The three crystal axes of the Orthorhombic System are perpendicular to each other but have different lengths. The shape is rectangular columnar or plate structure.The columnar crystallization of Topaz is a typical example of this crystal system.[1]
The symmetry feature of this crystal system is that there is no higher symmetry axis, but the total number of secondary symmetry axis and symmetry plane is not less than three.The crystal takes the three mutually perpendicular quadratic axes or the normal of the symmetry plane as the crystal axis.Axis angle α=β=γ=90 °, but axis unit a ≠ b ≠ c.Therefore, orthorhombic system has stronger heterogeneity.The refractive index of parallel c-axis, b-axis and a-axis are different, that is, they have three main refractive indexes.The common crystal forms belonging to this crystal system include rhombic columns, rhombic bipyramids and their polymorphs.Common gemstones include topaz, olivineEmeraldEtc.[2]
Symmetrical characteristics
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No higher orderAxis of symmetry，LtwoThe number of or P is more than one, and there are three types of symmetry. Common crystals are mostly distributed in 3Ltwo3PC symmetrical type.
Crystal orientation
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With mutually perpendicular 3LtwoX, Y, Z axis.Right Ltwo2P symmetric type, then LtwoIs the Z axis, and the normal of 2P is the X and Y axes.The crystal constant is characterized by a ≠ b ≠ c, α=β=γ=90 °.
Common shape number
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There are 7 kinds of monomorphs in the orthorhombic system, and only 3 kinds are the most common. Their orientation and shape number are shown in Figure 1.
Fig. 1 Orientation and shape number of common simplex in orthorhombic system
The most common rhombohedral crystals are shown in Figure 2.[3]
Fig. 2 Common polymorph crystals in orthorhombic system
Chlorite
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Copper chloride ore is also called copper chloride ore.
Chemical composition: CuCItwo·3Cu（OH）two。
Crystal system and crystal form: rhombic crystal system.Cylindrical or tabular crystals.It is usually fibrous, granular, kidney shaped, sandy or compact.
Physical characteristics: green, black green or emerald green.The stripes are apple green.Transparent Half transparent.Glass luster.Conchoidal fracture。Complete cleavage.It is brittle.Mohs hardness3~3.5.The relative density is 3.75~3.77.
Optical characteristics: refractive index Ng=1.880，Nm=1.861，Np=1.831, strong dispersion.
Use: It can be used as raw material for copper smelting and mineral specimen when it is gathered in large quantities.
Occurrence: It is often associated with malachite and other secondary copper minerals, and coexists with calcite and grape stone.
Origin: Chile, the United States, Bolivia, Namibia, Zaire, the former Soviet Union, China (Yunnan, Sichuan and other places).[4]
Stibnite
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Chemical composition: SbtwoSthree。Containing a small amount of AsBi、Pb、Fe、Cu，Sometimes it contains Ag, Au and other mechanical admixture.
Crystal system and crystal form: rhombic crystal system.The crystal is needle shaped and long columnar, with obvious longitudinal stripes on the crystal plane.Crystals often bend.The aggregates are radial, shelf or granular.
Physical characteristics: the color and streak color are lead gray or steel gray.The crystal surface often has dark blue color.Metallic luster。Opaque.The cleavage is parallel (010) and complete, and there are often transverse lamellar double crystal lines on the cleavage surface.It is brittle.Mohs hardness2~2.5.The relative density is 4.5~4.6.
Application: It is an important mineral raw material for antimony smelting.Antimony is mainly used to make wear-resistant alloys, bearings, movable type lead and bullet heads.Antimony compounds are used in enamel, paint, pigment, medicine, matches, explosives and other industries.
Occurrence: It mainly occurs in low-temperature hydrothermal deposits.It is often associated with cinnabar, barite, calcite, etc.
Origin: the former Soviet Union, Japan, China (Hunan and other places).[4]
