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Why is Geode sedimentary I always wondered why Geode is
sedimentary chemical. It has
very large crystals and it
looks like it has to be
igneous intrusive. Can you
explain to me in detail why
that is?
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Geodes begin as bubbles in volcanic rock and can also be formed by tree roots or kind of mud balls and other things in sedimentary rock. Thats why they say its sedimentary chemical. With time the outer shell gets hard and water containing silica precipitation forms on the inside walls of the geode's cavity. The silica precipitation can contain minerals such as quartz, and others, I guess you know the rest. |
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Geodes form by precipitation of minerals from solution. That would make it a chemical sedimentary rock. They are not often found in intrusive igneous rocks because the geode needs a vug, or a cavity within the host rock in order to form. Intrusive igneous rocks rarely have vugs, but do sometimes have large fractures or joints that can be crystal lined similar to a geode. Geodes are more likely to be found in extrusive igneous rocks that have vugs formed by gas that comes out of solution as the rock cools rapidly (in comparison to intrusive igneous rocks). These vugs eventually may fill with fluid which carries dissolved calcite, quartz, and other minerals. As the fluid becomes oversaturated different minerals will drop out of solution forming layers of the geode and finally forming crystals within the center of the geode. The similar things that form in sedimentary rocks are often referred to as concretions, and these are seldom hollow. They may be composed of calcite, chert, or other minerals, and will often have concentric rings within them. However, they do not form within a vug or cavity, but instead appear to grow outward. Septarian concretions have crystalline filled fractures inside them that makes them quite attractive. Concretions appear in most cases to form from a nucleus and grow outward, unlike a geode's growth inward. |
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