Instrument data from three buoys in Lake Superior provide a reliable record of water temperatures since about 1980, and the information also shows that, with water temperatures running in the mid-to-upper 60s (and even warmer closer to shore), âwe are at record temperatures for this time of year,â according to Austin.
The talc is formed by the alteration of sedimentary magnesium carbonate rocks (dolomite and magnesite) at elevated temperature and pressure below the earth's surface. While the magnesium is fixed in situ, the silica is transported by silica-containing hot fluids to react with the Magnesium- bearing carbonates to form talc. These altered rocks are then talc-rich dolomites or magnesites. This type of deposits usually deliver the massive talcs accompanied by carbonates, chlorite and some quartz.
Basically, all rocks containing talc are metamorphic rocks and talc is thus present as a secondary mineral. Talc is also called steatite â or, in chemical terms, hydrated magnesium silicate. It is the main component of soapstone. Its crystals usually develop massive, leafy aggregates with laminar particles. Ground talc is called talcum. Talc is the softest mineral, occupying place 1 in the Mohs` hardness scale. Its silicate layers lie on top of one another without having a chemical bond but are bound to each other by weak van der Waals forces.