Rhombic system

Iodo-pentammino-iridium Iodide, [Ir(NH3)5I]I2, crystallises in short six-sided prisms and oetahedra belonging to the rhombic system. It is obtained by heating aquo-pentammino-iridium iodide to 100° C. 

J. C. G. de Marignac showed that the crystals are trimorphous, for they exist in three different forms (i) Thick plates belonging to the rhombic system—Pig. 16 (ii) twinned pyramids belong to the monoclinic system—Fig. 17 and (iii) thick plates also belonging to the monoclinic system—Fig. 18. Aq. soln. redden litmus,... 

The properties of ammonium hydrocarbonate.—Ammonium hydrocarbonate occurs in the form of a powder, in transparent or opalescent crystals, and in translucent crystalline masses. The crystals have been measured by H. Rose, W. H. Miller, H. St. C. Deville, C. F. Rammelsberg, F. Riidorff, etc. they belong to the rhombic system, and have the axial ratios a b c— 0 6726 1 0 3998. H. Rose claimed to have once made monoclinic crystals of ammonium hydrocarbonate, isomorphous with the potassium salt, by evaporating a soln. of the normal salt in vacuo, but this observation has not been confirmed. In his 1852 paper, H. St. C. Deville stated that he believed that he had prepared crystals of the hydrocarbonate belonging to a different system to the ordinary salt, but in his 1854 paper he attributed little weight to this observation, and stated only one hydrocarbonate exists, but since the crystalline form of this salt is not isomorphous with the corresponding potassium hydrocarbonate, he stated that both these salts will prove one day to be dimorphous, implying that the unknown form of the one carbonate will prove to be isomorphous... 

After the last portion of ethylene urea has been added, the solution is poured into ice water. The nitroethylene urea thus precipitated is filtered, carefully washed and thrown into boiling water. On hydrolysis carbon dioxide is evolved. Boiling is continued until all the gases have been removed, and then the solution is cooled down. Ethylenedinitramine crystallizes in the rhombic system as white, lustrous crystals, s.g. 1.75 which after filtration are washed with cold water and dried at 50°C. 

Rutile and anatase crystallize in the tetragonal system, brookite in the rhombic system. The melting point of TiOz is ca. 1800 °C. Above 1000 °C, the oxygen partial... 

The natural niobites and tantalites axe usually black, and form iso-morphous, prismatic crystals, belonging to the rhombic system. They are easily fusible and very brittle, presenting an uneven fracture. Their density increases from 5-2 to 8 2 with increase in tantalum content.6 When heated to redness in vacuo they evolve small quantities of gas, which consists of carbon dioxide, nitrogen and oxygen.7 Small quantities of helium have also been found occluded in them. 

The pentahydrate, FeAs03.5H,0, found with the preceding salt, crystallises in the rhombic system. 

---