There are several different choices for Mineral Powders to feed your lawn: 

Granite Dust - Containing 3-5% potassium, this powder is often used by organic farms as a slow release form of potassium. This is not a very expensive option. 

Greensand - Greensand, also known as glauconite, is a very expensive but good source of potassium. It contains about 8-10% potassium and slowly releases it into the soil. There are many organic gardeners who swear by this as an autumn fertilizer. 

Gypsum - Used for its high calcium content, about 22%, gypsum is a widely used organic fertilizer. It is shown to improve sodic soils more than clay soils, but it is shown to be beneficial in any soil it is applied to. 

Langbeinite - This is  form of potassium sulfate extracted from the Great Salt Lake in Utah through a specialized process.  Potassium is used during seed establishment or to give your lawn an extra boost going into winter. Langbeinite is less water soluble than other forms of potassium fertilizer, making it a slow release form potassium. Be careful when using this product. It contains 22% potassium, 18% magnesium, and 27% sulfur. Using products containing magensium and/or sulfur without a soil test runs the risk of soil compaction from too much magnesium or an acidic pH due to too much sulfur. 

Limestone - Popular in the East for its pH balancing quality, limestone is a regular component of lawn care. Before applying to your yard, you should run a soil test to ensure there are no negative side effects. Dolomitic lime is prefered over calcitic lime by organic gardeners because it has a lower magnesium content. Limestone contains about 30% calcium and dolomitic lime contains around 6-15% magnesium. Because it is slow to move through the soil, it is recommended to apply lime in the fall. Limestone is available in powder or pellet form. The pellets move quicker into the soil due to a quicker dissolve rate than the powder. Note: If applying powdered limestone, be sure to wear a mask to avoid breathing it in and NEVER use a rotary spreader. Be sure to use a drop spreader.

Rock Phosphates - Rock phosphates can contain up to 30% phosphorus but only 2% may be available to the soil in a given year. 

Zeolites - Used frequently in Europe, Zeolites are beginning to make their way into organic lawn care in the US as a soil amendment. They contain 3.2% potassium, 2.5% calcium, aluminum and silicone.