Andalusite
Andalusite is an anhydrous aluminum silicate mineral with the chemical formula Al₂SiO₅, belonging to the nesosilicate group and existing as one of three polymorphs alongside kyanite and sillimanite.[1][2] It crystallizes in the orthorhombic system, typically forming prismatic crystals that are vitreous to pearly in luster, with a Mohs hardness ranging from 6.5 to 7.5 and colors including pink, yellow, brown, green, or red, often exhibiting strong pleochroism in gem-quality specimens.[1][2] Andalusite is notable for its structural feature of aluminum in five-fold coordination and is commonly found in low-pressure, low- to high-temperature metamorphic environments, such as schist, gneiss, hornfels, and contact metamorphism zones of pelitic rocks, as well as in some peraluminous granites and placer deposits.[2][3] A distinctive variety, chiastolite, features cross-shaped inclusions of carbonaceous material or iron oxides aligned along the crystal length.[2] Industrially, andalusite is prized for its high alumina content (over 60% Al₂O₃) and excellent thermal and volumetric stability, making it a key raw material in refractory products for steelmaking, glass, cement, and ceramics production, where it withstands temperatures up to 1,800°C without significant expansion or contraction.[4][1]Overview
Definition and Composition
Andalusite is an aluminum nesosilicate mineral with the chemical formula Al₂SiO₅, classified within the nesosilicates group due to its isolated SiO₄ tetrahedra in the structure.[5] This composition reflects its role as a key member of the aluminosilicate minerals, where aluminum and silicon are coordinated in a framework stabilized by oxygen atoms.[5] The ideal chemical composition of andalusite corresponds to 62.92% Al₂O₃ and 37.08% SiO₂ by weight.[5] In natural occurrences, it often includes minor impurities such as iron (up to 0.36% as Fe₂O₃) or manganese, which substitute for aluminum in the lattice without significantly altering the overall formula.[5] Andalusite crystallizes in the orthorhombic system with space group Pnnm.[5] The unit cell parameters are a = 7.7980(7) Å, b = 7.9031(10) Å, c = 5.5566(5) Å, and Z = 4.[5] It forms one of the three polymorphs of the Al₂SiO₅ group, alongside kyanite and sillimanite.[5]Polymorphs
Andalusite, kyanite, and sillimanite are three polymorphs sharing the chemical formula Al_2SiO_5, each exhibiting distinct crystal structures that determine their thermodynamic stability under varying pressure and temperature conditions.[1] Andalusite represents the low-pressure, low-temperature stable form in this system, while kyanite forms under high-pressure conditions and sillimanite under high-temperature conditions.[6] These polymorphs coexist in a pressure-temperature phase diagram where their stability fields converge at a triple point, delineating the boundaries for phase transitions in the Al_2SiO_5 system.[7] The stability field of andalusite is restricted to pressures below approximately 4 kbar and temperatures between 500 and 600 °C, corresponding to relatively shallow crustal depths and moderate thermal regimes typical of contact metamorphism.[8] In contrast, kyanite's field extends to higher pressures exceeding 4 kbar at similar temperatures, reflecting its denser packing suitable for deeper burial, whereas sillimanite dominates above 600 °C across a broader pressure range, favored by increased thermal energy that promotes its less dense framework.[9] Phase transitions between these polymorphs are generally sluggish due to the reconstructive nature of the structural changes, often requiring overstepping of equilibrium boundaries in natural settings.[10] Structurally, andalusite features chains of edge-sharing AlO_6 octahedra aligned parallel to the c-axis, which are cross-linked by double chains of alternating isolated SiO_4 tetrahedra and AlO_5 trigonal bipyramids, resulting in an orthorhombic symmetry with relatively open packing.[11] This arrangement contrasts with kyanite's more compact structure, where chains alternate between SiO_4 tetrahedra and AlO_6 octahedra, and sillimanite's framework, which incorporates chains of AlO_4 tetrahedra alongside SiO_4.[12] These differences in polyhedral coordination and linkage dictate the density variations and stability preferences among the polymorphs. At elevated temperatures around 1200 °C, andalusite undergoes an irreversible reconstructive transformation to mullite (approximately 3Al_2O_3·2SiO_2) and a silica-rich phase, driven by the breakdown of its silicate framework into a more stable aluminosilicate structure suitable for refractory applications.[13] This conversion is kinetically hindered below this threshold but becomes rapid under prolonged heating, highlighting andalusite's role as a precursor in high-temperature ceramic processing.[14]Physical and Optical Properties
Crystal Structure and Habit
Andalusite crystallizes in the orthorhombic crystal system with space group Pnnm and point group 2/m 2/m 2/m.[5][15] Its unit cell parameters are a = 7.7980(7) Å, b = 7.9031(10) Å, c = 5.5566(5) Å, and Z = 4.[5] This atomic arrangement features isolated silica tetrahedra linked by aluminum octahedra and additional aluminum in five-fold coordination, characteristic of the nesosilicate structure.[5] The typical crystal habit of andalusite is prismatic, forming euhedral crystals or columnar aggregates with nearly square cross-sections elongated parallel to the c-axis , reaching lengths up to 20 cm.[5][15] Common morphological forms include {110} prisms and {001} pinacoids, often modified by interactions with surrounding minerals.[15] Crystals may also occur as fibrous, compact, or massive aggregates.[5] Twinning in andalusite is rare and occurs on the {101} plane, sometimes resulting in a lamellar appearance.[5][15] Andalusite crystals frequently contain inclusions such as graphite or other minerals, which can render them nearly opaque.[5] X-ray powder diffraction confirms the orthorhombic structure, with key d-spacings including a strong peak at 5.55 Å (100% intensity), followed by 4.53 Å (80%), 2.77 Å (70%), 3.93 Å (30%), and 3.53 Å (30%).[15]| d-spacing (Å) | Relative Intensity (%) |
|---|---|
| 5.55 | 100 |
| 4.53 | 80 |
| 2.77 | 70 |
| 3.93 | 30 |
| 3.53 | 30 |
| 2.27 | 30 |
| 2.18 | 30 |
| 2.17 | 40 |