"Type IV" tapes using pure metal particles (as opposed to oxide formulations) were introduced in 1979 by 3M under the trade name Metafine.
The metal-formulated audio cassette hit the scene as the answer to a number of problems with previous tape formulations. The standard Ferric-oxide (Type I) tape had typically rendered poor high frequency definition. Two subsequent solutions had been developed: the first
(Type Il) being a chrome (CrO2) Formulation, which rendered much better frequency reproduction and very low noise (hiss) at the expense of some output level and low-frequency solidity. The second solution, ferro-chrome (Type III), was an attempt to restore some low-frequency firmness to the bright and airy sound of chrome, by adding a second coating layer (ferric) to the existing chrome formulation. Fundamentally, it was a technically imperfect innovation, and it wasn't significant enough an improvement over standard chrome. 
Enter the Type IV tape, with a completely new formulation, of metal particles. The advantages were quickly recognized. This was a very hard-wearing tape, which could take much higher sound levels than a chrome, whilst reproducing equal or better high frequency definition, and a firmer bass — but, without losing the spacious midrange in the way Ferro-chromes could.
The disadvantages were quickly recognized too; excess wear on the tape heads, and excess cost in the retail price being the most significant. In order to get 20% better sound, you have to spend 80% more.
The metal audio tape was, however, considered worth persevering with. Efforts were made to solve the problem of wear, and in the course of the early 1980s the Type IV cassette did start to catch on.
By the mid 1980s, metal tapes had been adopted by a lot of enthusiasts, but they remained too expensive to be bought in bulk by the average consumer.
Metal cassettes (IEC Type IV) also use 70 µs equalization.