Gypsum quality

Quality of gypsum

Gypsum quality if mainly determined by its purity. Gypsum (CaSO₄.2H₂O) purity is determined by the sulfur (S) content expressed as a percentage of the sample weight. When pure, gypsum will contain 18.6% sulfur.

Gypsum purity = S% ÷ 18.6 x 100

Most gypsum mines in southern Western Australia contain 14-18% sulfur, equating to gypsum purity of 75-97%.

The finer the particle size, the more reactive gypsum is. Gypsum is a moderately soluble salt (2.0-2.5 g/L) and as such, particle size is not an important quality attribute compared to sparingly soluble soil amendments such as lime (0.014 g/L). Gypsum is considered to have acceptable fineness when more than 50% of the gypsum sample passes through a 2 mm sieve.

Legislation around minimum standards for the sale of gypsum differ between states. Gypsum is suitable for agricultural purposes if:

• its purity is greater than 53.7% (South Australia) – 65% (New South Wales),
• has less than 15% water content,
• has greater than 50% of particles pass through a 2 mm sieve (SA and NSW), and
• contains less than 1.2% chloride (NSW).

Western Australia’s Agricultural and Veterinary Chemicals (Western Australia) Act 1995 has no reference to gypsum.

Gypsum deposits, WA

In Western Australia, most of the gypsum used in agriculture is mined from dry salt lakes (playas). These lakes have formed from infilled ancient drainage systems. Salts, including gypsum, crystallise as the salt water evaporates. Hench much of the gypsum found in Western Australia is associated with salt lakes and in environments with low rainfall and high evaporation.

Often, wind erosion of the salts within the dry lake bed results in gypsum-rich lunettes (crescent-shaped deposits) forming on the southern and eastern margins of the salt lakes. There are three main types of gypsum found in lake bed deposits.

• Selinite is the crystalline form of gypsum.
• Gypsite, also known as kopi, is in the form of a powder.
• Gypsarenite is a porous granular material also known as seed gypsum.

Both gypsite and gypsarenite are commonly found together in the lunettes and are the main types of gypsum mined for agriculture. Mined gypsum from these sources is commonly 70-90% pure. Impurities include clay, sand and other salts. Shell remnants from prehistoric snails (Coxiella sp.) that are up to 40 000 years old are often found in lunettes.