Alumohydrocalcite

Alumohydrocalcite is a hydrated calcium aluminium carbonate that forms chalky white to pale blue fibrous crusts and spherulites in the oxidised zones of limestones, dolomites and bauxite deposits.铝水钙石是一种含水钙铝碳酸盐，在石灰岩、白云岩与铝土矿的氧化带中形成白垩白至浅蓝色的纤维状壳层与球晶。

It was first described in 1926 from Khakassia in Siberia and is classed as a grandfathered species, since it entered the literature long before the International Mineralogical Association was founded.它于 1926 年首次描述于西伯利亚哈卡斯，因远早于国际矿物学协会成立而被列为沿用名矿物种。

The mineral is soft, with a Mohs hardness of only 2.5, and it is best known for the chromian varieties that develop a deep violet colour near weathering chromite.该矿物质软，莫氏硬度仅 2.5，最著名的是在蚀变铬铁矿附近发育的深紫色含铬变种。

Discovery and naming

• Alumohydrocalcite was first described in 1926 from material at Potekhina Village near Sorsk in the Khakassia Republic of Siberia, Russia, which remains the type locality.铝水钙石于 1926 年首次描述，标本来自俄罗斯西伯利亚哈卡斯共和国索尔斯克附近的波捷希纳村，该地至今仍是模式产地。
• The name is purely compositional, combining references to aluminium, to water in the hydrated structure, and to calcium carbonate in the calcite-like component.名称纯属成分命名，分别指铝、结构中的水，以及方解石式组分中的碳酸钙。
• Because the description predates the founding of the International Mineralogical Association in 1959, the species is grandfathered rather than formally approved.由于该描述早于 1959 年国际矿物学协会成立，该矿物种列为沿用名而非正式批准。
• Type material is preserved in the A. E. Fersman Mineralogical Museum of the Russian Academy of Sciences in Moscow, under catalogue number 77088.模式标本保存在莫斯科俄罗斯科学院费尔斯曼矿物学博物馆，编号 77088。
• The hydration state was long reported as a trihydrate, and current reference data give four water molecules per formula unit, the older value being retained in much of the descriptive literature.该矿物的水化状态长期报道为三水合物，现行参考数据给出每个化学式单位含四个水分子，而旧值仍保留在大量描述性文献中。

Occurrence and paragenesis

• Alumohydrocalcite forms at low temperature by the action of carbonated waters on allophane or dickite in dolomites and limestones.铝水钙石在低温下由含碳酸的水作用于白云岩与石灰岩中的水铝英石或迪开石而形成。
• Chromian varieties typically develop where serpentinite-derived solutions react with altering chromite, and the chromium content reaches several weight per cent Cr₂O₃.含铬变种通常发育于蛇纹岩来源的溶液与蚀变铬铁矿反应的部位，其氧化铬含量可达数质量分数。
• Associated minerals include allophane, dickite, gibbsite, gypsum, aragonite, barite and iron–manganese oxides.共生矿物包括水铝英石、迪开石、三水铝石、石膏、文石、重晶石以及铁锰氧化物。
• It occurs as fibres and needles up to 2.5 mm long, commonly arranged in radial aggregates and spherulites, and also as felt-like crystal linings and powdery to chalky masses.矿物呈长可达 2.5 mm 的纤维与针状晶体，常组成放射状集合体与球晶，亦呈毡状晶簇衬里及粉末状至白垩状块体。
• Well-crystallised material is known from a dolomite quarry near Bergisch Gladbach in Germany, from the Kalem quarry near Birresborn in the Eifel, and from Ramsbeck in North Rhine-Westphalia.结晶良好的样品见于德国贝吉施格拉德巴赫附近的白云石采石场、艾费尔地区比勒斯博恩附近的卡勒姆采石场，以及北莱茵—威斯特法伦的拉姆斯贝克。
• Chromian material has been found at the Ruben mine at Nowa Ruda in Silesia, Poland, at the Akenobe mine in Hyogo Prefecture, Japan, and east of Mount Hamilton in Santa Clara County, California.含铬样品发现于波兰西里西亚诺瓦鲁达的鲁本矿、日本兵库县明延矿，以及美国加利福尼亚州圣克拉拉县汉密尔顿山以东地区。
• Further localities include Ladomirov in Slovakia, Bicske-Csordakut in Hungary, Scarborough in North Yorkshire and Weston Favell in Northamptonshire in England, Montcada in Barcelona Province, Spain, and Chitral in Pakistan.其他产地包括斯洛伐克拉多米罗夫、匈牙利比奇凯—乔尔达库特、英格兰北约克郡斯卡伯勒与北安普敦郡韦斯顿法维尔、西班牙巴塞罗那省蒙特卡达，以及巴基斯坦吉德拉尔。

Crystal structure

• Alumohydrocalcite is triclinic, with point group 1̄ and space group P1̄ in the chromian material.铝水钙石属三斜晶系，含铬样品点群为 1̄，空间群为 P1̄。
• The cell parameters are a = 6.498(3) Å, b = 14.457(4) Å and c = 5.678(3) Å, with α = 95.83(5)°, β = 93.23(3)° and γ = 82.24(3)°, and Z = 2.晶胞参数为 a = 6.498(3) Å、b = 14.457(4) Å、c = 5.678(3) Å，α = 95.83(5)°、β = 93.23(3)°、γ = 82.24(3)°，Z = 2。
• The structure contains layers of AlO₆ octahedra linked by carbonate groups and calcium polyhedra, with water molecules and hydroxyl groups in the interlayer space.其结构由 AlO₆ 八面体层经碳酸根团与钙多面体联结而成，水分子与羟基位于层间。
• The strong reflections of the X-ray powder pattern occur at 6.25 Å (100), 6.50 Å (70), 3.23 Å (60), 2.039 Å (50), 2.519 Å (40), 7.21 Å (30) and 2.860 Å (30).X 射线粉晶谱的强反射位于 6.25 Å（100）、6.50 Å（70）、3.23 Å（60）、2.039 Å（50）、2.519 Å（40）、7.21 Å（30）与 2.860 Å（30）。
• The species is the aluminium analogue of grguricite and is closely related to para-alumohydrocalcite, which shares the same composition but a different stacking sequence.该矿物种是格鲁里奇特石的铝类似物，并与副铝水钙石关系密切，后者成分相同但堆垛序列不同。
• Chromium enters the octahedral sites in place of aluminium, and a chromian sample corresponds to Ca(Al₁.₅₇Cr₀.₄₃)(CO₃)₂(OH)₄·4H₂O.铬进入八面体位替代铝，一个含铬样品的化学式为 Ca(Al₁.₅₇Cr₀.₄₃)(CO₃)₂(OH)₄·4H₂O。

Physical properties

• Cleavage is perfect on {100} and imperfect on {010}, and the mineral is brittle.解理沿 {100} 完全，沿 {010} 不完全，矿物性脆。
• Hardness is 2.5 on the Mohs scale, and the measured density of 2.21 to 2.24 g/cm³ is close to the calculated value of 2.213 g/cm³.莫氏硬度为 2.5，实测密度 2.21 至 2.24 g/cm³，与计算值 2.213 g/cm³ 接近。
• On boiling in water the mineral decomposes to calcite and hydrous aluminium oxide, which makes it unstable in hot aqueous environments.在沸水中该矿物分解为方解石与含水氧化铝，因此在高温水环境中不稳定。
• Colour ranges from chalky white through pale blue, pale yellow, cream and grey, and chromian material ranges from pale rose through brownish pink to dark violet.颜色由白垩白到浅蓝、浅黄、奶油色与灰色，含铬样品则由浅玫瑰色经褐粉色至深紫色。
• Lustre is vitreous to pearly and earthy, and the mineral ranges from transparent to opaque.光泽为玻璃光泽至珍珠光泽与土状光泽，矿物呈透明至不透明。
• Analysis of material from Bergisch Gladbach gives CO₂ 24.2, Al₂O₃ 31.3 and CaO 17.8 wt% with the balance water, close to the ideal composition.贝吉施格拉德巴赫样品的分析给出 CO₂ 24.2、Al₂O₃ 31.3 与 CaO 17.8 质量分数，其余为水，接近理想组成。

Optical properties

• Alumohydrocalcite is biaxial negative, with α = 1.485–1.502, β = 1.553–1.563 and γ = 1.570–1.585, measured in sodium light.铝水钙石为二轴晶负光性，在钠光下测得 α = 1.485–1.502、β = 1.553–1.563、γ = 1.570–1.585。
• The measured 2V of 64° exceeds the calculated range of 50° to 55°, an unusually large discrepancy that has been attributed to the poorly ordered interlayer water.实测 2V 为 64°，大于计算值 50° 至 55°，这一异常大的差异被归因于层间水有序度低。
• Dispersion is very strong, with r greater than v, and the optical orientation places X along b with the extinction inclined 6° to 10°.色散很强，r 大于 v，光性方位中 X 平行 b，消光角 6° 至 10°。
• In transmitted light the chromian varieties appear pale violet, while the chromium-free material is colourless.在透射光下含铬变种呈浅紫色，而不含铬的样品为无色。
• The low refractive indices, the strong dispersion and the ready decomposition in water distinguish the mineral from other calcium aluminium carbonates such as hydrocalumite.折射率低、色散强以及在水中易分解等特征，可将该矿物与钙水铝石等其他钙铝碳酸盐区分开。

Formation and alteration

• Alumohydrocalcite crystallises in the near-surface oxidation zone, in the narrow pH window where carbonated groundwater still carries dissolved aluminium.铝水钙石在近地表氧化带结晶，所处 pH 窗口狭窄，要求含碳酸地下水仍携带溶解铝。
• Its formation requires the prior breakdown of allophane or dickite, which supplies the aluminium that is otherwise immobile at near-neutral pH.其形成须先有水铝英石或迪开石的分解，以提供在近中性 pH 下本不活动的铝。
• On heating or on contact with boiling water it alters to calcite plus hydrous alumina, so it is not stable under hydrothermal conditions.受热或与沸水接触时它转变为方解石加含水氧化铝，故在热液条件下不稳定。
• Chromian varieties form where serpentinite weathering releases both chromium and aluminium, producing the violet coloration that is diagnostic in the field.含铬变种形成于蛇纹岩风化同时释放铬与铝的部位，产生可作为野外鉴定依据的紫色。

Uses

• Alumohydrocalcite has no industrial use and is not exploited as an ore of aluminium, since it occurs only in small, impure masses.铝水钙石无工业用途，也不作为铝矿石开采，因其仅呈小块状且不纯。
• Its importance is mineralogical: the species is a marker of low-temperature alteration in bauxite and carbonate terrains and helps date the weathering history of a deposit.其意义在于矿物学方面，该矿物种是铝土矿与碳酸盐岩区低温蚀变作用的标志，有助于判断矿床的风化历史。
• The purple chromian material is prized by collectors as a rare colour variety, and it is the form in which the species most often appears in mineral collections.紫色含铬样品因色彩稀见而受收藏者青睐，也是该矿物种在收藏中最常见的形态。
• Recent work treats the mineral as an analogue for aluminium-bearing corrosion products and for the behaviour of aluminium in cement and in carbonate systems.近期研究将该矿物视为含铝腐蚀产物以及水泥和碳酸盐体系中铝行为的类比物。

Related minerals and varieties

• Alumohydrocalcite is the aluminium member of a small group of hydrated calcium aluminium carbonates that also includes grguricite and para-alumohydrocalcite.铝水钙石是含水钙铝碳酸盐小家族的铝成员，同族还包括格鲁里奇特石与副铝水钙石。
• Para-alumohydrocalcite has the same composition but a different structure, and the two can only be separated by X-ray diffraction.副铝水钙石成分相同但结构不同，两者只能借助 X 射线衍射区分。
• The chromian variety, with Cr₂O₃ reaching 8.3 wt% in one analysis, approaches a hypothetical chromium end member and is the most distinctive variety of the species.含铬变种在某个分析中 Cr₂O₃ 达 8.3 质量分数，趋近假想铬端元，是该矿物种最具辨识度的变种。
• Dresserite and dundasite are the barium and lead analogues of the same structural family and occur in comparable low-temperature weathering environments.德雷塞尔石与邓达斯石是同一结构族的钡与铅类似物，产出于相似的低温风化环境。
• Hydrocalumite and gibbsite are the associated aluminium-bearing phases most easily confused with alumohydrocalcite in the oxidised zone.钙水铝石与三水铝石是氧化带中最易与铝水钙石混淆的共生含铝物相。