1-Talc and graphite are two of the lowest minerals on the hardness scale. They are also described by terms like greasy or soapy. Both have a crystal structure characterized by sheet-structures at the atomic level, yet they don't behave like micas. What accounts for their unusual physical properties

They have weak chemical bonds between them therefore their sheets can easily slip.

Explanation:

Talc is structurally similar to the micas, having weakly bonded sheets.  These sheets are held together only by van der Waals forces therefore, they can slip past each other easily. Hence, talc is extremely soft,  greasy, soapy, and can function as a high-temperature lubricant.

Graphite, consists of carbon atoms arranged in a hexagonal network to form sheets that are about one atom thick. The sheets are held together by weak Van Der Waals forces hence they can slide over one another when subjected to a small  force. So graphite is not hard and is slippery.

Therefore, the  unusual properties of these substances stems from the fact that the bonds between the sheets are weak and a very low stress can cause a slip between the sheets.

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