I have a large Calcite xls (about 10x10cm) that is completely covered with fibrous Bavenite. When I bought the specimen it was unpacked in a box and compeltely settled with dust. I don't know how I could clean it without damaging the Bavenite.
Hope you can help me.
I would as a first step just swish it around in room temperature water and see if that helped clean it any. Ultrasonic cleaning in water should also be ok, but you should probably keep an eye on it. With the ultrasonic working, partially dip the specimen (an unimportant part of the specimen) into the vibrating water and keep a close look on what is happening. If clouds of the fibrous crystals come off the specimen pull the specimen immediately out and let it dry and examine the "cleaned" area and see if you think it has improved the specimen. If it has, then continue and clean the entire specimen. If you have a decent binocular microscope, before cleaning, examine the specimen and then after each cleaning thing you do to make sure it is cleaning the specimen and not damaging it.