碳酸钙是一种无机化合物，化学式为CaCO₃，是石灰石、大理石等的主要成分。碳酸钙基本上不溶于水，但溶于盐酸。它是地球上常见物质之一，存在于霰石、方解石、白垩、石灰岩、大理石、石灰华等岩石内，亦为某些动物骨骼或外壳的主要成分。碳酸钙还是重要的建筑材料，工业上用途甚广。方解石是一种碳酸钙矿物，天然碳酸钙中最常见的就是它。

Calcite [碳酸钙] is an ionic salt called calcium carbonate or CaCO₃. It forms under reasonably deep marine conditions from the gradual accumulation of minute calcite shells shed from micro-organisms called coccolithophores [球石藻].

Carbonate is used in the names of some substances that are formed from carbonic acid, which is a compound of carbon dioxide and water.

Chalk [白垩] is a soft, white, porous [能渗透的], sedimentary carbonate rock, a form of limestone composed of the mineral calcite [方解石]. pore: 毛孔。

Chalk has greater resistance to weathering and slumping than the clays with which it is usually associated, thus forming tall, steep cliffs where chalk ridges meet the sea. Chalk hills, known as chalk downland, usually form where bands of chalk reach the surface at an angle, so forming a scarp [陡坡; 悬崖] slope. Because chalk is well jointed it can hold a large volume of ground water, providing a natural reservoir that releases water slowly through dry seasons.\

Chalk is mined from chalk deposits both above ground and underground. Chalk mining boomed during the Industrial Revolution, due to the need for chalk products such as quicklime and bricks. Some abandoned chalk mines remain tourist destinations due to their massive expanse and natural beauty. Quicklime [石灰 =lime] is a white powder that is made by heating limestone.

Most people first encounter chalk in school where it refers to blackboard chalk, which was originally made of mineral chalk, since it readily crumbles and leaves particles that stick loosely to rough surfaces, allowing it to make writing that can be readily erased.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 [石膏] (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".

Several traditional uses of chalk have been replaced by other substances, although the word "chalk" is often still applied to the usual replacements. Tailor's chalk is traditionally a hard chalk used to make temporary markings on cloth, mainly by tailors. It is now usually made of talc [滑石] (magnesium silicate, 硅酸镁).

Chalk was traditionally used in recreation. In field sports, such as tennis played on grass, powdered chalk was used to mark the boundary lines of the playing field or court. If a ball hits the line, a cloud of chalk or pigment dust will be visible. In recent years, powdered chalk has been replaced with titanium dioxide. [二氧化钛，钛白] In gymnastics, rock-climbing, weightlifting and tug of war [拔河], chalk - now usually magnesium carbonate - is applied to the hands and feet to remove perspiration and reduce slipping.

Chalk may be used for its properties as a base [可与酸化合成盐的物质(碱是其中的一种)]. In agriculture, chalk is used for raising pH in soils with high acidity. ph=7为中性。>7为酸性，<7为碱性。

六级/考研单词: calcium, shallow, marine, gradual, accumulate, shed, compound, carbon, dioxide, chalk, limelight, compose, slump, clay, thereby, steep, cliff, ridge, reservoir, deposit, subway, revolve, abandon, destination, powder, encounter, blackboard, crumble, particle, manufacture, seldom, tailor, recreation, gymnasium, tug