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<br />5 <br />Aggregate Industries – Morrison Quarry GEI Consultants, Inc. <br />AM-07 Geotechnical Stability Exhibit November 2020 <br /> <br /> <br />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 C) <br />descriptions are based on the United States Bureau of Reclamations Engineering Geology <br />Field Manual (USBR, 2001). <br /> <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 W1). <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 /> <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.