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🔗 Physics 🔗 Chemistry 🔗 Geology 🔗 Rocks and minerals

A quasiperiodic crystal, or quasicrystal, is a structure that is ordered but not periodic. A quasicrystalline pattern can continuously fill all available space, but it lacks translational symmetry. While crystals, according to the classical crystallographic restriction theorem, can possess only two-, three-, four-, and six-fold rotational symmetries, the Bragg diffraction pattern of quasicrystals shows sharp peaks with other symmetry orders—for instance, five-fold.

Aperiodic tilings were discovered by mathematicians in the early 1960s, and, some twenty years later, they were found to apply to the study of natural quasicrystals. The discovery of these aperiodic forms in nature has produced a paradigm shift in the fields of crystallography. Quasicrystals had been investigated and observed earlier, but, until the 1980s, they were disregarded in favor of the prevailing views about the atomic structure of matter. In 2009, after a dedicated search, a mineralogical finding, icosahedrite, offered evidence for the existence of natural quasicrystals.

Roughly, an ordering is non-periodic if it lacks translational symmetry, which means that a shifted copy will never match exactly with its original. The more precise mathematical definition is that there is never translational symmetry in more than n – 1 linearly independent directions, where n is the dimension of the space filled, e.g., the three-dimensional tiling displayed in a quasicrystal may have translational symmetry in two directions. Symmetrical diffraction patterns result from the existence of an indefinitely large number of elements with a regular spacing, a property loosely described as long-range order. Experimentally, the aperiodicity is revealed in the unusual symmetry of the diffraction pattern, that is, symmetry of orders other than two, three, four, or six. In 1982 materials scientist Dan Shechtman observed that certain aluminium-manganese alloys produced the unusual diffractograms which today are seen as revelatory of quasicrystal structures. Due to fear of the scientific community's reaction, it took him two years to publish the results for which he was awarded the Nobel Prize in Chemistry in 2011. On 25 October 2018, Luca Bindi and Paul Steinhardt were awarded the Aspen Institute 2018 Prize for collaboration and scientific research between Italy and the United States.

🔗 Geology

Geothermal gradient is the rate of increasing temperature with respect to increasing depth in Earth's interior. Away from tectonic plate boundaries, it is about 25–30 °C/km (72–87 °F/mi) of depth near the surface in most of the world. Strictly speaking, geo-thermal necessarily refers to Earth but the concept may be applied to other planets.

Earth's internal heat comes from a combination of residual heat from planetary accretion, heat produced through radioactive decay, latent heat from core crystallization, and possibly heat from other sources. The major heat-producing isotopes in Earth are potassium-40, uranium-238, uranium-235, and thorium-232. At the center of the planet, the temperature may be up to 7,000 K and the pressure could reach 360 GPa (3.6 million atm). Because much of the heat is provided by radioactive decay, scientists believe that early in Earth history, before isotopes with short half-lives had been depleted, Earth's heat production would have been much higher. Heat production was twice that of present-day at approximately 3 billion years ago, resulting in larger temperature gradients within the Earth, larger rates of mantle convection and plate tectonics, allowing the production of igneous rocks such as komatiites that are no longer formed.

🔗 Disaster management 🔗 Geology 🔗 Mining 🔗 Lakes

Lake Peigneur (locally pronounced [pæ̃j̃æ̹ɾ]) is a brackish lake in the U.S. state of Louisiana, 1.2 miles (1.9 kilometers) north of Delcambre and 9.1 mi (14.6 km) west of New Iberia, near the northernmost tip of Vermilion Bay. With a maximum depth of 200 feet (60 meters), it is the deepest lake in Louisiana.

It was a 10-foot-deep (3 m) freshwater body, popular with sportsmen, until an unusual man-made disaster on November 20, 1980 changed its structure and the surrounding land.

🔗 Astronomy 🔗 Geology 🔗 Geology/Meteorites 🔗 Solar System

The Perseids are a prolific meteor shower associated with the comet Swift–Tuttle. The meteors are called the Perseids because the point from which they appear to hail (called the radiant) lies in the constellation Perseus.

🔗 California 🔗 Skepticism 🔗 Geography 🔗 Alternative Views 🔗 Geology

Sailing stones (also known as sliding rocks, walking rocks, rolling stones, and moving rocks), are a geological phenomenon where rocks move and inscribe long tracks along a smooth valley floor without human or animal intervention. The movement of the rocks occurs when large ice sheets a few millimeters thick and floating in an ephemeral winter pond start to break up during sunny days. Frozen during cold winter nights, these thin floating ice panels are driven by wind and shove rocks at speeds up to 5 meters per minute.

Trails of sliding rocks have been observed and studied in various locations, including Little Bonnie Claire Playa in Nevada, and most famously at Racetrack Playa, Death Valley National Park, California, where the number and length of tracks are notable.

🔗 Astronomy 🔗 Geology

The Eltanin impact is thought to be an asteroid impact in the eastern part of the South Pacific Ocean during the Pliocene-Pleistocene boundary around 2.51 ± 0.07  million years ago. The location was at the edge of the Bellingshausen Sea 1,500 km (950 mi) southwest of Chile. The asteroid was estimated to be about one to four km (0.6 to 2.5 mi) in diameter and the impact would have left a crater approximately 35 km (22 mi) across.

🔗 Geology 🔗 Rocks and minerals

Cummingtonite ( KUM-ing-tə-nyte) is a metamorphic amphibole with the chemical composition (Mg,Fe²⁺
)
₂(Mg,Fe²⁺
)
₅Si
₈O
₂₂(OH)
₂, magnesium iron silicate hydroxide.

Monoclinic cummingtonite is compositionally similar and polymorphic with orthorhombic anthophyllite, which is a much more common form of magnesium-rich amphibole, the latter being metastable.

Cummingtonite shares few compositional similarities with alkali amphiboles such as arfvedsonite, glaucophane-riebeckite. There is little solubility between these minerals due to different crystal habit and inability of substitution between alkali elements and ferro-magnesian elements within the amphibole structure.

🔗 Physics 🔗 Meteorology 🔗 Chemistry 🔗 Geology 🔗 Limnology and Oceanography 🔗 Materials

Ice is water frozen into a solid state. Depending on the presence of impurities such as particles of soil or bubbles of air, it can appear transparent or a more or less opaque bluish-white color.

In the Solar System, ice is abundant and occurs naturally from as close to the Sun as Mercury to as far away as the Oort cloud objects. Beyond the Solar System, it occurs as interstellar ice. It is abundant on Earth's surface – particularly in the polar regions and above the snow line – and, as a common form of precipitation and deposition, plays a key role in Earth's water cycle and climate. It falls as snowflakes and hail or occurs as frost, icicles or ice spikes.

Ice molecules can exhibit eighteen or more different phases (packing geometries) that depend on temperature and pressure. When water is cooled rapidly (quenching), up to three different types of amorphous ice can form depending on the history of its pressure and temperature. When cooled slowly correlated proton tunneling occurs below −253.15 °C (20 K, −423.67 °F) giving rise to macroscopic quantum phenomena. Virtually all the ice on Earth's surface and in its atmosphere is of a hexagonal crystalline structure denoted as ice I_h (spoken as "ice one h") with minute traces of cubic ice denoted as ice I_c. The most common phase transition to ice I_h occurs when liquid water is cooled below 0 °C (273.15 K, 32 °F) at standard atmospheric pressure. It may also be deposited directly by water vapor, as happens in the formation of frost. The transition from ice to water is melting and from ice directly to water vapor is sublimation.

Ice is used in a variety of ways, including cooling, winter sports and ice sculpture.

🔗 Environment 🔗 Meteorology 🔗 Astronomy 🔗 Middle Ages 🔗 Middle Ages/History 🔗 Geology 🔗 Glaciers 🔗 Solar System

The 774–775 carbon-14 spike is an observed increase of 1.2% in the concentration of carbon-14 isotope in tree rings dated to 774 or 775, which is about 20 times as high as the normal background rate of variation. It was discovered during a study of Japanese cedar trees, with the year of occurrence determined through dendrochronology. A surge in beryllium isotope ¹⁰
Be, detected in Antarctic ice cores, has also been associated with the 774–775 event. It is known as the Miyake event or the Charlemagne event and it produced the largest and most rapid rise in carbon-14 ever recorded.

The event appears to have been global, with the same carbon-14 signal found in tree rings from Germany, Russia, the United States, Finland and New Zealand.

The signal exhibits a sharp increase of around 1.2% followed by a slow decline (see Figure 1), which is typical for an instant production of carbon-14 in the atmosphere, indicating that the event was short in duration. The globally averaged production of carbon-14 for this event is calculated as Q = 1.3×10⁸ ± 0.2×10⁸ atoms/cm².

🔗 Biology 🔗 Palaeontology 🔗 Geology

The Boring Billion, otherwise known as the Mid Proterozoic and Earth's Middle Ages, is the time period between 1.8 and 0.8 billion years ago (Ga) spanning the middle Proterozoic eon, characterized by more or less tectonic stability, climatic stasis, and slow biological evolution. It is bordered by two different oxygenation and glacial events, but the Boring Billion itself had very low oxygen levels and no evidence of glaciation.

The oceans may have been oxygen- and nutrient-poor and sulfidic (euxinia), populated by mainly anoxygenic purple bacteria, a type of chlorophyll-based photosynthetic bacteria which uses hydrogen sulfide (H₂S) instead of water and produces sulfur instead of oxygen. This is known as a Canfield ocean. Such composition may have caused the oceans to be black- and milky-turquoise instead of blue. (By contrast, during the much earlier Purple Earth phase the photosynthesis was retinal-based.)

Despite such adverse conditions, eukaryotes may have evolved around the beginning of the Boring Billion, and adopted several novel adaptations, such as various organelles, multicellularity, and possibly sexual reproduction, and diversified into plants, animals, and fungi at the end of this time interval. Such advances may have been important precursors to the evolution of large, complex life later in the Ediacaran and Phanerozoic. Nonetheless, prokaryotic cyanobacteria were the dominant lifeforms during this time, and likely supported an energy-poor food-web with a small number of protists at the apex level. The land was likely inhabited by prokaryotic cyanobacteria and eukaryotic proto-lichens, the latter more successful here probably due to the greater availability of nutrients than in offshore ocean waters.