You can make sure it is halloysite using XRD as they are chemically identical to kaolinite and it's hard to distinguish them otherwise. Halloysite tubes do not need to be "exfoliated", in fact unrolling the tubes would ruin the properties. You simply grind them using low energy so as not to damage the tubes. SEM should confirm you have suceeded.

Could treated in ultrasonic and density seperation achieve the goal?

Prior to the treatments, grinding the sample (should be limited based on needs) need to be done.

The ore is hard like marble so no, that doesn't work. BTW I have quite a bit of experience with halloysite so speaking from first hand experience.

Thank you for your response

I found description as follow: 

"Raw halloysite was purified as the method introduced in the

reference [28]. Typical procedure is described below. A

10 wt% water solution of halloysite was prepared by slow

addition water to dry halloysite. Then 0.05 wt% sodium

hexametaphophate was added in the solution while stirring.

The solution was stirred for 30 min and standing for 20 min

under room temperature. The clay aggregate and impurities

was precipitated in the bottom and were removed by

filtration. The upper solution was carefully collected and

the resulting HNTs was separated by centrifugation and dried

at 80°C in air for 5 h."

in: Mingxian Liu & Baochun Guo & Mingliang Du & Yanda Lei & Demin Jia, Natural inorganic nanotubes reinforced epoxy resin nanocomposites, J Polym Res (2008) 15:205–212

What are your opinions about this?

One cannot make a "water solution" of a solid in a liquid because solution applies only to dissolved species. One must grind the halloysite before it can be dispersed in water. Sodium hexametaphosphate is a very effective dispersant for kaolin group minerals but then you will not isolate pure halloysite but will instead have particles coated in a layer of sodium hexametaphosphate. Halloysite always occurs with kaolinite and this method will not separate the two.

Selective ultrasonic treatment will disperse particles cleaning the halloysite nanotubes

exterior and is particularly good at removing iron, which is often associated with the ore.

Aeration can be beneficial once the ore has been crushed using ultrasonic cavitation.