I've been looking for the source of Maria Thun's assignment of the four elements to the zodiacal trigons: Fire/Aries-Leo-Sagittarius; Earth/Taurus-Virgo-Capricornus; Air/Gemini-Libra-Aquarius; Water/Cancer-Scorpio-Pisces.
That led me to read Claudius Ptolemy's BOOK I of Tetrabiblios, Schmidt translation, looking for the source of the assigment of the Elements to the zodiacal Trigons: This is not found in Ptolemy. Since Ptolemy (90-168 AD) was summarizing the astrological canon in his time in this book, it appears that the element/trigon assignments above may not be ancient.
Would anyone perhaps know the origin of these assignments?
Element/Trigon assignments
From Malcolm Gardner's response to Allan Balliett it would seem that Maria Thun (Just died on 8th Feb 2012 - God bless her) got this from Guenther Wachsmuth. You can read about this here.