You may omit the comma following a short introductory phrase: On Thursday the committee decided the dispute. In 1954 the Supreme Court desegregated the public schools. But if you omit this comma once, you should do the same throughout the paper for all short introductory phrases.
But, over time the differences seen among the ever growing number of these visitors has led to many more categories than three. At the present there are approximately fifty types of meteorites recognized. They still fall mainly into the three original families; the irons, the stony-irons and the stones.
Iron meteorites, of course, are nealy 100% metal, although many contain the iron sulfide mineral troilite.Pallasites, a rare type of stony-iron meteorite, consist of olivine grains embedded in an iron-nickel metal matrix.inally, some rare meteorites do not contain any appreciable metal and consequently have low concentrations of Ni. Most of the meteorites known as achondrites are poor in metal. In other words, many of the rarest types of meteorites contain little or no metal and have low nickel concentrations, just like Earth rocks.