Chalk
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Chalk is mined for use in industry, such as for quicklime, bricks and builder's putty, and in agriculture, for raising pH in soils with high acidity. It is also used for \"blackboard chalk\" for writing and drawing on various types of surfaces, although these can also be manufactured from other carbonate-based minerals, or gypsum.
Chalk is a fine-textured, earthy type of limestone distinguished by its light color, softness, and high porosity.[1][2] It is composed mostly of tiny fragments of the calcite shells or skeletons of plankton, such as foraminifera or coccolithophores.[1] These fragments mostly take the form of calcite plates ranging from 0.5 to 4 microns in size, though about 10% to 25% of a typical chalk is composed of fragments that are 10 to 100 microns in size. The larger fragments include intact plankton skeletons and skeletal fragments of larger organisms, such as molluscs, echinoderms, or bryozoans.[3][4][5]
Chalk is typically almost pure calcite, .mw-parser-output .template-chem2-su{display:inline-block;font-size:80%;line-height:1;vertical-align:-0.35em}.mw-parser-output .template-chem2-su>span{display:block;text-align:left}.mw-parser-output sub.template-chem2-sub{font-size:80%;vertical-align:-0.35em}.mw-parser-output sup.template-chem2-sup{font-size:80%;vertical-align:0.65em}CaCO3, with just 2% to 4% of other minerals. These are usually quartz and clay minerals, though collophane (cryptocrystalline apatite, a phosphate mineral) is also sometimes present, as nodules or as small pellets interpreted as fecal pellets. In some chalk beds, the calcite has been converted to dolomite, CaMg(CO3)2, and in a few cases the dolomitized chalk has been dedolomitized back to calcite.[3]
Chalk is highly porous, with typical values of porosity ranging from 35 to 47 per cent.[3] While it is similar in appearance to both gypsum and diatomite, chalk is identifiable by its hardness, fossil content, and its reaction to acid (it produces effervescence on contact).[5]
In Western Europe, chalk was formed in the Late Cretaceous Epoch and the early Palaeocene Epoch (between 100 and 61 million years ago).[6][7] It was deposited on extensive continental shelves at depths between 100 and 600 metres (330 and 1,970 ft), during a time of nonseasonal (likely arid) climate that reduced the amount of erosion from nearby exposed rock. The lack of nearby erosion explains the high purity of chalk. The coccolithophores, foraminifera, and other microscopic organisms from which the chalk came mostly form low-magnesium calcite skeletons, so the sediments were already in the form of highly stable low-magnesium calcite when deposited. This is in contrast with most other limestones, which formed from high-magnesium calcite or aragonite that rapidly converted to the more stable low-magnesium calcite after deposition, resulting in the early cementation of such limestones. In chalk, absence of this calcium carbonate conversion process prevented early cementation, which partially accounts for chalk's high porosity.[3] Chalk is also the only form of limestone that commonly shows signs of compaction.[8]
Flint (a type of chert) is very common as bands parallel to the bedding or as nodules in seams, or linings to fractures, embedded in chalk. It is probably derived from sponge spicules[4] or other siliceous organisms as water is expelled upwards during compaction. Flint is often deposited around larger fossils such as Echinoidea which may be silicified (i.e. replaced molecule by molecule by flint).[9]
Chalk is so common in Cretaceous marine beds that the Cretaceous Period was named for these deposits. The name Cretaceous was derived from Latin creta, meaning chalk.[10] Some deposits of chalk were formed after the Cretaceous.[11]
The Chalk Group is a European stratigraphic unit deposited during the late Cretaceous Period. It forms the famous White Cliffs of Dover in Kent, England, as well as their counterparts of the Cap Blanc Nez on the other side of the Dover Strait. The Champagne region of France is mostly underlain by chalk deposits, which contain artificial caves used for wine storage.[3] Some of the highest chalk cliffs in the world occur at Jasmund National Park in Germany and at Møns Klint in Denmark.[12]
Most people first encounter chalk in school where it refers to blackboard chalk, which was originally made of mineral chalk, since it readily crumbles and leaves particles that stick loosely to rough surfaces, allowing it to make writing that can be readily erased. Blackboard chalk manufacturers now may use mineral chalk, other mineral sources of calcium carbonate, or the mineral gypsum (calcium sulfate). While gypsum-based blackboard chalk is the lowest cost to produce, and thus widely used in the developing world, use of carbonate-based chalk produces larger particles and thus less dust, and it is marketed as \"dustless chalk\".[20][5]
Coloured chalks, pastel chalks, and sidewalk chalk (shaped into larger sticks and often coloured), used to draw on sidewalks, streets, and driveways, are primarily made of gypsum rather than calcium carbonate chalk.[21]
Chalk and other forms of limestone may be used for their properties as a base.[23] Chalk is a source of quicklime by thermal decomposition, or slaked lime following quenching of quicklime with water.[24] In agriculture, chalk is used for raising pH in soils with high acidity.[25] Small doses of chalk can also be used as an antacid.[26] Additionally, the small particles of chalk make it a substance ideal for cleaning and polishing. For example, toothpaste commonly contains small amounts of chalk, which serves as a mild abrasive.[27] Polishing chalk is chalk prepared with a carefully controlled grain size, for very fine polishing of metals.[28]
In southeast England, deneholes are a notable example of ancient chalk pits. Such bell pits may also mark the sites of ancient flint mines, where the prime object was to remove flint nodules for stone tool manufacture. The surface remains at Cissbury are one such example, but perhaps the most famous is the extensive complex at Grimes Graves in Norfolk.[31]
Chalk may also be used as a house construction material instead of brick or wattle and daub: quarried chalk was cut into blocks and used as ashlar, or loose chalk was rammed into blocks and laid in mortar.[34][35] There are still houses standing which have been constructed using chalk as the main building material. Most are pre-Victorian though a few are more recent.[36]
Chain styles and call the last one as a method with a string argument. Order doesn't matter, and later styles take precedent in case of a conflict. This simply means that chalk.red.yellow.green is equivalent to chalk.green.
chalkStderr contains a separate instance configured with color support detected for stderr stream instead of stdout. Override rules from supportsColor apply to this too. supportsColorStderr is exposed for convenience.
Confirming the commitment of the Marca Corona Line 1741 for experimentation, this concrete porcelain tile collection combines delicate tones with bold shapes: on one hand it explores all the cold and chalky nuances, on the other it bets on the rhomboidal format (18.7x32.4cm), which includes a large variety of installation schemes, thus allowing installers to create endless spectacular decorative compositions.
Over 20 years ago we set out to make the best possible chalk for rock climbing, and Super Chalk was the result. Since then, billions of pitches have been climbed, problems sent, and projects finished. Sure, we could triple the price, but we'd rather you put your money towards what matters - climbing.
Children and adults enjoy this annual event where community members join professional artists in using the walkways around the Fine Arts Garden as a colorful canvas. Begun in 1990, our festival is a modern expression of a Renaissance tradition from 16th-century Italy in which beggars copied paintings of the Madonna by Raphael and his contemporaries using chalk on the plazas outside cathedrals. Watching the chalk artists and enjoying the entertainment is free.
Celebrate creativity with a night of arts, eats, and beats! Chalk of the Town is a new community art festival that showcases the visual and performing arts. Chalk artists will adorn the sidewalks with their work while attendees can shop from local artisans, enjoy food & beverage, and be entertained by roving musicians. Come on out for a great night that will truly be the \"chalk of the town.\"
Tokyo Powder continues to be the most high friction chalk and best food grade magnesium carbonate we have EVER discovered. We enjoy it so much that we wanted to make more Tokyo Powder products available to you.
Tokyo Powder Pure is a high friction, super fine, PREMIUM chalk. It has the highest purity rating of magnesium carbonate and, when paired with BOOST, delivers a friction so high it almost feels like cheating.
Tyvek Rolldown Chalk Bucket - SilverOur crowd favorite Rolldown Chalk Bucket is better than ever. Outfitted with Tyvek material, this space-saving, collapsible design and top velcro closure/buckle secures your chalk when not in use. Now available in a Silver colorway.
Tyvek Rolldown Chalk Bucket - SeafoamOur crowd favorite Rolldown Chalk Bucket is better than ever. Outfitted with Tyvek material, this space-saving, collapsible design and top velcro closure/buckle secures your chalk...
Tyvek Rolldown Chalk Bucket - PinkOur crowd favorite Rolldown Chalk Bucket is better than ever. Outfitted with Tyvek material, this space-saving, collapsible design and top velcro closure/buckle secures your chalk when not in use. Now available in a Pink colorway.
Step 2. Angle your chalk at a small angle \\(\\theta\\) down from the perpendicular. The correct angle is somewhere between \\(0^\\circ\\) and \\(45^\\circ\\), but you may need to experiment with what works.
Murray City reserves the right to disqualify any artwork that is deemed inappropriate. All artwork must be