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2.2.1 Rock Types <br /> Granitic and biotite gneiss are the two most common rock types in the South Quarry and <br /> West Quarry. Sillimanitic gneiss is also present in lessor amounts, with its distribution <br /> shown in geologic maps Figure 3 and Figure 4. The rock descriptions below are applicable <br /> to both the South and the West Quarry, although most of the discontinuity information is <br /> based on observations made during annual site inspections at the South Quarry. <br /> The alphanumeric descriptors for rock hardness (H), weathering (W), and joint(S,R and Q <br /> descriptions are based on the United States Bureau of Reclamations Engineering Geology <br /> Field Manual (USBR, 2001). <br /> Granitic and Biotite Gneiss <br /> The granitic and biotite gneiss units are very similar and differ mostly in their mineral <br /> composition. Granitic gneiss is primarily composed of feldspars and quartz (light white and <br /> pinkish colored) and biotite, pyroxene, and amphibole (dark colored minerals). Biotite <br /> gneiss contains a notable lack of feldspars (pink), less quartz (white), and the addition of <br /> sillimanite (white). Both units are generally fine to medium grained, thinly foliated <br /> (foliations less than 1 cm thick), very hard to extremely hard (H2 to H1), and slightly <br /> weathered to fresh(W3 to W 1). <br /> Pegmatite <br /> The pegmatites tend to be randomly oriented features and have variable widths, ranging from <br /> 1-inch to 3-feet wide. The pegmatites are primarily medium coarse to coarse grained and <br /> light colored (pink to white)with feldspar, quartz, and mica minerals. The pegmatite is very <br /> hard to extremely hard (H2 to H1) and is slightly weathered to fresh (W3 to W1). <br /> Sillimanitic Gneiss <br /> Sillimanitic gneiss is generally fine to medium grained and thinly foliated (generally less <br /> than 1 cm thick). It is hard to soft(H3 to H6) and slightly weathered to intensely weathered <br /> (W3 to W7)with localized zones where it is decomposed (W9). <br /> Sillimanitic gneiss can be characterized as having quartz and sillimanite in alternating layers <br /> with minor biotite (dark). Portions of the sillimanitic gneiss appear to have been subjected to <br /> a second phase of low-temperature low-pressure metamorphism called metasomatism. <br /> Metasomatism is a chemical alteration that causes the replacement of one mineral with <br /> another of different mineralogical and chemical composition. During this process, <br /> hydrothermal fluids chemically alter the sillimanite (Al2SiO5), stripping Silicon (Si)from the <br /> sillimanitic gneiss, resulting in creation of a new minerals that include pyrophyllite, and <br /> possibly chlorite, vermiculite, sudoite and hydromuscovite. All the resulting minerals are <br /> considered phylosillicates and are very soft on the Mohs harness scale (e.g., values of 1 to 2 <br /> vs 6.5 to 7.5 for sillimanite). These phylosillicates have a lower strength, are highly friable, <br /> and result in poor-quality rock. The sillimanitic gneiss has a lower strength and durability <br /> than the other gneissic units and is physically unsuitable for use as aggregate. Photo 2-1 <br /> shows an outcrop of banded sillimanitic gneiss near the crushing plant. <br /> 5 <br /> Aggregate Industries—Morrison Quarry GEI Consultants, Inc. <br /> AM-07 Geotechnical Stability Exhibit November 2020 <br />