Written by Carson Seo
Edited by Renee Lee
Mineral crystals found in nature are one of nature’s great beauties, possessing orderly, rigid, geometric structures along with a diverse selection of colors. Luray Caverns, for example, is home to a striking visual of the crystal stalagmites and stalactites extruding out of the cave’s interior. While people generally treat these solids as a delicacy to be found in grand natural sites, crystals can also be commonly found in more routine, accessible locations, ranging from the gravel on the ground to the open rivers and streams [1]! However, some questions remain: how do the aesthetic, geometric structures of mineral crystals form and develop? What factors and influences determine the form of a crystal?
The geometric nature of mineral crystals can be partially understood by understanding the chemical characteristics of minerals. Geologists define minerals , as inorganic, crystalline compounds that are naturally formed and have a definite chemical composition [2]. Let’s break down this definition. Fundamentally, a compound is a substance made up of multiple elements that are bound together in fixed ratios. Furthermore, the fact that minerals have a definite chemical composition means that each mineral is defined by a singular, specific chemical formula. For example, the table salt we all consume on a daily basis is defined by the chemical formula for sodium chloride, NaCl. An inorganic compound refers to a compound that is composed of elements other than carbon. Crystalline materials are structures that are made up of crystals, and the crystalline character is the most defining trait of minerals. Crystals are essentially solids that are made up of a repeating, consistent arrangement of atoms [2]. In chemical terms, this regular structure is called a crystal lattice. This regular, solid arrangement atoms form is part of the reason why natural mineral crystals appear in defined geometric shapes. For example, salt is in fact a crystal, since at the atomic level, sodium chloride is made up of repeating units of sodium ions and chloride ions neighboring one another.
The environment in which a mineral crystal grows also has a significant influence on the development of a crystal’s shape. Mineral crystals tend to develop more defined geometric structures under adequate heat and pressure. Metamorphic environments can provide such conditions and allow mineral crystals to form and stabilize inside rocks that are buried under the Earth’s crust [3]. Minerals also readily form in igneous surroundings, where minerals cool and solidify after being displaced from eruptions of molten rock or lava that occur due to buildup of internal heat and pressure [3]. Large environments, such as open cavities, with large amounts of available space also allow crystals to form full geometric surfaces [4]. Ample space is necessary for crystals to continuously add atoms to their structure within an environment. Additionally, aqueous environments, such as rivers, can also be receptive towards crystal formation, since dissolved mineral ions can precipitate out of these environments as solid crystals [5].
From different chemical and environmental influences, a variety of minerals crystallize on our planet, from salt to quartz. These minerals come in a wide range of geometric shapes, called the system of the mineral, such as right-angled cubic and tetragonal, triangular trigonal , and asymmetrical triclinic crystals [4]. Minerals are categorized by their composition. For instance, one of the largest mineral categories is silicates, which includes the mineral crystals of feldspar and quartz [2]. All silicates are made of a structure called the silica tetrahedron, a pyramid-like structure made from four oxygen atoms and one silicon atom. Some other mineral categories include halides, which are made of metal atoms bound to halogens, and carbonates, which are made of carbonate ions (CO32-) being bound to other elements, like calcium or iron [2]. Now, I have one last request. The next time you go outdoors and discover a mineral crystal, please remember to appreciate our planet’s effort in shaping its aesthetic.
Carson Seo ’30 is in the College of Agriculture and Life Sciences. He can be reached at gs826@cornell.edu.
Sources:
[1] https://www.thestonecircle.co.uk/post/where-to-find-crystals-and-minerals-in-your-home-town?currency=USD
[2] https://courses.lumenlearning.com/suny-earthscience/chapter/minerals-and-mineral-groups/
[3] https://www.amnh.org/exhibitions/permanent/gems-minerals/how-do-minerals-form
[4] https://legacycrystalsandminerals.com/blogs/crystal-and-mineral-knowledge-base/why-crystals-form-different-shapes-a-science-guide?srsltid=AU7gw4WV2PjagOx1FKtdKzA36xFsIUdJXBskyGpUqPgfyegIGkqXJrRF
[5] https://www.garviespointmuseum.com/mineral-crystallization.php
