Montmorillonite

Montmorillonite is a 2:1 layered aluminosilicate of the smectite group with the formula (Na,Ca)₀.₃₃(Al,Mg)₂(Si₄O₁₀)(OH)₂·nH₂O.蒙脱石是蒙皂石族的 2:1 型层状铝硅酸盐，化学式以钠钙、铝镁、硅氧层与水合水表示，层间水量可变。

It is monoclinic with perfect basal cleavage and a Mohs hardness of only 1 to 2, and it is the main constituent of bentonite.它属单斜晶系，底面解理完全，莫氏硬度仅 1 至 2，是膨润土的主要成分。

The mineral is the commonest alteration product of volcanic ash, and its ability to take up water between the layers makes it swell and gives it a wide range of industrial uses.该矿物是火山灰最常见的蚀变产物，其层间吸水能力使之膨胀，并因此获得广泛的工业用途。

Discovery and naming

• Montmorillonite was first recognised in 1847 at Montmorillon in the Vienne department of western France, after which it is named.蒙脱石于 1847 年首次在法国西部维埃纳省的蒙莫里永被识别，名称即取自该地。
• Because the species was described before 1959, it is classed as grandfathered by the International Mineralogical Association and needed no re-approval.该矿物种在 1959 年之前已被描述，故被国际矿物学协会列为沿用原名，无须重新批准。
• The 2021 compilation of approved mineral symbols assigned the abbreviation Mnt.2021 年的批准矿物符号汇编为其分配缩写 Mnt。^[1]
• The species gives its name to the montmorillonite group of clays, and it is the type member of the dioctahedral smectites.该矿物种为蒙脱石类黏土提供名称，并且是二八面体蒙皂石中的模式成员。

Occurrence and paragenesis

• Montmorillonite forms by the alteration of volcanic tuff and ash, and the resulting beds of bentonite are its most important geological occurrence.蒙脱石由火山凝灰岩与火山灰蚀变形成，由此产生的膨润土层是其最重要的地质产出。
• It also forms in pegmatite dykes and in the wall rocks of hydrothermal ore deposits, where it replaces feldspar and volcanic glass.它也形成于伟晶岩脉与热液矿床的围岩中，交代长石与火山玻璃。
• The mineral develops under alkaline conditions with poor drainage, where magnesium, calcium, sodium and potassium remain in the soil instead of being leached away.该矿物在碱性且排水不良的条件下发育，此时镁、钙、钠与钾留在土壤中而不被淋失。
• In caves it can accumulate from the weathering of aluminosilicates in the bedrock, and high bicarbonate concentrations and long times favour its formation.在洞穴中，它可由基岩中铝硅酸盐的风化而富集，较高的碳酸氢根浓度与漫长的时间有利于其形成。
• Its usual associates are cristobalite, zeolites, biotite, quartz, orthoclase, dolomite, amphiboles, pyroxenes, olivine, calcite, gypsum, pyrite and limonite.常见共生矿物有方石英、沸石类、黑云母、石英、正长石、白云石、角闪石、辉石、橄榄石、方解石、石膏、黄铁矿与褐铁矿。
• It is one of the commonest clay minerals and has been recorded from more than two thousand localities worldwide, so only the reference occurrences matter.它是最常见的黏土矿物之一，全球已记录两千余处产地，故只有参考性产地具有意义。
• American material regarded as standard comes from Bayard and Santa Rita in New Mexico, Chambers in Arizona, Belle Fourche in South Dakota and the Upton and Clay Spur districts of Wyoming.被视为标准的美国材料来自新墨西哥州贝亚德与圣丽塔、亚利桑那州钱伯斯、南达科他州贝尔富什，以及怀俄明州阿普顿与克莱斯珀地区。

Crystal structure

• The structure is of the 2:1 type: two tetrahedral silica sheets sandwich one octahedral alumina sheet, and the sandwich is one layer of the mineral.结构属 2:1 型：两层硅氧四面体片夹一层铝氧八面体片，这一三明治即为矿物的一个层。
• Montmorillonite is monoclinic, space group C2/m, with a = 5.17, b = 8.94 and c = 9.95 Å, β = 99.54° and one formula unit per cell.蒙脱石属单斜晶系，空间群 C2/m，晶胞参数 a = 5.17、b = 8.94、c = 9.95 埃，β = 99.54 度，每胞一个化学式单位。
• The octahedral sheet is dioctahedral: aluminium fills two of the three octahedral sites, and magnesium replaces part of that aluminium.八面体片为二八面体型：铝占据三个八面体位中的两个，镁又替代其中部分铝。
• Each substitution of magnesium for aluminium leaves one excess negative charge on the nearby oxygens, and that charge is what attracts the interlayer cations.每一次镁替代铝都在邻近氧上留下一个多余负电荷，正是这一电荷吸引层间阳离子。
• The interlayer cations are sodium, calcium and potassium, and they are exchangeable, which is the origin of the cation exchange capacity of the clay.层间阳离子为钠、钙与钾，且可交换，这正是该黏土阳离子交换容量的来源。
• Water also enters the interlayer space, and the basal spacing therefore varies with humidity and with the cation present rather than being fixed.水同样进入层间空间，故底面间距随湿度与所存阳离子而变，并非定值。
• Individual particles are plates about one micrometre across and 0.96 nanometres thick, and resolving one requires a magnification of some 25,000 times.单个颗粒是直径约一微米、厚 0.96 纳米的薄片，分辨单颗粒需约两万五千倍放大。
• Where the layer charge comes mainly from the octahedral sheet the mineral is montmorillonite, and where it comes mainly from the tetrahedral sheet it is beidellite.层电荷主要来自八面体片时即为蒙脱石，主要来自四面体片时则为贝得石。
• Interstratification with chlorite, muscovite, illite, cookeite and kaolinite is common and is one reason why clay identification needs several methods.与绿泥石、白云母、伊利石、锂绿泥石及高岭石互层十分常见，这也是黏土鉴定需要多种方法的原因之一。

Physical properties

• Hardness is 1 to 2 on the Mohs scale and the measured density lies between 2 and 3 g/cm³, both values depending on the water content.莫氏硬度 1 至 2，实测密度在 2 至 3 g/cm³ 之间，两者都随含水量变化。
• Cleavage is perfect on {001}, the plane between the layers, and fracture is uneven.解理在层间的 {001} 面上完全，断口不平。
• The colour ranges from white through pale pink and blue to yellow, red and green, and the tint depends on the exchangeable cation and on iron impurities.颜色由白色经淡粉与蓝色到黄、红、绿，色调取决于可交换阳离子与铁杂质。
• The lustre is dull and earthy and the mineral is translucent, never transparent in the ordinary massive material.光泽暗淡土状，矿物半透明；常见的块状材料从不透明到半透明之间。
• The powder pattern of sodium-saturated material treated with glycerol gives a strong basal reflection at 17.6 Å, which is the standard test for the species.经甘油处理的钠饱和材料粉晶图样在 17.6 埃给出强底面反射，这是该矿物种的标准检验。
• Chemical analysis of Montmorillon material gives 51.14 percent silica, 19.76 percent alumina and 3.22 percent magnesia, with 14.81 percent of water lost below 110 °C.蒙莫里永材料的化学分析给出二氧化硅 51.14 百分比、氧化铝 19.76 百分比、氧化镁 3.22 百分比，110 摄氏度以下失去的水为 14.81 百分比。

Optical properties

• Montmorillonite is biaxial negative with a small 2V of 10° to 25°, which is typical of the fine-grained smectites.蒙脱石为二轴晶负光性，2V 很小，仅 10 至 25 度，这是细粒蒙皂石的典型特征。
• The refractive indices are α = 1.492 to 1.503 and β and γ = 1.513 to 1.534, and the birefringence is about 0.020 to 0.030.折射率为 α 1.492 至 1.503、β 与 γ 1.513 至 1.534，双折射率约 0.020 至 0.030。
• Pleochroism is distinct for a clay: X is colourless to pale brown or yellow-green, Y dark brown to olive-green and Z brown to olive-green.对黏土而言多色性明显：X 为无色至淡褐或黄绿，Y 为深褐至橄榄绿，Z 为褐至橄榄绿。

Formation and alteration

• The bentonite beds that supply most commercial montmorillonite formed when volcanic ash fell into alkaline lakes and altered in place.提供大部分商品蒙脱石的膨润土层，是火山灰落入碱性湖泊后就地蚀变而成。
• In caves the mineral transforms to palygorskite under dry conditions and to halloysite-10Å where the water is acidic, below about pH 5.在洞穴中，该矿物在干燥条件下转变为坡缕石，在水呈酸性（pH 约低于 5）时转变为多水高岭石-10 埃。
• The cation exchange capacity of the clay is due to the magnesium-for-aluminium substitution, and acid treatment increases the accessible surface and the capacity.该黏土的阳离子交换容量源于镁替代铝，酸处理则提高可及表面与容量。^[2]
• With burial and heating the interlayer water is lost, the layers collapse and the mineral converts towards illite, which is why deeply buried bentonites are no longer swelling clays.随埋深与升温，层间水失去、层片塌陷，矿物向伊利石转化，故深埋的膨润土不再具膨胀性。

Uses

• Montmorillonite is a component of drilling mud, where it makes the slurry viscous enough to cool the bit and carry cuttings to the surface.蒙脱石是钻井泥浆的组分，使浆液具有足够黏度以冷却钻头并把岩屑带至地面。
• It is used as a soil additive to hold water in drought-prone soils, in the construction of earthen dams and levees, and in foundry sand.它用作土壤添加剂以在易旱土壤中保水，也用于土质堤坝与堤防的建造以及铸造用砂。
• Montmorillonite clays have served as cracking catalysts for more than sixty years, and acid-treated varieties are used as acid catalysts.蒙脱石类黏土作为裂化催化剂已有六十余年，经酸处理的品种则用作酸催化剂。
• Sodium montmorillonite swells to several times its original volume, and bentonite is therefore used as a seal for water wells and as a liner for landfills.钠蒙脱石可膨胀至原体积的数倍，膨润土因此被用作水井封孔材料与垃圾填埋场衬层。
• Minor uses include an anticaking agent in animal feed, a retention and drainage aid in papermaking, a base for clumping cat litter and a component of cosmetics.次要用途包括动物饲料抗结块剂、造纸助留助滤剂、结团猫砂的基料以及化妆品组分。
• In medicine the purified clay is taken orally as diosmectite for acute diarrhoea and applied externally for contact dermatitis.在医药上，精制黏土以蒙脱石散形式口服用于急性腹泻，外用则用于接触性皮炎。
• The clay also removes arsenic from wastewater, and its adsorptive capacity for heavy metals is the basis of most environmental applications.该黏土还能除去废水中的砷，其对重金属的吸附能力是多数环境应用的基础。

Related minerals and varieties

• Beidellite is the aluminium smectite in which the layer charge sits mainly in the tetrahedral sheet, and it forms a series with montmorillonite.贝得石是层电荷主要位于四面体片的铝蒙皂石，与蒙脱石构成一个系列。
• Nontronite is the iron analogue, saponite the magnesium trioctahedral analogue and hectorite the lithium-bearing trioctahedral analogue.绿脱石是铁的类似物，皂石是镁的三八面体类似物，锂蒙脱石则是含锂的三八面体类似物。
• Illite is the non-swelling mica-like clay to which montmorillonite converts on burial, and the two are the end members of a diagenetic sequence.伊利石是蒙脱石埋藏后转变而成的非膨胀云母状黏土，两者构成成岩序列的两端。
• Calcium montmorillonite, ferrimontmorillonite and askanite are recognised varieties, distinguished by the dominant interlayer cation or by iron content.钙蒙脱石、铁蒙脱石与阿斯卡石是已被承认的变种，以层间优势阳离子或含铁量相区别。
• In the Strunz-mindat scheme the species stands at 9.EC.40 among the phyllosilicates with mica-like layers, with Dana number 71.3.1a.2 and Hey's reference 16.7.9.在 Strunz-mindat 方案中，该矿物种位于具云母状层片的层状硅酸盐 9.EC.40，丹那编号 71.3.1a.2，海氏参考号 16.7.9。