Apophyllite can be a beautiful mineral, forming in lustrous, transparent crystals that are well-formed and occasionally very large. Though it is found worldwide in
volcanic zeolite environments, the Indian
traprock quarries have produced enormous quantities of this mineral in all different shapes, sizes, and colors, making Apophyllite easily obtainable and very affordable.
Apophyllite was originally regarded as a single mineral, with a variable ratio of fluorine to
hydroxyl. In 1978, the
IMA sub-classified into two distinct minerals:
Fluorapophyllite and
Hydroxyapophyllite. In 1981, an additional rare member with sodium replacing the potassium was added to the group, and called
Natroapophyllite.
In a controversial move, the
IMA has recently changed the scientific naming of these minerals for easier cataloging and alphabetical indexing. The new mineral names are Apophyllite-(KF) [instead of Fluorapophyllite], Apophyllite-(KOH) [instead of Hydroxyapophyllite], and Apophyllite-(NaF) [instead of Natroapophyllite]. Although use of the original names is now discouraged by the IMA, these names are still used and referenced, with the new names frowned upon by many collectors. The reality is that most collectors rarely sub-classify Apophyllite specimens, and simply label them all as Apophyllite.
Apophyllite-(KF) is the most common and abundant of the group. Most specimens labelled simply as "Apophyllite" are of the Apophyllite-(KF) form. Apophyllite-(KOH) is less common, but is still the dominating form of Apophyllite in several localities, including most of the Virginia occurrences. Apophyllite-(NaF) is very rare, and is found sparingly at only a few localities.
Apophyllite almost always occurs together with
zeolites, especially in
traprock environments. Apophyllite appears very similar to the zeolites, and is sometimes even confused with them. However, the physical structure of Apophyllite is different, with
tetrahedrons aligning in sheets as a
phyllosilicates, as opposed to the zeolites which are
tectosilicates.
Carletonite is a rare blue mineral that is similar to Apophyllite, and it only occurs in Mont Saint Hilaire, Quebec, Canada. Many classify Carletonite as group member within the Apophyllite group.
Apophyllite is named from a combination of the Greek word "
apo" - to be off, and "
phyllos" - leaf, alluding to the property of this mineral that it exfoliates (flakes apart like a leaf) when heated due to loss of water in its structure.
