and various outcrops along the highways, county roads, and Forest Service roads. Hand samples of the minerals, or the rocks containing visible minerals, were collected at each site. The minerals collected and identified at the various locations were limited to those clearly visible with the naked eye or with the aid of a hand lens.

The second part of the project consisted of laboratory studies using a petrographic microscope, X-ray diffractometer, and the scanning electron microscope to identify positively the individual minerals.

It was not the purpose of this project to identify all the minerals in Latah County. Many minerals other than those included in this thesis exist, and it would require a lifetime of work to locate all of them. Hopefully, this project will inspire further studies, and be a valuable educational tool and a source of enjoyment for science teachers, students, and the general public.

The thesis provides a brief introduction to the geology of Latah County, followed by 18 mineral-collecting localitites. Each of the 18 localitites stands alone with a brief geologic setting, road log, location map, and mineral descriptions. Individual descriptions are provided for each location because mineral appearances, such as forms, habits, and colors may vary owing to subtle structural properties. Because each section is self-contained, each can be "Xeroxed" and taken into the field.

Certain mineral associations occur together because rocks are merely associations of minerals. For instance, basalt, a dark-colored, fine-grained rock is comprised of such primary minerals as Ca-rich plagioclase, olivine and augite. Secondary minerals include calcite, aragonite, siderite, goethite, opal, or zeolite. Granite and syenite contain quartz, feldspar, and mica as primary minerals, and accessory minerals such as hornblende, actinolite, zircon, titanite, apatite, and magnetite. Metamorphic rocks will vary mineralogically depending on parent material and temperature and pressure conditions. The most common minerals are amphibole, pyroxene, muscovite, biotite, talc, serpentine, Ca-rich plagioclase, garnet, staurolite, kyanite, sillimanite, andalusite, and chlorite.