THE FELDSPATHOID GROUP
The feldspathoids are not a typical mineral group like other mineral groups that are related by structure and chemistry.
Their relation to each other is due primarily to their relationship to the feldspar group of minerals.
Feldspathoids are low silica igneous minerals that would have formed feldspars if only more silica (SiO2) were present in the original magma.
The aluminum to silicon ratio is nearly 1:1 in most of the feldspathoids but is closer to 1:3 in most of the feldspars.
As would be predicted, feldspathoids are not found in igneous rocks that contain primary quartz.
Similar to the zeolites, the feldspathoids have large openings in the crystal structure.
These openings are mostly separated from each other and do not lend themselves to the movement of ions and molecules that is allowed in the zeolite's structure.
The openings do allow the structure to contain large ions like chlorine, carbonate and sulfate.
The lower density common to the minerals of this group is evidence of the openess of their structure.
The sodalite group is a group within a group.
It is composed of minerals with a similar isometric structure and related chemistry.
It is named for its most common member, sodalite.
The following are some of the more common feldspathoid minerals:
- Cancrinite (Sodium Calcium Aluminum Silicate Carbonate)
- Leucite (Potasium Aluminum Silicate)
- Nepheline (Sodium Potasium Aluminum Silicate)
Hauyne(Sodium Calcium Aluminum Silicate Sulfate)
Lazurite (Sodium Calcium Aluminum Silicate (Sulfate Sulfide Chloride))
Nosean(Sodium Aluminum Silicate Sulfate)
Sodalite (Sodium Aluminum Silicate Chloride)