This is a lightweight bag of vermiculite. Vermiculite lightens your soil and helps to hold moisture so your plants don't dry out as quickly. It is environmentally inert. 

Vermiculite is a hydrous phyllosilicate mineral which undergoes significant expansion when heated. Exfoliation occurs when the mineral is heated sufficiently, and commercial furnaces can routinely produce this effect. Vermiculite forms by the weathering or hydrothermal alteration of biotite or phlogopite. The more you know!

You should NOT use vermiculite with succulents as it holds moisture. Use perlite instead. Vermiculite absorbs water and is a key ingredient in many aspects of mushroom growing and fruiting -- something you definitely don't want to use with your cactii!

Vermiculite usage suggestions: 

Seed germination: Vermiculite, alone or mixed with soil or peat, is used to germinate seeds; very little watering is required. When vermiculite is used alone, seedlings should be fed with a weak fertilizer solution when the first true leaves appear.

Bulb storage: Pour vermiculite around bulbs placed in container. If clumps are dug, allow to dry for a few hours in the sun and then place in cartons or bushel baskets and cover with vermiculite. The absorptive power of vermiculite acts as a regulator that prevents mildew and moisture fluctuation during the storage period. It will not absorb moisture from the inside of stored tubers, but it does take up free water from the outside, preventing storage rot.

Soil conditioner: Where the native soil is heavy or sticky, gentle mixing of vermiculite—up to one half the volume of the soil—is recommended. This creates air channels and allows the soil mix to breathe. Mixing vermiculite in flower and vegetable gardens or in potted plants will provide the necessary air to maintain vigorous plant growth. Where soils are sandy, mixing of vermiculite into the soil will allow the soil to hold the water and air needed for growth.

Packing medium: Vermiculite is widely used as packing material for fragile items. It's super lightweight, so it won't increase shipping costs, and it is very absorbent so it wicks moisture and odour away from the product to keep it dry and smelling fresh.