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Due to the fact that feldspars and feldspathoids form approx. 60% of the Earth’s crust, they are readily available on each and every continent in abundance. Major quantities are sourced for glass and ceramic production. Special grades of feldspars are used for ceramic veneers in dentistry.
World production of feldspars and nepheline syenites accounts for 20 million tons per year (2011); biggest producers are Turkey, Italy, and China (USGS 2012).
Nepheline syenite is not that common - so suitable qualities for glass, ceramics, and filler applications are produced in Canada (580Ttpa), Norway (350Ttpa), and China (50Ttpa) (USGS 2010) apart from syenites used as aggregates for road construction and similar uses.
Feldspars and nepheline syenite are weathering and chemical resistant against weak acids. Particle shape typically is angular/nodular due to the fracture habit (uneven - flat surfaces, fractured in an uneven pattern) which results in low surface area and as a consequence low oil absorption of the milled materials. Particle sizes from rocks down to few hundreds of nanometers commercially are available. pH typically is in the range of 6-9 for feldspars and at 9-10 for nepheline syenites.
The following Figs. 1, 2, 3, and 4 show typical particle shapes of milled feldspar and nepheline syenite products. Surfaces and structures of the grains in a feldspar flour depend on the chemical/mineralogical composition of the mineral and are a function of the genesis of the deposit.
Fig. 1 SEM of feldspar flour: 44% orthoclase, 45% albite, 4% anorthite, and 7% quartz
Fig. 2 SEM of feldspar flour: 85% albite, 3% orthoclase, 4% anorthite, and 8% quartz
Fig. 3 SEM of feldspar flour: 87% orthoclase, 7% albite, 4% kaolinite, and 2% quartz
Fig. 4 SEM of nepheline syenite flour: 25% nepheline, 73% feldspars, and 2% analcime and natrolite
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