MINED EarthCache

Queensland has an extensive history of mining starting in 1827 at “the Limestone Hills” (Ipswich) with the quarrying of limestone. The limestone was burnt in kilns to produce quicklime which was used as a mortar for building. Today, mining in Queensland is primarily of coal although a wide variety of minerals and other rocks are mined.

At the Miners Memorial Monument site are 20 examples showcasing some of the materials, both minerals and rocks, that are mined in Queensland. Here, together with large examples of the various materials are plaques that provide an insight as to how the materials are mined and what they are used for.

We are aware of some of the various minerals and rocks, but what are rocks?
Rocks are formed when minerals (the building blocks) are combined in nature to form the rocks. There are three classifications of rocks: Igneous, Sedimentary and Metamorphic. The classifications are based on the way in which the rocks are formed.

Igneous rocks are formed from the cooling of magma (a very hot liquid found underneath the Earth's surface) due to volcanic activity. The name igneous comes from the Latin word 'igneous' which means fire. That is, igneous rocks occur when molten rock solidifies either below the earth's crust or on the earth's surface. Different cooling rates create different types of rocks.

Igneous rocks mined in Queensland include:

Magnetite is a very common iron oxide (Fe3O4) mineral that is found in igneous, metamorphic, and sedimentary rocks. It is the most commonly mined iron ore with the highest iron content (72.4%). Magnetite is very easy to identify. It is one of just a few minerals that are attracted to a common magnet, it is also the most strongly magnetic mineral found in nature.

Granite is a light-colored igneous rock with grains large enough to be visible with the unaided eye. It forms from the slow crystallization of magma below the earth's surface. Granite usually has a red, pink, gray, or white color with dark mineral grains visible throughout the rock.

Granite is the best-known igneous rock. Many people recognize granite because it is the most common igneous rock found at Earth's surface and because granite is used to make many objects that we encounter in daily life. These include counter tops, floor tiles, paving stone, curbing, stair treads, building veneer, and cemetery monuments. Granite is used all around us - especially if you live in a city.

Rhyolite is an extrusive igneous rock with a very high silica content. It is usually pink or gray in color with grains so small that they are difficult to observe without a hand lens.

Many gem deposits are hosted in rhyolite. The thick granitic lava that forms rhyolite often cools quickly while pockets of gas are still trapped inside the lava. As the lava quickly cools, the trapped gas is unable to escape, and forms cavities known as "vugs." Later, when the lava flow has cooled and hydrothermal gases or ground water move through, material can precipitate in the vugs. This is how some of the world's best deposits of red beryl, topaz, agate, jasper, and opal are formed. Gem hunters have learned this and are always on the lookout for vuggy rhyolite.

Rhyolite is a rock that is rarely used in construction or manufacturing. People have used rhyolite to manufacture stone tools, particularly scrapers, blades, and projectile points. It was probably not their material of choice, but a material used out of necessity.

Sedimentary rocks are types of rock that are formed by the accumulation or deposition of small particles and subsequent cementation of mineral or organic particles on the floor of oceans or other bodies of water on the Earth's surface. Sedimentation is the collective name for processes that cause these particles to settle in place. The particles that form a sedimentary rock are called sediment, and may be composed of geological detritus (small pieces of trash or waste material) or biological detritus (organic matter). Before being deposited, the geological detritus was formed by weathering and erosion from the source area, and then transported to the place of deposition by water, wind, ice, mass movement or glaciers, which are called agents of denudation. Biological detritus was formed by bodies and parts (mainly shells) of dead aquatic organisms, as well as their fecal mass, suspended in water and slowly piling up on the floor of water bodies (marine snow).

Sedimentary rocks mined in Queensland include:

Sandstone is a type of clastic sedimentary rock formed from sand grains that have undergone cementation. They take on black, brown, gray, pink, red or white color. Sandstones may have either fine or coarse grains which makes them easy to distinguish visually.

Quartz (mature) sandstones are light in color and are mainly made of round and well-formed quartz grains. On the other hand, greywackes (immature) sandstones are made of angular-shaped grains of different minerals.

Uluru is basically arkose sandstone (a sandstone rich in feldspar). Uluru is naturally grey, but the iron content of the rock is "rusting" at the surface, resulting in the distinctive red iron oxide coating.

Siltstone is a sedimentary rock composed mainly of silt-sized particles. It forms where water, wind, or ice deposit silt, and the silt is then compacted and cemented into a rock.

Silt-sized particles range between 0.0039 and 0.063 millimeters in diameter.

Silt accumulates in sedimentary basins throughout the world. It represents a level of current, wave, or wind energy between where sand and mud accumulate. Sedimentary structures such as layering, cross-bedding, ripple marks, erosional contacts, and fossils provide evidence of these environments.

Siltstone occurs in a wide range of colors. It is usually gray, brown, or reddish brown. White, yellow, green, red, purple, orange, black, and other colors occur. The color is caused by the composition of the grains, the composition of the cement that binds them together, and stains produced by contact with subsurface waters.

Metamorphic rocks are a class of rocks that are derived from igneous, sedimentary or other metamorphic rocks that have altered their form (recrystallized) as a result of changes in their physical environment. Metamorphism comprises changes both in mineralogy and in the fabric of the original rock.

In general, these alterations are brought about either by the intrusion of hot magma into cooler surrounding rocks or by large-scale tectonic movements of Earth’s lithospheric plates that alter the pressure-temperature conditions of the rocks. Minerals within the original rock respond to the changing conditions by reacting with one another to produce a new mineral that is thermodynamically stable under the new pressure-temperature conditions.

In contrast to the formation of igneous rocks, metamorphic rocks do not crystallise from a silicate melt, although high-temperature metamorphism can lead to partial melting of the host rock.

Metamorphic rocks mined in Queensland include:

Greenstone is a common generic term for valuable, green-hued minerals and metamorphosed igneous rocks and stones which early cultures used in the fashioning of hardstone carvings such as jewelry, statuettes, ritual tools, and various other artifacts. The term also includes jade and jadeite, although these are perhaps more frequently identified by these latter terms. The greenish hue of these rocks generally derives from the presence of minerals such as chlorite, hornblende, or epidote.

Greenstone minerals were presumably selected for their color rather than their chemical composition. In archaeology therefore, having a loosely applied general term is at least partially influenced by the observation that ancient cultures often used and considered these various green-hued materials as interchangeable.

 Marble is a metamorphic rock formed when limestone is subjected to high pressure or heat. In its pure form, marble is a white stone with a crystalline and sugary appearance, consisting of calcium carbonate (CaCO₃). Usually, marble contains other minerals, including quartz, graphite, pyrite, and iron oxides. These minerals can give marble a pink, brown, gray, green, or variegated coloration. While true marble forms from limestone, there is also dolomitic marble, which forms when dolomite [CaMg(CO₃)₂] undergoes metamorphosis.

To earn your smiley  you must correctly answer the following questions.

1. What material are the descriptive and memorial plaques mounted on? This can be determined from the information above and by comparing the rock examples to the material on which the plaques are mounted.

2. How would you describe the appearance and textural feel of the Siltstone example?

3. What is the predominate colour of the Greenstone example?

4. Many gems are found in igneous rocks, but what gem can be found in a sedimentary rock example here?   

Answers must be sent to the CO by private message.
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