By Dave Tucker September 22, 2010
If you like to collect mineral crystals, you’ll like this field trip. There is a road cut on a logging road west of Centralia and Chehalis with unusual well-formed crystals of the mineral augite. It is well-known to mineral and crystal collectors. The larger augite crystals are a centimeter or even more long; smaller crystals are more numerous and usually more well-formed. Crystals can be picked out of the outcrop, but you’ll find them more easily, and generally in better condition, if you pick through the sand bank below that has eroded from the exposure. I collected a large handful of nice shiny crystals in just a couple of minutes. An article by State Geologist Ray Lasmanis about this crystal-collecting locality appeared in Washington Geology in 1999 and can be found online here; go to page 20 for photos, geology, and a detailed map of the logging road system. That article has more detail than I will provide here. See below for directions from I-5. The weathered deposit is chock full of black, or slightly greenish, opaque augite crystals.
Use Exit 88, US 12 West, from I-5 just north of Centralia. Follow 12 west to Rochester; turn south (left) at the light at Albany Street SW. In two blocks, turn right on 185th, which curves southwest and becomes Marble St SW and eventually becomes Independence Road SW. Cross over the Chehalis River; in 1/10 mile turn left on Michigan Hill Rd. Turn right (west) at the intersection with Lincoln Creek Rd, which meanders through pleasant farm and pasture land. Turn sharp right (still on Lincoln Creek Rd) at the junction with Lepisto Rd. From this point it is 1.3 miles to the unsigned right on the gravel DNR mainline, L-1000. The road is drivable with a 2WD sedan. There is a yellow gate, but it is reportedly rarely closed. If you come to Barton Road, you have just missed L-1000. Stay on this main line; most spurs are gated. It is 3.7 miles to the obvious low outcrop, more or less. It will be on the right. The location is (in WGS 83) N46°42.416′ W123°17.283′; elevation is 1189′ by GPS. See the map in the Washington Geology article (p. 20) for details of this road.
The crystals are found in a weathered, crystal lithic tuff. The ‘tuff’ part means that this is a sedimentary rock, so the crystals and rock fragments either fell from the air or through the water after an eruption as tephra, or were eroded from a nearby lava or tephra deposit and then deposited in water as a sedimentary deposit. This is a marine deposit, based on fossils found nearby (see the WA Geol article); I’ll propose that the crystals are probably associated with an underwater basaltic eruption “cloud”. The crystals and small rock fragments fell onto the seafloor and formed the tuff deposit. The rock is very weathered, and can be picked apart with a pocket knife or even crushed in your hands. The tuff is overlain by deep, orange-red soil that has had a long time to develop, since the Pleistocene glaciers didn’t get this far south to strip off that cover.
The tuff is found in the middle to upper Eocene marine Mackintosh Formation. The one map source I have found is a low quality hand-lettered open file report by the Washington DGER (Logan, 1987), which is not very useful for purposes of this discussion. You can find it yourself by going to the Department of Geology and Earth Resources 30 x 60 minute minute map index and clicking on the “Chehalis” map.
The Washington Agate and Mineral Society has a small webpage dedicated to this locality.
Augite appearance (habit) and occurrence
Augite is a very dark, essentially black mineral. Its name comes from a Greek word meaning ‘luster’. If it is able to form in magma without interference from other adjacent crystals, it can form prismatic crystals. Augite crystallizes fairly early during magma cooling. Well-formed crystals are virtually square or eight-sided in cross section- the angles between the sides in a nearly square crystal are diagnostic, if you have the means of measuring this; they are 87 and 93 degrees. These fully formed crystals have tapered end, so they superficially resemble quartz crystals except in color. A figure in the Washington geology article shows the shape of augite crystals. Augite grows in magmas with a fair amount of iron and/or magnesium and/or calcium and/or sodium. It is most common in the darker, more mafic igneous rocks: basalt and its plutonic equivalent gabbro, or in andesite. It is ubiquitous in Mount Baker andesites, for instance, and in virtually every other Cascade andesite, where it forms small (1-3 mm) shiny black or very dark green crystals.
Augite is an inosilicate in the clinopyroxene group in the silicate mineral class Klein and Hurlbut, 1999). The chemical formula is (Ca,Na)(Mg,Fe,Al)(Si,Al)2O6 . Don’t let that mess scare you- it is pretty simple, really. The element symbols in the sets of parentheses mean that the elements present in augite’s crystal structure vary considerably. All silicate minerals have Si (silicon) and O (oxygen) as their basic composition; quartz is the simplest of these, with no other elements added: SiO2 .The augite chemical formula means that in addition to the SiO2 base, there is calcium and/or sodium in the ratio of 1:1. Magnesium, iron, and/or aluminum (1:1), are also required to cause augite to grow in magma. The last parenthetical set in the formula means that some aluminum (Al) can substitute for some of the silicon in the basic structure with oxygen, and that there is twice as much Si (or some Al) as there is of those other elements. There are 6 times as many oxygen atoms as the others, except only 3 times as much oxygen as silicon (or aluminum) atoms.
Klein, C, and Hurlbut, C.S. Jr., 1999, Manual of Mineralogy (21st edition). Wiley and Sons, NY.
Lasmanis, R., 1999, Augite crystals from Doty Hills, Lewis County, Washington: Washington Geology, vol. 27, no. 2/3/4, December 1999, p. 20
Logan, R.L., 1987, Geologic map of the Chehalis River and Westport Quadrangles, Washington: Washington Division of Geology and Earth Resources, Open File Report 87-8.