I look to Dr Carolyn Dean for this kind of information. Here is what she says (in her e-booklet How To Change Your Life With Magnesium) about magnesium oxide:
“Magnesium oxide is the cheapest form of magnesium but it’s only 4 percent absorbed, the rest goes right through the intestines making it a fairly powerful laxative. It is used in many bowel‐cleansing products for its purging effect. Be very careful when taking magnesium oxide because it can cause diarrhea... Magnesium flushing through the intestines can pull out beneficial nutrients and good bacteria.”
Here’s what she says about magnesium citrate
“Magnesium citrate powder is the most commonly used form of magnesium. Mixed with water, either warm or room temperature, it is better absorbed than in pill or capsule form. Magnesium chloride is much like magnesium citrate, but in the powder form, it has a very strong taste."
And here’s what she says about Angstrom Liquid Minerals.
“I’m convinced angstrom minerals are going to revolutionize the mineral supplement industry...
"According to Joseph B. Marion; Anti‐Aging Manual (1999) ‘Most minerals (from food) are 8‐12% absorbable, chelated minerals are 40% absorbable, and liquid Angstrom minerals are 99% absorbable, being 7000 times smaller than Red blood cells, 1 Million times smaller than a colloidal mineral, smaller than ionic, with negative‐charge for added absorption.’ I’m not convinced that chelated minerals are that well absorbed but I appreciate his insight of over a decade ago about the importance of angstrom minerals.....
"It seems that only in the past few years have physicists determined that the mineral ion channels that allows minerals to enter cells are only 4‐5 angstroms in diameter. This information changed the face of cell fluid dynamics....
"The dosage of angstrom minerals is about 5‐10 times less than dirt‐minerals. That means a 60 mg dose of magnesium [of Angstrom Liquid Magnesium] is [equivalent to] about 300‐600 mg of dirt‐based magnesium. I judge the amount I need by my symptoms.”