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Lapis Lazuli History
Lapis lazuli has been popular through most of recorded human history. Mining for lapis occurred in the Badakhshan Province of northeastern Afghanistan as early as 7000 BC. The lapis was used to make beads, small jewelry items and small sculptures. These have been found at Neolithic archaeological sites dating back to about 3000 BC in…
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Composition and Properties of Lapis
In addition to lazurite, specimens of lapis lazuli usually contain calcite and pyrite. Sodalite, hauyne, wollastonite, afghanite, mica, dolomite, diopside, and a diversity of other minerals might also be present. To be called “lapis lazuli,” a rock must have a distinctly blue color and contain at least 25% blue lazurite. Calcite is often the second most abundant mineral present in lapis…
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Geologic Occurrence of Lapis Lazuli
Lapis lazuli forms near igneous intrusions where limestone or marble has been altered by contact metamorphism or hydrothermal metamorphism. In these rocks, lazurite replaces portions of the host rock and often preferentially develops within certain bands or layers. Afghanistan is the world’s leading source of lapis lazuli. Some parts of the country have been actively mined for thousands of years.…
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What is Lapis Lazuli?
Lapis lazuli, also known simply as “lapis,” is a blue metamorphic rock that has been used by people as a gemstone, sculpting material, pigment, and ornamental material for thousands of years. High quality lapis lazuli can be a costly gem. The most desirable specimens have a rich, solid blue color and perhaps a few reflective pieces of gold pyrite.…
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Characteristics of Hornfels
Hornfels often retains the stratification, large-scale geometry, and also some textural characteristics of the protolith. The changes of contact metamorphism that convert rocks to hornfels can include recrystallization, cementation, silicification, partial melting, and more. The result is often a dense, hard, fine-grained rock that is generally homogenous and exhibits a semi-conchoidal fracture. Hornfels can be…
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Parent Rocks and Protoliths
Hornfels is not a rock that is “deposited”. Instead it is a rock type that forms when an existing rock is metamorphosed. The original rock that was metamorphosed is usually referred to as the “parent rock” or “protolith”. A variety of sedimentary, igneous, and metamorphic rocks can be the protolith of hornfels. Common protoliths of hornfels include…
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What is Hornfels?
Hornfels is a fine-grained metamorphic rock that was subjected to the heat of contact metamorphism at a shallow depth. It was “baked” by heat conducted from a nearby magma chamber, sill, dike, or lava flow. Common temperatures for the formation of hornfels range from about 1300 to 1450 degrees Fahrenheit (700 to 800 degrees Celsius). Because directed pressure does not play a…
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Gabbro as an Ore
Gabbro sometimes contains economic amounts of some relatively rare metals. Gabbros containing significant amounts of the mineral ilmenite are mined for their titanium content. Other gabbros are mined to yield nickel, chromium, or platinum.
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Uses of Gabbro
Gabbro can be polished to a brilliant black luster. Brightly polished gabbro is used to make cemetery markers, kitchen counter tops, floor tiles, facing stone, and other dimension stone products. It is a highly desirable rock that stands up to weathering and wear. In the dimension stone industry, gabbro is sold under the name “black…
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Gabbro in Continental Crust
On the continents, gabbro can be found within thick lava flows of basaltic composition, where slow cooling allows large crystals to form. Gabbro will also be present in the deep plutons that form when magma chambers that feed basaltic eruptions crystallize. Large volumes of gabbro are present beneath extensive flood basalts such as the Columbia…