Direct answer: Mineral tar is an older or loose term for dark tar-like fossil materials, especially bitumen, asphaltum, and petroleum-derived substances that resemble tar.
The phrase appears because earlier writers often grouped black sticky materials by appearance before modern petroleum chemistry created sharper categories. Natural bitumen, asphaltum, petroleum residue, coal tar, and wood tar can all look related to the naked eye, even when their origins differ.
Mineral tar is usually a terminology problem rather than a modern product category. It may appear in older books about waterproofing, roads, roofs, ancient materials, or natural asphalt deposits. The useful question is what the author meant in that exact period and industry.
Mineral tar should not be treated as a precise synonym for coal tar or pine tar. It points toward fossil hydrocarbon materials. In modern writing, bitumen and asphalt are usually clearer terms.
In short: An older, ambiguous phrase. Usually points to fossil hydrocarbon materials. May overlap with bitumen, asphaltum, or petroleum residue. Modern articles should define the exact material whenever possible.