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Precious stones
Published in Francis P. Gudyanga, Minerals in Africa, 2020
Sapphire is blue gemstone variety of the mineral corundum α-Al2O3. Trace amounts of iron, titanium, chromium, copper, or magnesium are responsible for the gemstone’s colours of blue, yellow, purple, orange, or green, respectively. The sapphire with pink or red tint is due to chromium impurities and it is called ruby which is the first variety of corundum discussed in 6.4. Padparadscha is the third gem-variety of corundum with a pinkish orange colour. Sapphire and rubies often occur together.
An Introduction to Crystal Structures
Published in Elaine A. Moore, Lesley E. Smart, Solid State Chemistry, 2020
Elaine A. Moore, Lesley E. Smart
The mineral corundum (α-Al2O3) is the basis for ruby and sapphire gemstones, with their colour depending on the impurities (see Chapter 8). It is very hard—second only to diamond on the Mohs scale. This structure can be described as an hcp array of oxygen atoms with two thirds of the octahedral holes occupied by aluminium atoms (Figure 1.41). As we have seen before, geometrical constraints dictate that octahedral coordination of the aluminium atoms precludes tetrahedral coordination of the oxygen atoms. However, it is suggested that this structure is adopted in preference to other possible ones because the four aluminium atoms surrounding an oxygen atom approximate most closely to a regular tetrahedron. The structure is also adopted by Ti2O3, V2O3, Cr2O3, α-Fe2O3, α-Ga2O3, and Rh2O3. Crystal structure of rutile, TiO2. (a) Unit cell, (b) parts of two chains of linked [TiO6] octahedra, and (c) projection of structure on base of unit cell. Key: Ti, blue; O, grey.
Introduction
Published in Roshan L. Aggarwal, Anant K. Ramdas, Physical Properties of Diamond and Sapphire, 2019
Roshan L. Aggarwal, Anant K. Ramdas
Both ruby and sapphire are corundum (α-aluminum oxide) crystals. The birthstone months for ruby and sapphire are July and September, respectively. Ruby is red (∼675–741 nm), whereas sapphire is typically blue (∼450–482 nm) but also occurs in yellow (∼538–600 nm) and green (∼482–538 nm) colors. The red (∼675–741 nm) color of ruby is due to chromium. Yellow sapphires are due to iron. Blue and green sapphires are due to iron and titanium pairs. The physical properties of ruby and sapphire are the same.
Synergic effect of some waste pozzolans on the mechanical and shielding properties of geopolymer concretes
Published in Radiation Effects and Defects in Solids, 2023
Barış Bayrak, Ali Öz, Esra Kavaz, Gökhan Kaplan, Oğuzhan Çelebi, Haluk Görkem Alcan, Abdulkadir Cüneyt Aydın
Strong peaks of NASH gel were observed in all mixtures in the range of 25–30o. It was determined that the NASH gel had a sharper peak in the M3 mixture. It is followed by M4 and M1 mixes. The NaOH solution used in the mixtures provided the formation of many crystalline phases. One of these crystal structures is the zeolitic phases (Na-P zeolite (Na3.6Al3.6Si12.4O3214H2O)). These structures were also observed in SEM images. CH (portlandite) peaks were observed at 37o in geopolymer mixtures. CH is due to the presence of GBFS used in geopolymer mixtures. The CH peaks also indicate that the pozzolanic activity is not fully completed. Due to insufficient curing time, the reaction of CaO compounds with SiO2 and Al2O3 has not been completed. In particular, a sharper CH peak was obtained in the M4 mixture. Although 150 kg/m3 of MK is used in the M4 mixture, the amount of CH is higher due to the low curing temperature. The strong quartz peaks obtained in the mixtures are due to SF and QP. The reason for the corundum peak obtained at 67o is GBFS and MK. Corundum is a trigonal crystalline aluminum oxide compound.
A novel dense Al2O3-Ti2O3 slag synthesized while ferro-titanium alloy making
Published in Journal of Asian Ceramic Societies, 2022
Qingyao Zheng, Yong Li, Chenhong Ma, Xiaofang Wu, Hezhen Li, Jialin Sun
The paper proposes a new process to produce ferro-titanium alloy, which can produce ferro-titanium alloy and at the same time obtain a slag material with high added value. The material has broad application prospects for the following reasons. (1) It exhibits the corundum-like high-temperature properties and, therefore, can be used as a refractory material. (2) Dispersing Ti2O3 among corundum grains can increase their toughness to produce an ideal abrasive material [13]. (3) The presence of titanium compounds can be controlled through process optimization. While the reaction Ti2O3 + O2 + Al2O3 → Al2O3 + Al2TiO5 occurs in an oxidizing atmosphere, the TiC/TiN/TiCN corundum-based composite material is formed in a reducing atmosphere [14]. (4) The Al2O3–Ti2O3 composite is easy to obtain and environmentally friendly; furthermore, it increases the utilization rate of natural resources.
Fabrication, characterization, fouling behavior and performance study of ceramic microfiltration membranes for oily wastewater treatment
Published in Journal of Asian Ceramic Societies, 2019
Yaser Rasouli, Mohsen Abbasi, Seyed Abdollatif Hashemifard
The mechanical strengths of mullite, MA 50%, MAZ 20% and MZ 20% are 18.3, 24.6, 22.2 and 19.8 MPa, respectively. As mentioned in section 4.1.2 above concerning XRD analysis, α-alumina powder in the MA membrane structure is transformed into the corundum crystalline phase at a temperature of 1300°C. It is well known that the main characteristics of the corundum crystalline phase are hardness, high mechanical strength and anti-abrasion properties. The mechanical strength of MA membranes was, therefore, higher than that of other membranes due to greater formation of the corundum phase. It can be expected that the mechanical strength of MAZ membranes will be lower than that of MA membranes because of the lower corundum phase formation due to lower content of the α-alumina powder. The mechanical strength of MZ membranes obtained was higher than that of the mullite membranes because of their natural zeolite content and the resultant formation of more crystalline phases in their structure.